How are they measuring it? With what kind of balance?
For the uber-geeks on the SDMB, I have a challenge for ya:
Say you throw some deli meat onto a Mettler precision balance. The balance was recently calibrated by a Mettler rep. The display reads 534.123575 g.
Does this mean you have 534.123575 g of meat? Yes or no? Explain your answer. 
I’m in Alberta and deli meats are sold by the gram (usually in 100 gram increments). Also, although meat prices are usually in price per kilogram, more expensive stuff (like most seafood) usually has prices per 100 grams. Also, bulk food in the grocery store (self-serve, where you scoop it into a bag yourself) is usually has prices per 100 grams.
Well, I’m not sure what gotcha you’re aiming for, here, but I can see two possible objections. First, no real scale (especially one exposed to pesky things like air) that can measure hundreds of grams is going to be able to reliably measure to a precision of micrograms, and I’d be surprised if any real deli scale actually has that many digits on its display. Second, depending on the specific instrument, calibration might not change what’s shown on the display, but actually consist of a table or graph showing how much it’s off by for any given displayed value.
A knot is a unit of speed, equal to one nautical mile per hour.
Yes, indeed they are. How are they weighed? On a scale that weighs in grams, I suppose.
Once, back when I was in Canada, I went to buy some seafood and asked for 500 grammes of shrimp. The guy at the counter started piling some shrimps on the scale… and then stopped.
“I’m sorry, I can’t give you 500 grammes of shrimp, this scale is in kilos.”
Well, I’m not sure what gotcha you’re aiming for, here, but I can see two possible objections. First, no real scale (especially one exposed to pesky things like air) that can measure hundreds of grams is going to be able to reliably measure to a precision of micrograms, and I’d be surprised if any real deli scale actually has that many digits on its display. Second, depending on the specific instrument, calibration might not change what’s shown on the display, but actually consist of a table or graph showing how much it’s off by for any given displayed value.
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Also, Crafter_Man didnt’ mention location. And, considering that the word balance is a bit unfortunate for devices that look more like spring scales to me, location can be quite important here. For all we know the deli meat measurement could take place on the moon, on a “balance” that was recently calibrated on earth.
Nah, here is what I was getting at: precision balances are calibrated with the assumption that you’re measuring the mass of steel. If what you’re measuring is *not *steel you’ll get a wrong answer.
Why? Mass is mass. How does the balance “know” the difference between steel and not-steel?
I used precision balances extensively, and I’ve never heard this before. Do you have a cite for this assertion, Crafter_Man, because it does not make any sense to me, either. Mass is mass.
The only issue that I have run into at times is with certain hygroscopic materials in a humid environment. The substance can pick up moisture from the air during the process of measurement.
What a great unit! I’ll have to remember that.
That’s a neat observation, too.
Buoyancy? If that many digits are valid, it might have an effect on something with low density.
Buoyancy will certainly have an effect: A microgram (the precision of that hypothetical scale) is about the mass of a cubic millimeter of air. So anything bigger than a cubic millimeter (as I would imagine anything weighing a half kilo would be) would have a detectable buoyancy.
In addition, any steel sitting on that scale would almost surely have magnetic forces on it as well as gravitational. But that doesn’t mean that the scale would be calibrated even for steel: Different kinds of steel react to magnetic fields in different ways. If you really need enough precision that you’re worried about magnetic effects, then you have to make sure that there are no magnetic fields in the area (which is a lot more trouble than anyone’s going to go to for pricing a chunk of meat).
:smack: Buoyancy certainly makes sense. An object less dense than steel (but with the same mass) would have a larger volume, displace more air, and thus have a greater buoyant force. Therefore, on a scale, the object would register less weight than a steel object of equivalent mass. This difference in weight would be detectable on a precise enough scale.
Conversely, an object more dense than steel would have a smaller buoyant force than a steel object of equivalent mass, and therefore register more weight than the steel object.
Yes, that’s correct. If the density of what you’re measuring is different than the density of steel, you must make a buoyancy correction when using an analytical balance (e.g. Mettler).
The whole time i was reading this thread everytime i heard someone say “what would you ever buy as a gram?” i kept thinking…hmmm an 1/8th of an ounce is 3.5 grams.
Yep. Researchers at NIST’s Mass Metrology Lab have spent decades addressing magnetic effects.
But as mentioned by you and others, buoyancy is the biggie. It may not matter for non-precision measurements, but it is a huge deal when making very precise measurements with analytical balances. The drug companies are well aware of this… when their machines fill capsules with drugs, they always perform a buoyancy correction when they measure the mass of the capsule.
Buoyancy is a strange thing with analytical balances, BTW. If you have access to such a balance, try this experiment:
Place an 8.5" X 11" sheet of paper on the balance. Record the result. Crumple the paper into a small ball. Place the crumpled paper ball on the balance and record the result.
The two measurements will be different.
Why?
In both cases, the paper is exposed to the atmosphere. Unless some gasses inside individual paper fibers become compressed (and frankly, that seems unlikely), I’m not seeing why crumpling a sheet of paper would have an effect. It seems more likely that any measured difference would be due to moisture or salts from your hands getting on the paper.
Have you actually done this?
Well to be honest, I haven’t tried it. But my coworker has, and he likes to relate this story. I have always assumed it’s true. I’ll give it a try, but I’ll use a Post-It note instead of a large piece of paper (the pan on our analytical balance is pretty small).