[voxmpg]

Hi,

Quick question:

Lets say you were to mix two different gases (say oxygen and carbon dioxide for instance) and you ran both from their respective canistors, to seperate rotameters, and then to a Y-block where they would be combined. If the flowrates are the same does that ensure that there will be 50% of each gas in the final mixture? If it takes different pressures to maintain the same flowrates does that effect the final mixture? And if so, is there a way to compensate for the difference in pressure so that you would know that there is 50% of each gas in the mixture?

Thanks a lot!

[Borek]

Flow rates being the same - if they measure volume/time unit - is not enough.

That's an abvious conclusion from ideal gas equation.

[voxmpg]

Flow rates being the same - if they measure volume/time unit - is not enough.

That's an abvious conclusion from ideal gas equation.

so is there a way to compensate for the difference in pressure?For example lets say it takes 25psi to have CO2 flow through the system at 20 lpm and it takes 35psi to have O2 flow at the same rate. How can i make sure that there is 50% volume of each gas in the mixture.

[typhoon2028]

Both gasses will occupy 100% of the end volume.  This is a property of a gas, they occupy the volume of the container. 

What is the mixing pressure?

Volumetric flowrates for gases does not seem like a good measuring method.  Gasses being a compressible fluid make this problem more difficult to solve.  In addition, is there a temperature change across the valve on the Y-block?

You should determine the desired final mass or molar ratio you want.  Since, mass is independent of temperature, volume, or pressure.

[wpenrose]

As the flow rates increase, there may be back pressures that can affect the mixing. Ideally, you want the gas pressures in the tubes prior to mixing to be near atmospheric. Keeping them at identical pressures above ambient while flowing can be done, but it's not a simple or inexpensive task.

Moreover, their temperatures have to be the same, or ambient. Coming straight from the cylinder, they may be cold if you don't take precautions.

It really depends on choosing the accuracy you need in an informed matter for the application. For instance, you needn't specify 0.1% if you're working with a sensor that's only accurate to 2%.

Rotameters aren't very accurate, either, and require constant tending.

This is the business I'm in. Read this, from my website:
http://www.customsensorsolutions.com/dilution.htm

Bill Penrose