More specifically, if I’m using a (diaphragm-type) dry gas meter -calibrated using ambient air- to measure a collected volume of H2, should I use any correction factor to determine the actual volume of H2 collected?
I’m sure that I’ve collected a different number of moles and I can correct for that when the time comes. But are the volumes accurate?
At a given temperature and pressure, the same number of molecules of an ideal gas will always be the same volume, no matter what sort of gas it is.
Now, of course, real gases are not ideal, and air is a little bit further from being ideal than hydrogen is… but both are really, really close to being ideal.
So a dry gas meter counts molecules?
No. 
The ideal gas law, which treats gasses as massless, non interacting points, says PV=nRT. So if you know the pressure going through the meter (P), and you the temperature of the gas going through the meter(T), and you know how big the chamber of the dry gas meter is (V) and you do, because you built it – then you know how much gas you have – n (moles).
Is a cubic foot of air the same as a cubic foot of hydrogen? It depends on what you mean by “the same.” Naturally, the hydrogen ft[sup]3[/sup] will certainly have less mass than the equivalent amount of air.
Dry gas meters aren’t terribly accurate as flow meters go, but they’re good enough for quasi-governmental (utility) work. The accuracy you can expect depends on where you are in the meter’s rated flow range (the middle is good, and the very bottom end isn’t). Their accuracy curve is generally linear for steady-state flows.
I used to work for a company that makes flow meters. We did thermal, laminar and Coriolis meters; I designed the Coriolis meters. Thermal and laminar flow meters are both really sensitive to different gasses, moreso than diaphragm meters. Coriolis meters, the most accurate type of flow meter, really do count molecules. That is, they’re true mass flow meters.
Thermal meters rely on knowing a gas’ heat capacity, among other things. Laminar meters depend upon a particular gas’ dynamic viscosity, kinematic viscosity and mass. Diaphragm meters don’t care as much what’s flowing through them.
The thing about hydrogen is that it loves to leak out, and that could affect your meter’s accuracy. Hydrogen will tend to diffuse through the diaphragms themselves, making your meter read low. It even diffuses through metal pressure vessels, embrittling them in the process. A balloon inflated with hydrogen will leak down faster than one filled with helium.
Where did you get your meter? Many diaphragm meters are only required to be within +/- 5% or 10% (depending on application), so keep that in mind. Also, hydrogen is a whole lot less viscous than air, so it will flow through your meter more easily (creating less pressure drop) and therefore reading a bit high. But diaphragm meters don’t have a ton of pressure drop to begin with, so it shouldn’t matter much.
The last big caveat is that diaphragm meters are really only accurate for steady-state flows. They don’t do well with flows that vary a lot in a relatively short period of time.
Why are you trying to measure hydrogen flow? If you’re doing chemistry, you probably want to drop $7,000+ on a Coriolis meter. But if you’re just trying to bill your neighbor who’s borrowing a cup of hydrogen for her fuel cell-powered car, the reading from your diaphragm meter is probably pretty close, even without a correction.
8 whole hours to find somebody who designs this esoteric stuff for a living.
I love this place. 
LSLGuy: One reaps what one sows, no?
I first encountered The Straight Dope when I was 13 years old, all the way back in 1986. Cecil’s column ran in the Washington (DC) City Paper, which, as a disaffected youth, I read religiously. I came across the first book of compiled columns a year later.
The intoxicating ideas I gleaned from the column and the books were that (a) the truth is out there, (b) it can be a ton of fun to find it, and (c) you can teach yourself almost anything. These were important life lessons, at least in my world.
When I was finishing up my undergrad degree in philosophy, I corresponded with Anthracite (who, I believe, is the same person as Una). She answered my questions about her experience in engineering and gave me some sage advice about grad school; I applied it and the benefit was enormous.
I’m just pleased that I can offer some useful information to the SD community; I’ve certainly received more than my fair share in exchange. I never expected my intimate knowledge of flow meters to be relevant to any discussion outside of my job.
I love hearing what you, LSLGuy, have to say about aviation and other things. Chronos, however misguided he may be on the subject of bicycle dynamics 
is a font of wisdom on the subject of physics in general. I’m not quite sure what MachineElf does professionally, but he’s sharp, asks great questions and is very well informed. I’m overlooking a zillion other contributors.
tl;dr: I love this place too. I was born the same year TSD began fighting ignorance. I’m amazed that there’s a thriving community built around it 44 years later.
Cheers!
Errata: In my longer post above, I messed up the sign convention. I wrote that diffusing hydrogen would cause the meter to read low and that hydrogen’s low viscosity would tend to make the diaphragm meter read high. I got those backwards: The diffusing hydrogen would cause the meter to report higher-than-actual flow rates, while the less=viscous nature of hydrogen would cause the meter to report lower-than-actual flow rates.
EdelweissPirate Has given a great reply.
However I wanted to point out that you are in dangerous territory. Most meter internals: diaphragms, gaskets, o-rings are not rated for Hydrogen service. You already probably know that from seeing helium filled ordinary balloons versus Mylar balloons.
So the diaphragm meter which does not leak in natural gas service will likely leak in Hydrogen service. Also not sure if the meter is electronically connected or if there are other electrical/electronic components around but those need to be checked too. I am sure you know what you are doing but no harm in mentioning.
I appreciate the concern about that *nasty *H2. I had the same concerns long, long ago in the GC lab when we used it as a carrier gas. And we had a little H2 generator that we filled with water every couple of days. “Nothing to worry about”, I was told. To that concern, I’ve recommended to the guys that they shouldn’t use our sampling equipment for this. But now we’re all curious to find out what would happen with the volume and (now) the flow rate.
To give more background and answer your queries:
We’ve old, Rockwell residential gas meters that are re-manufactured with a test-meter index to aid accuracy. These meters are rated for flow rates of natural gas and propane. Most all of our sources are combustion gas (almost air) so the calibrations we do in the lab are still valid for the accuracy required. We generally get a gamma of about 0.98-1.02 as referenced against our wet test meter which is periodically referenced to a bell prover.
As mentioned, we’re usually testing combustion sources but this client has a hydrogen source. The test period will be about 60 minutes at a (more or less) constant ~ .75 cfm (~21lpm). We’ll really just be using the meter for a volume check but there’s an orifice at the outlet to confirm the flowrate going through there.
In reviewing the calibration calculations this morning, I don’t see molecular weight required anywhere. We do make reference to ambient pressure and temp, as well as the temperature of the air in the meter itself and those values are used. The MW is used in the flow rate calcs, though.
Thanks everyone. I really wish that I could give back to the community even a fraction of what I’ve gotten from being around here.
Maybe you can. What’s your specialty?
Also don’t underestimate the value of asking good questions.
Shunpiker - I am not sure what you are doing but reading your post it looks like you will be having premixed combustion using Hydrogen as a fuel. If that is the case, and you have never done it before - you need to be aware of flame flashback when working with fast flames like those with Hydrogen / Syngas.
You can read morehere
Essentially, if the combustion nozzle is not designed for a hydrogen flame or a flashback arrestor is not used, the flame can travel backwards.
Here’s a cool video of flame flash back (except the hydrogen flame is not visible) Premixed Flame Blowoff vs Flash Back in Slow Motion - YouTube
That video was interesting. What was the little flash before it headed down the tube? I’m assuming it’s the breaking of the *boundary layer *or something. I’ll need to read up on it to understand what’s happening.
Ultimately what my sampler does is collect particles that are suspended in a gaseous flow stream. I’ll weigh the catch and use the measured volume to give a concentration of dust in the source. I’ve no idea if that’s actually what they’re doing or not.
I’m still obtaining more info from the client, but this specific task shouldn’t be related to a combustion process. I’m imagining a chemical plant that is manufacturing or transporting H2 in a pipeline. The client confirmed the H2 concentration was 100% and “does not reach the upper explosive limit of concentration. “ It still worries me enough to recommend he contact EdelweissPirate for a Coriolis meter.
This cracked me up!