Factors that can affect hygrometer precision

The hygrometers’ precision essentially depends on how insusceptible they are to the following three factors:

Linearity error
It's caused by the typical non-linearity of RH sensors. Hanna hygrometers compensate for the effects of this error. It is advisable however to calibrate the meter periodically to reduce the probability of this error reoccurring.

Temperature error
It's caused by the variation of the hygroscopic properties of the sensor's dielectric material as a function of temperature. In fact, the ratio between the quantity of water vapor present in the dielectric and the relative humidity is not directly proportional, but varies with temperature. Due to this temperature error, hygrometers normally do not operate at temperatures below -20 ºC (-4ºF).

Calibration error
It's caused by an incorrect calibration procedure. There are many "do-it-yourself" calibration kits on the market. Most are made of a receptacle containing two sealed chambers and two different types of salt. It is possible to simulate a particular RH value by filling each chamber with the correct distilled water and salt solution. The RH probe is first immersed in the low RH chamber and allowed to stabilize. The meter is then calibrated at the RH value of the chamber being used. The procedure is repeated with the high RH chamber. Since RH is dramatically affected by temperature changes kits do not provide accurate calibration due to the practical difficulties in performing the calibration at a constant temperature.

Climatic chambers that simulate different humidity levels are the ideal solution to calibrate hygrometers accurately. Hygrometers are also calibrated using two different levels of relative humidity in this calibration procedure, and then the accuracy is checked by simulating other RH values in the chamber. Hanna Service Centers are equipped with state-of-the-art calibration chambers to provide the best accuracy.

Dew Point

The dew point is defined as the lower temperature to which air must be cooled in order for condensation (saturation) to occur. The dew point is dependent on the concentration of water vapor present, and therefore the relative humidity. The graphs here can help to easily determine the dew point after measuring the relative humidity and temperature of the air.

To determine the dew point temperature, start by drawing a horizontal line at the measured temperature level until it intersects the measured relative humidity line.
From the intersection point, draw a vertical line down to the dew point axis, and read the dew point temperature.

Hygrometer calibration

The calibration is performed at the factory using state-of-the-art humidity chambers and adjusted at 3 different points (14%, 50%, 80%) against a NIST standard.

Relative Humidity calibration is a very delicate process that requires special equipment and trained personnel. Hanna now offers service contracts for all relative humidity meters. This supplies the user with the confidence that the meter is operating at 100% of its performance. Ask your dealer or the nearest Hanna office for more details.