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Gas Mixing & Blending: A Focus on Dilution

Oct 21, 2011

Dilution is a special application in the world of gas mixing.  Basically, you are diluting a concentration of gas in an already existing gas mixture. Let’s say you have a gas mixture of 10% Ar/90% N2 and you really need 5 Ar/95% N2 for your application. The solution is to add more N2 to the the mix, but how much?  Well, to answer the question of ‘how much’, we first need to know if the gas mix ratio is in % Mass or % Volume (See my earlier post on this issue to learn more). Lets assume it is % Mass. In that case, we need to generate a new mix of 50% of the original 10%Ar/90%N2 mix and dilute (or mix in) that with 50% N2.  The result is a new mix of 5%Ar/95%N2 Mix.

Most concentrations are however much smaller, more in 0.0001% region. In general, one expresses this in ppm values (ppm = parts per million). This means that 10000 ppm = 1%.

The example above was easy, next a quiz.  You have a gas of 1000 ppm of SO2 in a mix of 50% Ar with the rest N2.  You want to have 700 ppm of SO2 in the mix by diluting with Nitrogen. All values are in ppm and % of volume!  You have 2 x Smart-Trak 100 MFC’s of 0-1000 SCCM of Air with a setpoint of 4-20 mA each. You want to have a flow of 50 gr/h of the mix.

The first 5 people to email with the correct answers win a Sierra “Experience Our Passion For Flow” gift.

OK, Here are your Questions:

1)      What needs to be the setpoint for each MFC?

2)      What will be the calculated accuracy?

3)      What % of Mass will  the Ar in the mix be?

Would it not be nice to have an automated gas mix system that calculates this all for you? Sierra can provide this for you, contact us to learn more…

Paul de Waal, Managing Dir., Sierra Europe
Written By:
Paul de Waal, Managing Dir., Sierra Europe
Sierra Instruments

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