Temperature and humidity control together howto?

  • Thread starter Mehmet_ Ilker_YILMAZ
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Mehmet_ Ilker_YILMAZ

Dear list,

I am already reponsible to design and program a control system using PIDs in a plc to control relative humidity and temperature in a factory. As these two controlled variables affect each other I don't think it will be an easy task to program individual PIDs for these and run.

Can anyone help me about my doubt in this problem?


Electronics eng.
Will you be changing the temperature, or will it remain constant? If it will remain constant then this should not be difficult. If you can keep the temperature within a tolerance of +/- 1 or 2 degrees C, then just treat the humidity as an independent variable and don't complicate the issue.
Specifically, the indicated relative humidity will be affected by temperature swings. Temperature will not likely be affected much by your humidity control. Therefore if your process temperature can be expected to reach and hold steady-state, you should be ok. Two pids is the way to go. I have at least one plc set up this way.

Hakan Ozevin

Two separate PID controls are necessary.

Relative humidity change will somehow affect temperature (especially rate of change of temperature), however this should not alter your control algorithm; if temperature increase/decrease rate changes because of RH change, then PID controller of temperature will compensate this (which we are expecting from a closed loop control system).

In fact, you can estimate amount of RH change affecting temperature and feed this value to the temperature controller part (feedforward control), however this is unnecessary and impractical, because you cannot control RH and temp within a short time. These are variables changing at a slow rate.
What temp and humidity level are you try to hold ?
How large of an area?
We have some large areas humidity controlled but,
to maintain a low humidity below 15% it is a complex system. a/c,dehumidification and air locks.
hope this helps
What you are describing is a typical wetbulb situation. I would recommend you go to ASHRAE's website and ask that same question. Temperature and Humidity curves are described in a Psycrometric chart, if you can find one to download. The Chart and any help from ASHRAE members should help define the exact control philosophy you seek.