I'm a software developer (although math isn't my strong point). I've developed a device to monitor/control my clothes dryer by monitoring the intake air's humidity & temperature, and the exhaust's humidity and temperature. The plan was to compare the exhaust air's humidity against the intake air. Once the humidity in both was around the same, it would mean that no more moisture is evaporating from the clothes, ie: they're dry.

Unfortunately its not that straight forward. the air coming out of the dryer is much hotter, and because the sensor is giving me relative humidity values, I cannot compare it to the intake humidity (which is room temperature).

I'm looking for a formula that will allow me to compare the two humidities, taking their temperature into account. An "absolute" humidity I guess, or literally the amount of moisture in the air.

I've tried a couple of formulas I've found online but I'm not getting readings that look valid.

Note that the value doesn't need to be an official, absolute humidity value, it simply needs to allow me to compare the intake and exhaust humidities, at different temperatures (but everything else, such as air pressure, etc... being equal).

(Please feel free to suggest a more appropriate tag)

  • 3
    $\begingroup$ This is the problem of "psychrometrics" en.wikipedia.org/wiki/Psychrometrics and specifically the psychrometric chart is designed to do this, but I think they're created empirically so I don't know if you'll get simple formulas from them. $\endgroup$
    – Zander
    Aug 6, 2013 at 13:20

3 Answers 3


https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/ The formula on this link calculates AH from T and RH. Use the inlet and exhaust measurements to generate AHout and AHin, then express them as a ratio. If Tout is not too much above 40 Celsius, the accuracy should be fine.

  • $\begingroup$ Very interesting. Thank you. $\endgroup$
    – Dermot
    Sep 25, 2015 at 6:47

Thanks to @Zander I looked up the tables and came across a formula here http://forum.onlineconversion.com/showthread.php?t=567 to approximate the tables. The formula was:

g/m^3 =((0.000002*$B4^4)+(0.0002*$B4^3)+(0.0095*$B4^2)+( 0.337*$B4)+4.9034)*C$2

where C$2 is RH and $B4 is Temp in degC.

Here's a simple test application I created (I know this is a math forum, but I'm sure this will help someone) that allowed me to compare the output with the psychrometric tables. It seems quite accurate, certainly enough for my requirements. Thanks for pointing me in the right direction!

double temperature;
double relativeHumidity;
double absoluteHumidity;

Console.WriteLine("Absolute humidity calculator. Enter temperature as °C and relative humidity as");

        while (true)
                Console.Write("Temperature: ");
                temperature = Double.Parse(Console.ReadLine());
                Console.Write("Relative Humidity: ");
                relativeHumidity = Double.Parse(Console.ReadLine());
                absoluteHumidity = (
                    (0.000002 * Math.Pow(temperature, 4.0)) + 
                    (0.0002 * Math.Pow(temperature, 3.0)) + 
                    (0.0095 * Math.Pow(temperature, 2.0)) + 
                    (0.337 * temperature) + 4.9034) 
                    * relativeHumidity;

                Console.WriteLine(String.Format("Absolute Humidity: {0}", absoluteHumidity));

You may find the article "Principles of Humidity Measurement" in the section Humidity Sensor of E+E Elektronik useful. They also have a Humidity Calculator. Cheers, Peter


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