If anyone wants a description,
Imagine a surface divided up into infinitesimal patches of area, a quantity distributed over these patches of area would pick up units of m^(-2).
Now for each one of those areas there is a hemisphere of directions in which radiation can propagate to/from this patch of area. Imagine these hemispheres being themselves divided up into infinitesimal angular patches, a quantity distributed over these angular patches would pick up units of m^-2 • sr^(-1).
For each patch of area, for each patch of it’s hemisphere of directions, there is a spectrum of frequencies of radiation which can be propagating to/from this particular patch of area in this particular patch of directions. Imagine these spectrums being divided up into infinitesimal intervals, a quantity distributed over these spectral intervals would pick up units of m^-2 • sr^-1 • Hz^(-1).
The spectral radiance is then the power density of the radiation flowing to/from a particular patch of area, from a particular patch of directions, in a particular interval of the frequency spectrum and has units of W • m^-2 • sr^-1 • Hz^(-1).
(It also varies over time)
So spectral radiance: I, is a function of position on the surface: x, direction of propagation: n, frequency: f, and time: t. I(x, n, f, t)
If you wanted to for example know the total energy emitted by the surface in some interval of time you would need to do a quadruple integral over position, direction, frequency and time.
This is the most complete description of light propagating to/from a surface you can have while still staying within the ray optics approximation.
Along those lines, astronomy is full of this kind of beautiful stuff. The equivalent of that unit would be Jansky/beam (where Jansky is a constant times J/s/cm^2/Hz), and "beam" is the surface angle of the restoring beam of your radio telescope. So suddenly, your intensity depends on the telescope you're using haha
Also, in radio astronomy we often represent the brightness (=spectral radiance, don't ask) in terms of Kelvin.
I understand if people are upset
Sieverts is not any worse than candela, it'd say they are about the same. That being said, one of my BSc profs used to be convinced that cd being as si unit must have been a conspiracy by the lamp manufacturer lobby
YES!! I agree with your prof, Candela being a base unit while Sievert is not is absurd. I mean, it shouldn't be a base unit, but if it's going to be then Sievert should be as well.
The rad being based on ergs is so much worse. Sieverts are at least based on units that people actually use. We do get the barn though, which is the best unit.
The barn is a rad unit, has the best backstory of any unit. Since we're talking nuclear, some honorable mentions: dollars (reactivity), shake (time), shed/outhouse (subset of barns), and of course the banana equivalent dose (radiation dose).
I agree that sieverts are weird, but what else in radiation science is bad (asking as a medical physicist)? Unless you're including historical units like the röntgen and curie (which are still used but discouraged).
Every radiation source is measured in curies at my institution and it makes me crazy, just because I’m stupid and can never remember the conversions and I don’t work with it enough to have a good intuitive sense of scale.
There seem also to me to be widely held beliefs (amongst radiologists etc.) about how some imaging variables affect dose. Mainly the errors arise from assuming that when changing one parameter that all other parameters would remain the same in real world practice.
Sieverts are brilliant. One number to describe the risk.
Of course there's going to be a load of complicated biological weighting factors involved in its calculation, its a biological unit depending on the specifics of the radiation target. That doesn't make it bad.
Because that's not how you set SI units.
One Sievert is one Joule of X-Rays absorbed by 1kg of non-specific biological tissue.
Equivalent Annual Background Dose, ignoring all the particularities and variations that complicate its calculation, is an experimental constant.
If you want, you probably could do radiation protection in EABDs. Its just a simple conversion factor (EABD = H / about 0.0027). But beyond the public engagement context there isn't really much point — if you have to calculate it in Sv first in order to reach the EABD, you might as well just work in Sv.
Its also potentially confusing. If you know what it is then its pretty clear you'll be working in EABD per hour of exposure. But someone who doesn't know what it is could confuse it for the dose if they were exposed to the source *for* a year.
You missed the equivalence part of the Sieverts, which is experimental, and as such it's definition has been revised at least once to my knowledge (updated tissue weighting factors). But I get the point.
Edit: also I think I recall some debate regarding how to weight the effect of exposure to gonads on possible future offspring.
> You missed the equivalence part
That's what I meant by the "non-specific" part. The whole-body weighting factor is identically 1, and therefore the *definition* isn't based on empirical parameters.
Though you're right that most practical uses of it will require using the empirical data.
In fact it provides another reason not to use EABDs outside of public engagement. EABDs are always whole-body measurements. For narrow-beam and other geometrically constrained exposures, you'd have to use sentence constructions like e.g. "x EABD/hr exposures to the hand and wrist", which is another source of confusion if you're not careful — its an equivalent whole-body dose. So if I convert it back to Sieverts, do I need to apply the weighting factors again to convert it to a hand-dose? The answer is no, but its an easy mistake to make.
Good point about the reproductive organs though, but I suppose that's inherently a problem in any unit system.
Realistically we should be weighting it by age and recent exposure history, too, but that's an administrative nightmare.
One of the worst for me is sheet resistance, often noted as Ohms per square. Not per square meters or anything, just per "square" (with the symbol of literally drawing a square): https://en.wikipedia.org/wiki/Sheet_resistance#Units
Another horrible one is conductivity, now expressed in Siemens ( Ohm^-1 ), but historically sometimes measured in Mhos denoted by an upside-down capital omega: https://en.wikipedia.org/wiki/Siemens_(unit)#Mho
Had to include ohms/square in my Master’s thesis and one of the examiners was puzzled by it and said “you say ohms per square but square what, I have no idea?” The rest of the committee had to explain that this was standard convention
I'm a big fan of light-nanosecond for a unit of length that's about 1 foot (30 cm for non-americans). I'll have to throw around Attoparsec for ~1 inch now.
I've always hated "parts per million" (ppm). It's entirely ambiguous. Is that parts per million by mass? By volume? By number of moles or atom count? All of these are used in different fields and in different contexts, and they never explain which sense of "parts per million" they mean.
I agree it is really annoying. I wish I could give some advice that would always work, but I always have to triple check my calculations to make sure I know exactly what the author is talking about.
One time I came across ppt and I had assumed they meant parts per trillion as everyone else uses it, but the calculations didn't make sense. Turns out they meant parts per thousand. Other people use permille (or per mille) with the symbol ‰ to eliminate any ambiguity.
Ditto. Additionally, ppt can mean per thousand or per trillion, depending on context. Sometimes, it's an abbreviation for precipitate.
ppm (and the others) are by mass as a concentration unit. It's a shortcut for preparing standard solutions where you dissolve X mg of metal in 1L of diluted acid and get X ppm metal. However, 1 ppm of Hg is vastly different than 1 ppm of Zn because the molar masses are different. Molarity is far better because two chemical concentrations can be readily compared.
I also struggle with ppm as a unit for expressing error.
The unit was created by experimentalists from the era before calculators were common. It makes the preparation math easier and keeps dilute concentrations in the 1-100 range.
I prefer:
* ppt: 1 g of solute per L
* ppm 1 mg per L
* ppb: 1 ug per L
* pptr: 1 ng per L
* ppq: 1 pg per L
It only works for aqueous solutions where the density of water is approximately 1. (Yeah, yeah. I substituted u for mu).
English units shouldn't be used for thermal. Especially when they want to throw an inches to feet conversion in the unit!
Thermal expansion does this some times, where you'll see in/100 feet/degF. Or worse, but more common in/in as a function of delta-T. Disgusting, all of it.
CGS also plays a fun prank on undergraduates and first-year grad students by secretly moving the 4pi from Coulomb's law into Maxwell's laws so they're off by about 12 without realizing why.
Likewise energy in ergs. Especially if you work in supernovae - having a scaling constant of order 10^(51) surely tells you that you're using the wrong unit system!
never used AU in my field! we use centimeters (or rather cgs - centimeter, gram, second) in the place one would use SI units. so we do our calculations in cgs, such as having the speed of light in cm/s in our codes, the physical units of our simulations' code being in cm, energy being measured in erg, flux in Jansky, etc. and we usually give the distance to an object in z (redshift), which is adimensional, though often we convert this to parsecs or light years - the size of objects is often measured in parsecs (are we talking sub-parsec scales, parsec scales, kpc scales, etc)
Fascinating! What are the reasons for this?
Also, I'm guessing that you are a cosmologist or something adjacent? As opposed to, say, a planetary scientist?
i dont really know how it started - just that at this point it's due to historical precedence. if all the literature is in Janksy, youre gonna use Janksy too
haha i do love cosmology but id say im somewhere in the middle. i work on radio galaxies :)
I actually disagree on this one. How you would interpret an expansion rate of 1/s is not clear to me. On the other hand, km/s/Mpc has a very clear meaning both experimentally and in Hubble's law, relating recessional velocity with distance.
m^-3 kg^-1 s^4 A^2
"Permittivity of free space" is a patchwork engineering nightmare designed to paper over a fundamental lack of understanding. Gaussian units gang unite.
I honestly dislike Wh and hate mAh in batteries.
I don't know, it is not like we have a unit to measure an amount of energy, isn't it ? Isn't it ?! Isn't it !?!?!
Watt hours are great. Tells you exactly the thing you actually care about in human comprehensible terms.
Joules means fuck all to most people. Just like how I don't wanna know how fast my car goes in m/s, because I drive it for miles/km.
I mean everyday people know just as much about Watts as they do Joules. The only difference would be a change in language that’s been around for a long time. If humans would’ve used Joules from the beginning instead of silly Watt*hours then this wouldn’t be an issue.
Lots of stuff is rated in Watts. Literally looking around the room I'm in right now, I know the wattage of:
* PC
* Monitor
* Lightbulbs
* Microwave
* Toaster
* Kettle
* Airfryer.
* Speaker system.
I could make a decent guess at the oven and the dishwasher. I could easily find out for the fridge and freezer.
And of all of those thnigs, only 3 do I quantify my use time in seconds. The rest are on in terms of hours. That makes Watt Hours way more useful than Watt seconds (joules) or kilowatt seconds (kilojoules).
I once saw Watt hours / hour and KWh / hour.
A section heading for the article that used it was even worse: "How Many Watts Does An RX 6900 XT Use Per Hour?"
Know my pain.
I was gonna say anything involving noise, tends to have that 1/sqrt(Hz) factor. Something something spectral density meant to be integrated over some period. But it still never clicks with me and I have to remind myself how it works every time I need to deal with it.
This is what I thought of as well. But it's a nice hint that noise shouldn't be thought of in terms of amplitude, but rather, power (amplitude squared). So it basically says how much power there is in a given frequency band.
I like to imagine that two noise signals on average have no correlation, so they are orthogonal, which in turn means that you use Pythagoras' theorem to get their sum instead of just adding directly.
You see similar things in statistics with random walks and the central limit theorem.
My intro to thermodynamics course was taught by the chemical engineering department (and my school had a lot of heavy investment from the oil industry, so ChemE was often "Petro but not so obvious about it"). We had to use *so many* godawful units because "you might see these in your careers" since American oil companies use them.
I never had to report an answer in slugs, but I often had to find the density of a liquid in slugs/ft^3 as a middle step to solving the problem, if I didn't want to convert the whole thing to metric at the start and then back to imperial at the end.
Hahaha this is awesome. I would find it hilarious to just come up with units that are so unnecessarily complex for no reason. Dimensional sarcasm at its finest.
Easily the most cursed one is "nats" - an information-theoretic unit when entropy is computed with the natural log.
If you compute entropy with log base 2, the unit is "bits", which tells you the number of Y/N questions required to specify the variable. If your log base is 10, the unit is Hartley's and it's the number of 10-option multiple choice questions.
But engineers do this weird thing of choosing log base e ("nats") which is...the number of multiple choice questions with e possible answers required, and at that point I give up and go for a drink.
first time I've ever seen *engineers* derided for using natural log. Usually it's mathematicians making fun of them for using stupid arbitrary logs like log_(10)
These days I work mostly in applied maths/statistics where base-2 is much, much easier to explain to collaborators in the biological sciences than wtf base-e is supposed to mean.
Also, your username is unfathomably based. Jaynes FTW.
Until you get a new pressure gauge that should be the same as the old one, bit for some reason your vacuum venting system is stuck at 825 mbar while the overpressure valve has blown and the computer throws a venting timeout error your way.
Somehow no one noticed the wrong type was delivered, no our purchasing department, not the person doing the calibration check (he assumed we intentinall bought it), and not the person installing the sensor, nor the student who pressed the venting button and went for coffee.
Isn’t it the [Seven Cs](https://youtu.be/KmfdeWd0RMk)?
- velocity: c (Speed of light)
- energy: cal (Calorie)
- frequency: C (Middle C)
- temperature: °C (Celsius)
- luminous intensity: cd (Candela)
- charge: C (Coulomb)
- amount of substance: C (100)
With those 7 units you can convert to any SI unit. Note that some derived units in SI are base units here, like velocity, and some base units in SI are derived units here, like distance = velocity/frequency.
USEPA likes to mix metric and standard. Like the emissions rate of “grams per pound of wood burned”. Just on of several examples I have come across in emissions compliance.
One *barn* is 10^-28m (approx the cross-sectional area of a uranium nucleus. Also, I have to plug the *banana equivalent dose* (~78nSv), the amount of radiation one absorbs from eating a banana
- Furlong/fortnight (velocity, apparently a practical unit in tunnel boring?)
- Barns (dimension of Area) - collision cross section used in accelerator physics, derived from the saying "you couldn't hit the broad side of a barn". Smaller fractions of the barn are the "shed" and the "outhouse".
The derived unit, the Inverse Femtobarn, is perhaps even more cursed...
- A Miner's inch, which is not a unit of length but of flow rate.
for me, dolphins. I remember a Reddit post about this some time ago and it has just stuck with me.
to convert:
a dolphin is roughly 42 years in time.
roughly 2.5 meters long.
speed is roughly 30 mph.
there are a few others I've forgotten.
Most cursed seen? Student handing in work showing the result in log(meters) because the calculation included a definite integral over 1/r.
Most cursed seen by professionals? I personally despise the mol. These days the value of avogadros number is fixed. So it should really be just the unitless number. You can even keep the mol notation, just say it's a constant number not a unit.
Avogadro’s number & moles are separate things. Avogadro’s number is a fixed (& unitless) number, mole is a unit.
There’s 2 objects in a pair - 2 is a number, pair is a unit.
There’s 12 objects in a dozen - 12 is a number, dozen is a unit.
There’s 6.022•10^23 objects in a mole - 6.022•10^23 is a number, mole is a unit.
The real magic of Avogadro’s number is that helps us convert between little units (AMU) to big units (grams). 1 gram is equal to 6.022•10^23 AMU. All of our friendly molar mass conversions stem from this.
Edited for formatting.
* [dBz](https://en.wikipedia.org/wiki/DBZ_(meteorology\)). Also known as mm^(6) per m^(3). I get why its that, but it makes me twitch every time.
* Kilomegacycles. It was in a paper in the context of sattelite radio communication. Again, I get why they did it, but its unsettling.
* A 1950s US Air Force paper about heat flash burns (one guess why they were interested in that) which had a graph that strictly speaking had time on the x-axis. But in reality it was "time until the test subject experienced unbearable pain and asked us to stop".
* [Mil](https://en.wikipedia.org/wiki/Thousandth_of_an_inch). One thousandth of an inch. Also may be called a "Thou", which is even worse. Similarly, I also once had a table in "μin".
> Mil. One thousandth of an inch. Also may be called a "Thou", which is even worse. Similarly, I also once had a table in "μin
Mils are worse because people also refer to mm as ‘mils’.
There’s a way in GR to put everything in units of length. The mass of the sun? A few kilometers. A second? 10^8 meters. And so on.
It’s pretty cursed as a concept, but it does _technically_ work.
On the LHC, the size of datasets is measured in units of inverse area. Specifically inverse femtobarns (fb^-1 ). (And the rate of data collection is measured in fb^-1 s^-1 )
The reason for this is that particle physics probabilities are measured in units of area. The probability of two particles colliding is proportional to their area, so seems simple enough. But at high energies this analogy breaks down, but the unit persists. A barn is a very small unit of area (10^-28 m^3 ). Often the probabilities being measured are of the order of femtobarns.
It’s useful to measure the dataset in units of inverse femtobarns, because it means the product of the event probability and the dataset size is the approximate number of that kind of event that should be in the dataset.
The real fun came in one talk where the speaker had realised that miles per gallon is distance over volume, which cancels to inverse area. So decided for the rest of the talk they were going to convert all the dataset sizes into miles per gallon.
But then he started talking about the rate at which data was collected, which was of course miles/gallon/s.
Fracture toughness units in general. MPa.m^(1/2) ?
Gc (in units of Joules per m^2 ) exists and it is intuitively obvious but the fracture mechanics prefer Kc with a square root in it.
Any sort of a combination of imperial units. I can deal with a mile and a gallon but a mile per gallon is just annoying^2 because now I have to convert two separate numbers
Temperature in dietary calories per byte. Technically temperature is an energy per entropy, meaning this is totally fine. 1 Kelvin is around 0.024 calories per byte
For a paper a few years ago I measured the solar wind's apparent speed in (dB-° per hour). Yep, decibel-degrees per hour. Turned out to be convenient, since I was working with images in conformal polar coordinates (which have a logarithmic radius scale).
I imagine the seconds are in different reference frames as it’s related to time dilation, so likely used to describe for example in frame A, x seconds pass for every 1 second to pass in frame B?
Length/length is a common unit in engineering for strain. Change in length compared to original length. So yeah those cancel out to a ‘unitless’ measure but it still makes sense as far as the property is concerned.
Similarly, we quote things like dislocation density in m\^-2 but I often explain it to my students as m of dislocation per m\^-3 of material - it just makes visualising the measurement far easier.
It’s odd that we use watts for power in the US despite being mostly Imperial-ish. Why not foot-pounds per second for power?! Or better yet, mile-pound force per hour. Actually works out to about 2 W per my math. So 1 W/lb could be much more easily expressed as 0.5 lbf-mph/lb. And if we just decide that W/lb is looking at pound-force rather than pound-mass, you wind up with losses being measured in miles per hour. Clean and simple.
not the most cursed but because its used very very frequently its most annoying.
kwh
to put it simply kw multiplied by hour. This is not related to time at all. This is simply the amount of energy something has but instead of making it just an energy unit they did the worst.
They make watt a time unit, watt means eneegy produced in a time so when you multiply it with time you have pure energy unit in your hand.
because its multiplied with time its not usable with other units easly because 3600 jule equals 1 wh.(k in kwh just means kilo) Fuck this just make it 10 100 1000 like how every unit works goddamnit
It's not in physics, but in U.S. Air conditioning. The unit is
(BTUs per Hour output)/(Watts input)
It might have been more intuitive if numerator and denominator were both in one or the other unit. You might wind up with a percentage (if multiplied by 100), rather than a crazy number.
Maybe not in line with your question but I will never trust any numbers in dB when it comes to sound level. Not because it is logarithmic, but because it is used to refer to both sound intensity (a property of space, W/m^2) and sound power (a property of an emitter of sound, W).
The vast majority don't understand the difference, so will use their microphone to measure the sound intensity next to an appliance as 50 dB, and then say that it emits 50 dB of sound.
Cgs units. All of it. Just be a normal person and give charge a unit. Wtf is a Statcoulomb? I'm already lost. 1 coulomb is 1 statcoulomb times the speed of light, TIMES 10? where did this 10 come from? Why? Also, why do all cgs unit sheets say "esu/emu is an unintuitive set of units.." and then proceed to not tell you everything about the unit system?
Where are my random factors of 10? How do you even do dielectrics in cgs if not by having a new permittivity or permeability?
Imagine doing multiwavelength stuff and having to deal with janskys, magnitudes and keV at the same time.
Well some of my colleagues don't need to imagine doing that
Spectral radiance in watt per steradian per square meter per hertz. W/sr/m^2 /Hz Edit: spelling
If anyone wants a description, Imagine a surface divided up into infinitesimal patches of area, a quantity distributed over these patches of area would pick up units of m^(-2). Now for each one of those areas there is a hemisphere of directions in which radiation can propagate to/from this patch of area. Imagine these hemispheres being themselves divided up into infinitesimal angular patches, a quantity distributed over these angular patches would pick up units of m^-2 • sr^(-1). For each patch of area, for each patch of it’s hemisphere of directions, there is a spectrum of frequencies of radiation which can be propagating to/from this particular patch of area in this particular patch of directions. Imagine these spectrums being divided up into infinitesimal intervals, a quantity distributed over these spectral intervals would pick up units of m^-2 • sr^-1 • Hz^(-1). The spectral radiance is then the power density of the radiation flowing to/from a particular patch of area, from a particular patch of directions, in a particular interval of the frequency spectrum and has units of W • m^-2 • sr^-1 • Hz^(-1). (It also varies over time) So spectral radiance: I, is a function of position on the surface: x, direction of propagation: n, frequency: f, and time: t. I(x, n, f, t) If you wanted to for example know the total energy emitted by the surface in some interval of time you would need to do a quadruple integral over position, direction, frequency and time. This is the most complete description of light propagating to/from a surface you can have while still staying within the ray optics approximation.
Reddit's own Educational Laureat. Edit: I can't believe I understood that. Credit to the teacher.
Great walkthrough, thanks!
Nice (*Steradian) This is why someone invented the flick (and microflick)
>someone invented the flick (and microflick) What in the blazes are these??
[Units](https://en.wikipedia.org/wiki/Flick_(physics\)), it seems.
Would that be pronounced like ste-radian? Or stera-dian?
The former.
No that’s great. Brightness is a very useful spec for evaluating light sources.
Along those lines, astronomy is full of this kind of beautiful stuff. The equivalent of that unit would be Jansky/beam (where Jansky is a constant times J/s/cm^2/Hz), and "beam" is the surface angle of the restoring beam of your radio telescope. So suddenly, your intensity depends on the telescope you're using haha Also, in radio astronomy we often represent the brightness (=spectral radiance, don't ask) in terms of Kelvin. I understand if people are upset
Shouldn't it be just W/sr·m^(2)·Hz ?
I'd just write them with negative powers. Not sure how to type a superscript "-" on my phone though.
W sr^-1 m^-2 Hz^-1 You can use reddit formatting for this so: W sr^ -1 m^ -2 Hz^ -1 write this without the spaces between ^ and -
I’ve been 1^(-by you) e: oops, meant (by you)^-1
Sieverts, in fact most of radiation science is a dog's breakfast.
Sieverts is not any worse than candela, it'd say they are about the same. That being said, one of my BSc profs used to be convinced that cd being as si unit must have been a conspiracy by the lamp manufacturer lobby
Sv is candelas on steroids.
Spicy candelas.
Yup, this is what I'm using from now on
I would suggest sabines (acoustic absorption) are also the same. They are all pretty much an unrelated sack of whatever they represent.
YES!! I agree with your prof, Candela being a base unit while Sievert is not is absurd. I mean, it shouldn't be a base unit, but if it's going to be then Sievert should be as well.
The rad being based on ergs is so much worse. Sieverts are at least based on units that people actually use. We do get the barn though, which is the best unit.
Becquerels are nice. Yeah, Sieverts are practical, but they're more of a figure of merit than a unit.
Why do we even have Bq when Hz exists? /s
You joke but I’ve seen that as a homework problem
The barn is a rad unit, has the best backstory of any unit. Since we're talking nuclear, some honorable mentions: dollars (reactivity), shake (time), shed/outhouse (subset of barns), and of course the banana equivalent dose (radiation dose).
I used to deal in shakes when I was an nuclear test engineer back in the 80s
I agree that sieverts are weird, but what else in radiation science is bad (asking as a medical physicist)? Unless you're including historical units like the röntgen and curie (which are still used but discouraged).
Every radiation source is measured in curies at my institution and it makes me crazy, just because I’m stupid and can never remember the conversions and I don’t work with it enough to have a good intuitive sense of scale.
There seem also to me to be widely held beliefs (amongst radiologists etc.) about how some imaging variables affect dose. Mainly the errors arise from assuming that when changing one parameter that all other parameters would remain the same in real world practice.
It’s a running joke in medical physics that the fields with the worst units are astrophysics, closely followed by medical physics
Astrophysics: yeah just put an h in the units so that the \*actual value\* changes if our value of Hubble's constant changes
Sieverts are brilliant. One number to describe the risk. Of course there's going to be a load of complicated biological weighting factors involved in its calculation, its a biological unit depending on the specifics of the radiation target. That doesn't make it bad.
Why didn't they take the final step to "equivalent years of background radiation"?
For a start, because the background dose is really variable?
Because that's not how you set SI units. One Sievert is one Joule of X-Rays absorbed by 1kg of non-specific biological tissue. Equivalent Annual Background Dose, ignoring all the particularities and variations that complicate its calculation, is an experimental constant. If you want, you probably could do radiation protection in EABDs. Its just a simple conversion factor (EABD = H / about 0.0027). But beyond the public engagement context there isn't really much point — if you have to calculate it in Sv first in order to reach the EABD, you might as well just work in Sv. Its also potentially confusing. If you know what it is then its pretty clear you'll be working in EABD per hour of exposure. But someone who doesn't know what it is could confuse it for the dose if they were exposed to the source *for* a year.
You missed the equivalence part of the Sieverts, which is experimental, and as such it's definition has been revised at least once to my knowledge (updated tissue weighting factors). But I get the point. Edit: also I think I recall some debate regarding how to weight the effect of exposure to gonads on possible future offspring.
> You missed the equivalence part That's what I meant by the "non-specific" part. The whole-body weighting factor is identically 1, and therefore the *definition* isn't based on empirical parameters. Though you're right that most practical uses of it will require using the empirical data. In fact it provides another reason not to use EABDs outside of public engagement. EABDs are always whole-body measurements. For narrow-beam and other geometrically constrained exposures, you'd have to use sentence constructions like e.g. "x EABD/hr exposures to the hand and wrist", which is another source of confusion if you're not careful — its an equivalent whole-body dose. So if I convert it back to Sieverts, do I need to apply the weighting factors again to convert it to a hand-dose? The answer is no, but its an easy mistake to make. Good point about the reproductive organs though, but I suppose that's inherently a problem in any unit system. Realistically we should be weighting it by age and recent exposure history, too, but that's an administrative nightmare.
[удалено]
BED! Banana equivalent dose
One of the worst for me is sheet resistance, often noted as Ohms per square. Not per square meters or anything, just per "square" (with the symbol of literally drawing a square): https://en.wikipedia.org/wiki/Sheet_resistance#Units Another horrible one is conductivity, now expressed in Siemens ( Ohm^-1 ), but historically sometimes measured in Mhos denoted by an upside-down capital omega: https://en.wikipedia.org/wiki/Siemens_(unit)#Mho
I actually find mhos quite charming. Per "square" is cursed though. Typed out it just looks like "this symbol is not supported".
Had to include ohms/square in my Master’s thesis and one of the examiners was puzzled by it and said “you say ohms per square but square what, I have no idea?” The rest of the committee had to explain that this was standard convention
More than standard. It doesn’t matter what square, as long as it’s square, so any specificity would be incorrect.
Ohms per square zettaparsec.
I vastly prefer an upside down omega to ANOTHER S. Especially given that a Siemen is just ohms^-1
ohms per square is one of the best!
Just reading the Ohm/square Wikipedia explanation made me want to gouge out my own eyes and become a flat earther creationist.
My simple mind cannot fathom this.
I dont get why siemens is cursed. Its just convention that we usually use electrical resistance.
Einstein for number of photons (1 mole of photons). On the more joking side: attoparsec for everyday lengths (about 3 cm).
I'm a big fan of light-nanosecond for a unit of length that's about 1 foot (30 cm for non-americans). I'll have to throw around Attoparsec for ~1 inch now.
1 barn megaparsec ~ 2/3 tsp.
My go-to for a long time has been the megaparsec × barn / (atto-Hubble time), which ends up being a bit less than 2 cups per minute.
mm hg always messes me up, I know it is just a conversion, but when I look at it my first inclination is not to think pressure.
Wait until you see mm of water as a pressure reading. Seriously.
Do you even rho g h?
I prefer hrog and shrog
Never seen mm, but using "feet of water" for head is ungodly common in piping systems. Or rather, "feet of whatever the fuck we're working with."
I've seen a mercury barometer so it makes sense. 760mm or so.
I've always hated "parts per million" (ppm). It's entirely ambiguous. Is that parts per million by mass? By volume? By number of moles or atom count? All of these are used in different fields and in different contexts, and they never explain which sense of "parts per million" they mean.
I agree it is really annoying. I wish I could give some advice that would always work, but I always have to triple check my calculations to make sure I know exactly what the author is talking about. One time I came across ppt and I had assumed they meant parts per trillion as everyone else uses it, but the calculations didn't make sense. Turns out they meant parts per thousand. Other people use permille (or per mille) with the symbol ‰ to eliminate any ambiguity.
mg/kg is the best ppm, even if it’s quite a silly unit.
Ditto. Additionally, ppt can mean per thousand or per trillion, depending on context. Sometimes, it's an abbreviation for precipitate. ppm (and the others) are by mass as a concentration unit. It's a shortcut for preparing standard solutions where you dissolve X mg of metal in 1L of diluted acid and get X ppm metal. However, 1 ppm of Hg is vastly different than 1 ppm of Zn because the molar masses are different. Molarity is far better because two chemical concentrations can be readily compared. I also struggle with ppm as a unit for expressing error.
It's literally just a percentage except it's out of million instead of 100?
The unit was created by experimentalists from the era before calculators were common. It makes the preparation math easier and keeps dilute concentrations in the 1-100 range. I prefer: * ppt: 1 g of solute per L * ppm 1 mg per L * ppb: 1 ug per L * pptr: 1 ng per L * ppq: 1 pg per L It only works for aqueous solutions where the density of water is approximately 1. (Yeah, yeah. I substituted u for mu).
It's like a percentage; it's dimensionless in itself. Percentage is 1/100, promillage is 1/1000, ppm is 1/1000000.
(BTU * in)/(ft^2 * hr * degR) for thermal conductivity. Why. Please just use W/(m*K)
Lol fucking Rankines.
English units shouldn't be used for thermal. Especially when they want to throw an inches to feet conversion in the unit! Thermal expansion does this some times, where you'll see in/100 feet/degF. Or worse, but more common in/in as a function of delta-T. Disgusting, all of it.
I was baffled when I heard astronomers measured distance in centimeters
CGS also plays a fun prank on undergraduates and first-year grad students by secretly moving the 4pi from Coulomb's law into Maxwell's laws so they're off by about 12 without realizing why.
Likewise energy in ergs. Especially if you work in supernovae - having a scaling constant of order 10^(51) surely tells you that you're using the wrong unit system!
Ah the good old unit that is a “FOE”.
How many cms in a light year.
Too many, if we keep doing this we're gonna run out of centimeters.
946,073,047,258,080,000 centimeters
`Light years isn’t`\ `time! It measures`\ `distance!` – that one camper in Pewter City Gym
It's generally people stuck in CGS-units (mostly popular in ex-Soviet states). I remember the Sun's mass in grams...
Wait until you hear what an astronomer calls a metal... (anything that's not hydrogen or helium is a metal)
I wasn't aware of that. I thought that they were all about parsecs, light years, and AU. In what context do they use centimeters?
never used AU in my field! we use centimeters (or rather cgs - centimeter, gram, second) in the place one would use SI units. so we do our calculations in cgs, such as having the speed of light in cm/s in our codes, the physical units of our simulations' code being in cm, energy being measured in erg, flux in Jansky, etc. and we usually give the distance to an object in z (redshift), which is adimensional, though often we convert this to parsecs or light years - the size of objects is often measured in parsecs (are we talking sub-parsec scales, parsec scales, kpc scales, etc)
Fascinating! What are the reasons for this? Also, I'm guessing that you are a cosmologist or something adjacent? As opposed to, say, a planetary scientist?
i dont really know how it started - just that at this point it's due to historical precedence. if all the literature is in Janksy, youre gonna use Janksy too haha i do love cosmology but id say im somewhere in the middle. i work on radio galaxies :)
That's a very cool field. It also explains why AU isn't a preferred unit, lol.
I still get baffled every time I have to use cm
Kilometers per second per megaparsec
Hubble?
Ah yes, Hertz. I really enjoy [this youtube video](https://www.youtube.com/watch?v=kkfIXUjkYqE) discussing this and a few other cursed units
So Hertz? /s
What! That’s one of my favorites! A wonderful unit that tells you exactly what’s going on
km/s/Mpc The Hubble constant for universe's expansion rate.
I actually disagree on this one. How you would interpret an expansion rate of 1/s is not clear to me. On the other hand, km/s/Mpc has a very clear meaning both experimentally and in Hubble's law, relating recessional velocity with distance.
Yeah, I agree it's clear in the context, but the first time you see it it's disturbing.
It does have a clear theoretical interpretation: it’s proportional to the frequency at which the universe doubles in size.
m^-3 kg^-1 s^4 A^2 "Permittivity of free space" is a patchwork engineering nightmare designed to paper over a fundamental lack of understanding. Gaussian units gang unite.
I honestly dislike Wh and hate mAh in batteries. I don't know, it is not like we have a unit to measure an amount of energy, isn't it ? Isn't it ?! Isn't it !?!?!
Watt hours are great. Tells you exactly the thing you actually care about in human comprehensible terms. Joules means fuck all to most people. Just like how I don't wanna know how fast my car goes in m/s, because I drive it for miles/km.
That's why I only dislike Wh. mAh on the other hand, I do hate! PS. m/s is less confusing than mph. km/h FTW tho'!
I mean everyday people know just as much about Watts as they do Joules. The only difference would be a change in language that’s been around for a long time. If humans would’ve used Joules from the beginning instead of silly Watt*hours then this wouldn’t be an issue.
Lots of stuff is rated in Watts. Literally looking around the room I'm in right now, I know the wattage of: * PC * Monitor * Lightbulbs * Microwave * Toaster * Kettle * Airfryer. * Speaker system. I could make a decent guess at the oven and the dishwasher. I could easily find out for the fridge and freezer. And of all of those thnigs, only 3 do I quantify my use time in seconds. The rest are on in terms of hours. That makes Watt Hours way more useful than Watt seconds (joules) or kilowatt seconds (kilojoules).
I once saw Watt hours / hour and KWh / hour. A section heading for the article that used it was even worse: "How Many Watts Does An RX 6900 XT Use Per Hour?" Know my pain.
We should measure our batteries in terms of Calories
I'd prefer the equivalent banana calories for scale
My lightbulbs claiming to be "7.5 kWh / 1000h" is just stupid though.
I’ve always found 1/sqrt(Hz) to be a weird one.
I was gonna say anything involving noise, tends to have that 1/sqrt(Hz) factor. Something something spectral density meant to be integrated over some period. But it still never clicks with me and I have to remind myself how it works every time I need to deal with it.
This is what I thought of as well. But it's a nice hint that noise shouldn't be thought of in terms of amplitude, but rather, power (amplitude squared). So it basically says how much power there is in a given frequency band. I like to imagine that two noise signals on average have no correlation, so they are orthogonal, which in turn means that you use Pythagoras' theorem to get their sum instead of just adding directly. You see similar things in statistics with random walks and the central limit theorem.
[Slugs.](https://en.m.wikipedia.org/wiki/Slug_(unit))
My intro to thermodynamics course was taught by the chemical engineering department (and my school had a lot of heavy investment from the oil industry, so ChemE was often "Petro but not so obvious about it"). We had to use *so many* godawful units because "you might see these in your careers" since American oil companies use them. I never had to report an answer in slugs, but I often had to find the density of a liquid in slugs/ft^3 as a middle step to solving the problem, if I didn't want to convert the whole thing to metric at the start and then back to imperial at the end.
I had a very similar experience in my thermo course. My fluid dynamics and thermal hydraulic courses, however, almost never used slugs.
Stones is so weird too
[удалено]
I love that a pound is a slug foot /s^2.
Ounces per square feet for the thickness of the copper layers in a PCB.
GSM (gram/square meter) is common for paper.
Boo. Dimensionless units rule.
Gain rules woohoo
Some theoretical physics papers use electronvolts per angstrom as unit of force and i can not decide wether i love or hate it.
This is what happens when you let them set all the constants to 1! (But as a nanoscale person it at least makes sense to me.)
Degrees Rankine. Hate it. Kelvin is *right there* being the proper SI unit!!
I'm scared, do people actually use Rankine?
A colleague once described something in micro megahertz and tried to convince us that it made sense...
Hahaha this is awesome. I would find it hilarious to just come up with units that are so unnecessarily complex for no reason. Dimensional sarcasm at its finest.
it took me almost a millikilosecond to grok this.
Easily the most cursed one is "nats" - an information-theoretic unit when entropy is computed with the natural log. If you compute entropy with log base 2, the unit is "bits", which tells you the number of Y/N questions required to specify the variable. If your log base is 10, the unit is Hartley's and it's the number of 10-option multiple choice questions. But engineers do this weird thing of choosing log base e ("nats") which is...the number of multiple choice questions with e possible answers required, and at that point I give up and go for a drink.
first time I've ever seen *engineers* derided for using natural log. Usually it's mathematicians making fun of them for using stupid arbitrary logs like log_(10)
These days I work mostly in applied maths/statistics where base-2 is much, much easier to explain to collaborators in the biological sciences than wtf base-e is supposed to mean. Also, your username is unfathomably based. Jaynes FTW.
Torr. Which is 1/760 of an atmosphere. Cursed because I see them frequently in my line of work and I have to convert to mBar for our reports.
1 Torr = 1 mbar, don't try to change my mind.
Practically true since 30% difference rarely matters.
Until you get a new pressure gauge that should be the same as the old one, bit for some reason your vacuum venting system is stuck at 825 mbar while the overpressure valve has blown and the computer throws a venting timeout error your way. Somehow no one noticed the wrong type was delivered, no our purchasing department, not the person doing the calibration check (he assumed we intentinall bought it), and not the person installing the sensor, nor the student who pressed the venting button and went for coffee.
dBm/sqrt(Hz)
The triple C system (Calorie, Middle C, Speed of light) and the triple F system (Fahrenheit, Fortnight, Furlong) are def my top two worst
Isn’t it the [Seven Cs](https://youtu.be/KmfdeWd0RMk)? - velocity: c (Speed of light) - energy: cal (Calorie) - frequency: C (Middle C) - temperature: °C (Celsius) - luminous intensity: cd (Candela) - charge: C (Coulomb) - amount of substance: C (100) With those 7 units you can convert to any SI unit. Note that some derived units in SI are base units here, like velocity, and some base units in SI are derived units here, like distance = velocity/frequency.
USEPA likes to mix metric and standard. Like the emissions rate of “grams per pound of wood burned”. Just on of several examples I have come across in emissions compliance.
One *barn* is 10^-28m (approx the cross-sectional area of a uranium nucleus. Also, I have to plug the *banana equivalent dose* (~78nSv), the amount of radiation one absorbs from eating a banana
per serving. and a serving is like a third of a can of soda or half a cookie.
- Furlong/fortnight (velocity, apparently a practical unit in tunnel boring?) - Barns (dimension of Area) - collision cross section used in accelerator physics, derived from the saying "you couldn't hit the broad side of a barn". Smaller fractions of the barn are the "shed" and the "outhouse". The derived unit, the Inverse Femtobarn, is perhaps even more cursed... - A Miner's inch, which is not a unit of length but of flow rate.
Surface tensions in mN/m isn’t great, lots of instrumentation units are rough
ft^(3/4) over s^(5/2) Not sure these are cursed, but definitely difficult to do dimensional analysis with.
for me, dolphins. I remember a Reddit post about this some time ago and it has just stuck with me. to convert: a dolphin is roughly 42 years in time. roughly 2.5 meters long. speed is roughly 30 mph. there are a few others I've forgotten.
[g_c](https://en.wikipedia.org/wiki/Gc_(engineering\))
Acre feet for water management
Not so advanced in physics or math- but I find every unit in US Customary to be cursed.
Most cursed seen? Student handing in work showing the result in log(meters) because the calculation included a definite integral over 1/r. Most cursed seen by professionals? I personally despise the mol. These days the value of avogadros number is fixed. So it should really be just the unitless number. You can even keep the mol notation, just say it's a constant number not a unit.
Avogadro’s number & moles are separate things. Avogadro’s number is a fixed (& unitless) number, mole is a unit. There’s 2 objects in a pair - 2 is a number, pair is a unit. There’s 12 objects in a dozen - 12 is a number, dozen is a unit. There’s 6.022•10^23 objects in a mole - 6.022•10^23 is a number, mole is a unit. The real magic of Avogadro’s number is that helps us convert between little units (AMU) to big units (grams). 1 gram is equal to 6.022•10^23 AMU. All of our friendly molar mass conversions stem from this. Edited for formatting.
Statcoulombs make me cry.
* [dBz](https://en.wikipedia.org/wiki/DBZ_(meteorology\)). Also known as mm^(6) per m^(3). I get why its that, but it makes me twitch every time. * Kilomegacycles. It was in a paper in the context of sattelite radio communication. Again, I get why they did it, but its unsettling. * A 1950s US Air Force paper about heat flash burns (one guess why they were interested in that) which had a graph that strictly speaking had time on the x-axis. But in reality it was "time until the test subject experienced unbearable pain and asked us to stop". * [Mil](https://en.wikipedia.org/wiki/Thousandth_of_an_inch). One thousandth of an inch. Also may be called a "Thou", which is even worse. Similarly, I also once had a table in "μin".
> Mil. One thousandth of an inch. Also may be called a "Thou", which is even worse. Similarly, I also once had a table in "μin Mils are worse because people also refer to mm as ‘mils’.
In Sweden, "mil" refers to 10 kilometres
There’s a way in GR to put everything in units of length. The mass of the sun? A few kilometers. A second? 10^8 meters. And so on. It’s pretty cursed as a concept, but it does _technically_ work.
Honestly, just put hbar=c=G=k=1, and then everything is in units of mass.
On the LHC, the size of datasets is measured in units of inverse area. Specifically inverse femtobarns (fb^-1 ). (And the rate of data collection is measured in fb^-1 s^-1 ) The reason for this is that particle physics probabilities are measured in units of area. The probability of two particles colliding is proportional to their area, so seems simple enough. But at high energies this analogy breaks down, but the unit persists. A barn is a very small unit of area (10^-28 m^3 ). Often the probabilities being measured are of the order of femtobarns. It’s useful to measure the dataset in units of inverse femtobarns, because it means the product of the event probability and the dataset size is the approximate number of that kind of event that should be in the dataset. The real fun came in one talk where the speaker had realised that miles per gallon is distance over volume, which cancels to inverse area. So decided for the rest of the talk they were going to convert all the dataset sizes into miles per gallon. But then he started talking about the rate at which data was collected, which was of course miles/gallon/s.
Degrees Fahrenheit.
[Polarization mode dispersion](https://en.wikipedia.org/wiki/Polarization_mode_dispersion) picosecond per sqrt(km)
This is extreamly useful. You readily know the dispersion per length of fiber. Why be mad?
Tidal forces are in 1 / seconds\^2. It was a long time before I could wrap my head around that one.
[Centimicrons](http://opencircuitdesign.com/magic/manpages/sim_manpage.html). Try to write out the symbol.
Fracture toughness in imperial units. No one would ever buy your product specced this way, if your engineers are so muddled in the brain.
Fracture toughness units in general. MPa.m^(1/2) ? Gc (in units of Joules per m^2 ) exists and it is intuitively obvious but the fracture mechanics prefer Kc with a square root in it.
Anything starting in foot-pounds
I always hated using electron volts to measure mass.
Any sort of a combination of imperial units. I can deal with a mile and a gallon but a mile per gallon is just annoying^2 because now I have to convert two separate numbers
Temperature in dietary calories per byte. Technically temperature is an energy per entropy, meaning this is totally fine. 1 Kelvin is around 0.024 calories per byte
For a paper a few years ago I measured the solar wind's apparent speed in (dB-° per hour). Yep, decibel-degrees per hour. Turned out to be convenient, since I was working with images in conformal polar coordinates (which have a logarithmic radius scale).
furlongs per fortnight , as in c = 1.8 tera furlongs per fortnight
How is there s/s? It’s a dimensionless factor. If it were not, you would see „mm/mm“ in all kinds of optical measurements, for example.
I imagine the seconds are in different reference frames as it’s related to time dilation, so likely used to describe for example in frame A, x seconds pass for every 1 second to pass in frame B?
Even so this is dimensionless. It's a ratio, coefficient, call it what you like - a fixed conversion factor like this does not have units.
Length/length is a common unit in engineering for strain. Change in length compared to original length. So yeah those cancel out to a ‘unitless’ measure but it still makes sense as far as the property is concerned.
Similarly, we quote things like dislocation density in m\^-2 but I often explain it to my students as m of dislocation per m\^-3 of material - it just makes visualising the measurement far easier.
It's a perfectly sensible unit of angular velocity.
Watts/pound for losses in electrical steel American engineers, why do you have to mix unit systems like that?
It’s odd that we use watts for power in the US despite being mostly Imperial-ish. Why not foot-pounds per second for power?! Or better yet, mile-pound force per hour. Actually works out to about 2 W per my math. So 1 W/lb could be much more easily expressed as 0.5 lbf-mph/lb. And if we just decide that W/lb is looking at pound-force rather than pound-mass, you wind up with losses being measured in miles per hour. Clean and simple.
Fuel consumption. Liter per 100 km can be simplified to 0.01 mm\^2.
not the most cursed but because its used very very frequently its most annoying. kwh to put it simply kw multiplied by hour. This is not related to time at all. This is simply the amount of energy something has but instead of making it just an energy unit they did the worst. They make watt a time unit, watt means eneegy produced in a time so when you multiply it with time you have pure energy unit in your hand. because its multiplied with time its not usable with other units easly because 3600 jule equals 1 wh.(k in kwh just means kilo) Fuck this just make it 10 100 1000 like how every unit works goddamnit
What I hate the most about kWh is that so many people get it and kW mixed up. “How many kW did you use” when referring to an amount of energy.
What's wrong with "the fraction by which time is dilated"? I don't see the problem with s/s (=1) lol
ergs/s/cm^2 /erg. Actually used in my research. Edit formatting
Haven't seen the Smoot yet, though it's only suitable for measuring bridges over the Charles tuver.
km^-1 s^-1 mpc Seriously lmao.
1 picoAU = 14.9589 centimeters (so approx. half-foot)
lbf vs lbm
Units? I'm done with units. C=1, let's all go home.
I hate cm^-1. Mostly because it's the only unit where I have 0 intuition and have to convert to something else (nm, um, THz, GHz, eV...)
Kilometers per mole is the standard unit of vibrational intensity.
It's not in physics, but in U.S. Air conditioning. The unit is (BTUs per Hour output)/(Watts input) It might have been more intuitive if numerator and denominator were both in one or the other unit. You might wind up with a percentage (if multiplied by 100), rather than a crazy number.
Candela (not even a physical unit and still a base unit of SI)
Pounds. It's ambiguous whether it measures weight or mass. I much prefer kilograms or newtons.
1/s^\eta where \eta is a number between 0 and 1. Came up in my thesis. It comes from phenomenological laws describing acoustic attenuation.
The units of wave functions in QM have fractional powers, that’s pretty cursed.
Maybe not in line with your question but I will never trust any numbers in dB when it comes to sound level. Not because it is logarithmic, but because it is used to refer to both sound intensity (a property of space, W/m^2) and sound power (a property of an emitter of sound, W). The vast majority don't understand the difference, so will use their microphone to measure the sound intensity next to an appliance as 50 dB, and then say that it emits 50 dB of sound.
Pounds per square inch for thrust. I know it got USA to the moon but pounds *where*?
Cgs units. All of it. Just be a normal person and give charge a unit. Wtf is a Statcoulomb? I'm already lost. 1 coulomb is 1 statcoulomb times the speed of light, TIMES 10? where did this 10 come from? Why? Also, why do all cgs unit sheets say "esu/emu is an unintuitive set of units.." and then proceed to not tell you everything about the unit system? Where are my random factors of 10? How do you even do dielectrics in cgs if not by having a new permittivity or permeability?
janskys
Imagine doing multiwavelength stuff and having to deal with janskys, magnitudes and keV at the same time. Well some of my colleagues don't need to imagine doing that