Psychrometric method of air humidity calculation is based on the fact that the temperature difference between wet and dry thermometers depends on the air humidity.
The humidity error consists of two components: temperature measurement error and humidity error after measuring the temperature. If you have a look at any psychrometric table in which temperatures are indicated within 0,1 ° C, one can see that at about 70% humidity the error measurement of wet and dry temperature difference 0,2 °C leads to a calculation of 2% humidity error. At higher humidity the influence of thermometers inaccuracy decreases, at the lower  increases.
Therefore, when measuring humidity by a psychrometer, the temperature measurement accuracy should be paid a special attention.
The basic psychometric formula is:
F= [E´ A (tt´) P]/e,
where:
e  maximum vapor pressure at dry bulb;
E´  maximum vapor pressure at wet bulb;
A  psychrometer constant, depends on a flow rate;
P  atmospheric pressure, accepted to be equal 1000 hPa = 100000 Pa;
(tt´)  difference in the readings of dry and wetbulbs.
This formula allows to obtain the exact value of relative humidity. As it can be seen from the formula, besides temperature, pressure and psychrometer constant influence the result of calculation. For example:
Tdry = 20,0 °C, Twet = 16,5 °C.
True Value 
Traditional Formula 
Aspiration Psychrometer Diagram 
VIT2 Hygrometer Table 


Value 
64%  65%  70%  67% 
Error 
1%  6%  3%  
Cause of Error 
Formula Inaccuracy  No corrections for the flow rate  No corrections for the flow rate 
We assume that the pressure in the room is normal to atmospheric pressure, P = 1100 hPa, airflow v = 0,2 m/sec. Under such conditions, the true value of the relative humidity is 64%.
There is a variety of methods for determining the moisture content according to dry and wetbulb temperature indications.
The easiest way of RH calculation is from 100% to subtract the difference between dry and wetbulb temperatures, multiplied by 10. This method works very well under high humidity, normal atmospheric pressure, flow rate of 2 m/sec, and air temperature of 15 °C to 18 °C. Under other circumstances, this "traditional" method gives an error. In our example RH = 1003,5*10 = 65%, calculation error is 1%
Another traditional way to calculate RH is to use a psyhrometric table or chart. There is a widespread misconception that the psyhrometric table is always the one and the same. In fact, psyhrometric tables are formed for different hygrometer and psychrometer types with taking into account their design and a flow rate. For example, according to the chart of the aspiration psychrometer, relative humidity is 70%, the error is greater than in the traditional method. The error of 6% is explained due to the fact that the chart, as it is indicated in the device manual, is counted for the flow rate of 2 m/sec.
According to the table of VIT2 Hygrometer, relative humidity is 67%. 3% error is explained that VIT2 psychrometric table is planned for the flow rate of 0.5 to 1.0 m/sec as it is indicated on the front panel of the device.
Devices of TERA's production with the function of the humidity calculation allow the parameter "flow rate" to be entered into the device’s memory.
Anemometers are used to measure the flow rate of air. The flow rates measurement of less than 2 m/sec is possible only with an expensive digital anemometer. Relatively inexpensive mechanical anemometers operate in the range of 2 10 m/sec.
For customers who do not have an anemometer, we recommend to set the "flow rate" parameter so that the indications of device and standard hygrometer are the same.