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  • I have a tubing system where I need to precisely control relative humidity. All my sensors are giving me temperature [°C] and relative humidity [%].
  • Right now I have simple PID controller, and it's input is hooked up to the relative humidity values.

But I had an idea: wouldn't be my controller more precise if I would feed into the input absolute humidity values and not relative humidity?

My though process:

  1. Relative humidity is dependent on temperature, partial pressure of water vapor and equilibrium vapor pressure of water. So it is dependent on 3 values.
  2. Absolute humidity on the other hand stated by Wikipedia is not dependent on the temperature around it.

If I would compute absolute humidity from relative humidity and temperature and feed it into PID input. Would I achieve greater precision?

I don't know if computing the absolute humidity from relative humidity and temperature would transfer the dependency of temperature or it would eliminate it.

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    $\begingroup$ if you calculate absolute humidity based on relative humidity and temperature, then it is based on temperature... $\endgroup$
    – Solar Mike
    Commented Mar 6, 2021 at 23:01
  • $\begingroup$ I know but you have to take into account that if the temperature changes it changes also relative humidity. So if temperature changes > relative humidity changes > but the absolute humidity shouldn't. Even if you calculate it from temperature. Am I right? $\endgroup$
    – Jakub Szlaur
    Commented Mar 6, 2021 at 23:05

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You are right in that the you the relative humidity will be quicker and easier to control. The question is whether that matter for your problem.

Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air ($g/m^3$).

Relative humidity is the ratio of water vapor (moisture) in the air with respect to the maximum water vapour in the dry bulb temperature. It is expressed as a percentage.

So when you are trying to control the water vapour in a room, is easier than trying to control the water vapour in the room and the temperature.

The problem is this might not be relevant.

enter image description here

if you see the following psychrometric map the pin is at 5 [gr water per kg dry air] (far right axis). At 36 degrees this is equivalent to 12% RH, while at 28C is 20%. If you extend the line towards the left, you see that at 3[C] you are getting 100%RH, therefore the water in the air starts to precipitate and the room becomes wet.

Depending on your application, you might care or not care about RH. If you don't and you can live just with absolute humidity then just go for that.

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Creating an output from 2 inputs won't have more precision; it will have 2 sources of error.

If you are trying to control relative humidity, then using that as your control variable only makes sense.

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