Measurement of humidity


Thread Starter

Jury Efremov

I need measurement of air humidity with accuracy 0.1% in range 0-10% of relative humidity. Temperature of measured air 120-130 C.

Anyone can help me? Thanks.

Giuliano Coppini

In a word: IMPOSSIBLE. Stop wasting your time. Humidity is almost impossible to measure accurately except by using a sling and a wet (literally) bulb and dry bulb thermometer.
The only way I know of to measure humidity at these conditions is to use an instrument to measure the absolute moisture content and then calculate the humidity. Bear in mind that humidity in the normal use of the word is a measurement of moist air at ambient pressure ( 1 atm). Humidity is not defined at temperatures above 100 deg C for a pressure of 1 atm. If you really have air with a humidity measurement at 130 C you have a mixture of air and water at a pressure that would condense the water at that temperature. This is about 3 times atmospheric.

Your level of accuracy is also very difficult to achieve.

Regards Erich Mertz [email protected]
The closest I know about is a unit from Vaisala which is only accurate to +/-1 %RH. It will also require a tedious calibration routine on a regular basis.

See for more information: USA.pdf

Depending on what you are trying to accomplish it may be better to record the "absolute humidity", which is the "kg water/kg of dry air". This will be a more stable measurement than humidity, since it is not temperature dependent.

This is especially true if you are controlling an industrial heating/drying process and comparing the moisture of the incomming air to the exhaust air.

You can also use an aspirated RTD setup with a dry RTD and a wet RTD. You must use distilled water to wet the wick/sock for the wet RTD. Of course to use this in a control setup you must also have calculations in your controller to convert this to %Rh. I do not even think this will meet your accuracy requirement.

Darren Ash

victor gallegos

If you need measurement of air humidity in a more commun range and accuracy, try the Honeywell´s HIH 3602 monolithic IC humidity sensor with integral precision RTD in a TO-5 metal can.