Calibrated 2% Hydrogen sample gas accuracy

[onereddog]

hi all,
Real life industry question...
An accurately calibrated cylinder of compressed gas sample of Hydrogen (certified 2% H2 and air 98%) is used to calibrate a hydrogen sensor by turning on its top regulator sample tap. It can be used for numerous calibration runs until it is empty. As H2 does not mix with air and is very much lighter/less dense wouldn't the H2 rise to the top of the cylinder and come out of the sample tap first, thereby making the "calibrated" sample remaining even more air; i.e. uncalibrated? How does cylinder remain a perfect mix ratio of 2% H2 if the Hydrogen can escape first? There is no indication or advisory on the cylinder that its accuracy will deteriorate.

 

[Spinnor]

You wrote,

" As H2 does not mix with air ..."

Why do you think that is true?

Calculate the potential energy difference for any of the gas molecules to be at the top of the container verses the bottom and compare that with a typical kinetic energy of the molecules. Compare mV^2/2 with mgh , compare V^2/2 with gh

Using 500 m/s for V (see http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html ) and 1m for h

125,000 verses about 10

I don't think gravity comes into play.

From page 83, Modern Physics, Paul Tipler

Equilibrium height distribution of particles in a gravitational field,

n(h) = n_o*exp(-mgh/kT)

In your case the exponential is about exp(-10/125,000)

 

[onereddog]

" As H2 does not mix with air ..."

Why do you think that is true?

Aha! Thanks Spinnor,

I’m sure this has the makings of a good exam question!

What I meant to say was when H2 is mixed with air, it quickly separates and rises up; hence the absence of H2 in earth’s atmosphere. I assumed the same would occur in the cylinder.

The math is a bit beyond me but I guess the key factor here is the Kinetic energy of the gas molecules (Brownian motion?) override the effect of gravity & keep the “mix“ in the cylinder even.

Hypothetically, if the cylinder was large enough, would there be a point where gravity would have an effect and the gases would separate?

Regards..

 

[Borek]

For the separation process to be effective you need very high columns and very low temperatures, this is called cryogenic distillation then and has its commercial uses. But it almost doesn't work in normal circumstances.

Hydrogen doesn't have to quickly separate and rise up - from what I remember it runs into space from the upper parts of the atmosphere, that means mixing is enough to remove it completely, especially taking account scarcity of elemental hydrogen sources.

 

[rwisz]

I would like to address this question by first acknowledging that it is a valid concern and one that is important to consider in the calibration process. However, there are several factors that need to be taken into account in order to understand why the cylinder remains a perfect mix ratio of 2% H2.

Firstly, it is important to note that the calibration process involves using a small amount of gas from the cylinder, rather than emptying the entire cylinder at once. This means that the gas mixture inside the cylinder remains relatively stable, as the gas is only being released in small quantities.

Secondly, while it is true that hydrogen is lighter than air and would rise to the top of the cylinder, it does not escape immediately. This is because the gas inside the cylinder is under high pressure, which keeps all the gases, including hydrogen, in a homogeneous mixture.

Furthermore, the cylinder itself is designed to maintain a constant pressure and prevent any significant changes in the gas mixture. This ensures that the 2% H2 and 98% air ratio is maintained throughout the calibration process.

Lastly, it is important to note that the cylinder is certified to contain 2% H2 and 98% air. This means that it has undergone rigorous testing and quality control measures to ensure its accuracy. This certification ensures that the cylinder remains a perfect mix ratio of 2% H2 even after multiple calibration runs.

In conclusion, while it is a valid concern that the hydrogen may escape first, the design and certification of the cylinder, as well as the calibration process itself, ensure that the gas mixture remains accurate and reliable for calibration purposes.