The accuracy of a hygrometer depends mainly on the following four factors:
The error of linearity
The response of the sensor is not linear. The microprocessors of the Hanna Instruments hygrometers compensate for this error. Regular calibration of the devices is also advisable to compensate for this error.
The temperature effect
The ability of the capacitive sensor to measure relative humidity depends on the temperature. The amount of water vapour it measures and the value of the relative humidity displayed on the screen are not quite proportional. This temperature error explains that most hygrometers do not operate below -20°C.
Incorrect calibration may cause reading errors. Many chemical kits are available for self-realization of hygrometer calibrations. They often consist of two closed chambers with two chemical salts. Precise moisture values can then be simulated by dissolving each of the salts in a precise volume of distilled water. Just place the electrode in the first closed chamber simulating a low RH rate, and wait for stabilization. The same is done for the other room with a higher value. This type of calibration is however uncertain because it is difficult to maintain a constant temperature during its development. It is best to calibrate the hygrometers in climatic enclosures. The devices are calibrated in two points and then verified by simulating other humidity values. This method of calibration is more reliable.
The dew point is the temperature from which the water vapour condenses to go into liquid phase. The dew point depends on the amount of water vapour and is therefore implicitly related to the relative humidity.