Thermohygrometer Edgetech Dewmaster

I suggest you contact the manufacturer. The Edgetech is a chilled mirror hygrometer with advertised accuracy of +/- 0.2degC. These devices can give false readings if the gas flow rate through the sensor is not within specified limits. Under normal conditions it is a primary standard and should not require calibration.

On second thought, +/- 0.2 deg C is about 2.2% RH at the conditions you specify(20 deg.C, 90%RH). Therefore, your Dewmaster may be giving readings within its specified accuracy.

I cannot believe that any process needs more accuracy than this!! Plus the fact that you appear to know the error, so you could easily add/subtract the known error from the measured value to get almost 100% correct!!!

Where are you coming from?????

You can mix your own calibration solutions as follows:

1. Take LiCl and mix a saturated solution in a closed glass. After some hours you get 13% Relative Humidity above the solution`s surface at 20°C which can be measured via a small sensor`s opening. Give the sensor some time to stabilize.

2. Take NaCl and you get 75% RH. For most processes this is accuracy enough to make a 2-point-calibration.

3. If not, you should preferably use a different type of measurement (which is used by the weather frogs). The best accuracy is achieved by measuring two temperatures - the dry thermometer temp and the wet bulb temp with an air velocity going over the wet bulb of about 2 m/s. This is called "aspiration psychrometry", with the dry temperature and the temp-difference gou go into aspiration tables or programs and read the % RH. This gives you the best accuracy because the errors are reduced on those of temp-measurements, which can be done very exactly.

Regards Uwe

Certainly saturated salt solutions are useful for generating known humidities. For a better description see:
http://www.omega.com/temperature/z/pdf/z103.pdf
It can be assumed from the statement of the problem that the error is already known. Aspiration psychrometry has many known sources of errors and are not primary standards. Dewpoint instruments are considered primary standards. The Dewmaster described, has an accuracy of 0.2 deg.C. Other similar instruments can be obtained with accuracies of 0.1deg.C.