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PID control of humidity

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A naive question from a novice ...

I'm interested in using DAQFactory to control the humidity in my experimental system.  It is a flow-through system, and I need the total flow to be X.  I would like to input a setpoint humidity and then have the system send voltages to mass flow controllers to control two flows, wet air (a) and dry air (b), such that a + b = X.

Is this possible (i.e. to have two outputs)?

Thanks very much!


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Sure, its just math.  Are you using PID or script to do your controlled loop?  Lets assume just a controlled loop, and lets assume that your wet air is 90% humid and your dry air is 0%.  So, the maximum humidity you could achieve would be 90% with full flow from the wet, and no flow from the dry.  And lets assume you have a variable called H which is the desired humidy (along with your a and b channels for the flow controllers, and a variable X for total flow).  Really you just want to apply enough wet air to get the desired humidity and then fill the rest with dry.  Give 90% humid wet air, the amount of wet flow you need is simply H / 90 * X.  So, you'd do a script like this:

   a = H / 90 * X
   b = X - a[0] 

Note that you'll want to constrain H to 0 to 90 somewhere in the program, probably where the user specifies it, but you could also add some checks in the above script.

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Hi Sarah,

Did you use Daqfactory for this in the end? I have used it for exactly the same purpose and it's really very easy to program so I could send you my program as an example. The key to doing this practically though is to use the correct MFC for the 'wet flow' that isn't influenced by humidity. Normally, if you get condensed water in your MFC it will give some crazy readings

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