Two gases, one servo - Control by Constant Flow Rate

[willcharlton]

Hello,

I am designing a medical device that must control the flow of two gases simultaneously. The way these gases must be controlled is by maintaining a constant flow rate at the output of a device that mixes the two gases together. If the flow rate is 5 Liters/Minute then the valve controlling Gas A can allow 1 Liter/Minute while the other can allow 4 Liters/Minute, for example. Likewise, if the valve controlling Gas A can allow 2 Liters/Minute, the valve controlling Gas B can allow 3 Liters/Minute.

At present, my design achieves this control by using two, normally-closed, pwm-controlled solenoid proportional valves. By adjusting the duty ratio, we can achieve something close to this kind of control. However, the nature of this device is that Gas B rarely needs to be introduced into the mixture so Gas A is almost always at full flow rate. Subsequently, the proportional valve that controls Gas A is constantly burning 12 Watts of power which creates alot of heat and makes it very energy inefficient.

I am looking for a specific type of product that contains two valves which control the flow of gases in the same manner which I've just described. I imagine it would contain a servo motor that can be positioned to set the two valves. Does anyone know of a device such as I've described or know a way I can make one?

Here is what I had in mind. Sorry in advance for the art and penmanship...



My expertise is in electrical, not mechanical.

 

[tpetar]

Of course this exist and often used at welding. I mean at fine TIG welding.
I used only once/twice when I needed, always I use AC TIG with one container with helium or argon.
When have two gases in two gas container linked via mixture valve, on each side just set flow rate and no problem.

However, the nature of this device is that Gas B rarely needs to be introduced into the mixture so Gas A is almost always at full flow rate.

I dont see the problem, you need just a good electromagnetic valve (On/Off) not servo motor.
You dont adjust mixture (what I see from Your post. Mixture is predefined with flowmeters on each side and You just controls electromagnetic valve when to open or close).
Consider power consuption of electromagnetic valve and right type.
Always use or try to use brass parts (always check reaction of gas and parts material).
Solution for decresing power consuption is use of Impulse Electromagnetic Valve!!!!! (Change state only on impulse).

Be VERY careful if You working with Oxygen!!!