There are many ways to approach the calibration and adjustment of relative humidity instruments. In a perfect world all instruments would be checked and certified in accredited laboratories to the highest standards. In the real world we must balance time, cost, technical requirements, expertise, and the unique needs of each individual application.
Users often ask manufacturers for a recommended calibration interval. This is a reasonable starting point when thinking about how often to calibrate your instruments, but in practice the performance of an instrument may degrade at different rates depending on the conditions it’s being used in, so it’s important to establish an appropriate calibration interval.
As far as performing the calibration itself, there are several options open to you: you can set up your own calibration lab, send your instruments to a third party for lab calibration, or perform a field check or one-point in-situ calibration yourself.
One-point in-situ calibration uses a working standard to calibrate a fixed field instrument without having to take it out of service – for example, when a technician uses a hand-held device to check a wall-mounted transmitter. The main advantage of this method is that it is both fast and easy. There is no downtime for the field instrument and no additional labor required for dismounting and reinstallation. However, the drawback of this method is the lack of a second calibration point to indicate the correct dynamic response of the field instrument. If the measured conditions are stable over a short period, it may be adequate to gently breathe on the sensor and observe the instrument’s response and recovery; if the measured conditions are highly variable, we recommend a multi-point calibration.
The Vaisala HUMICAP Handheld Humidity and Temperature Meter HM70 is designed for demanding humidity measurement in spot-checking applications. It is also ideal for field checking and calibration of Vaisala's fixed humidity instruments. The following transmitters can be calibrated using the HM70: HMDW60/70, HMW61/71, HMD/W20/30, HMP130, HMP230/240 series, HMT320/330 series, HMM210 series, and the DMW19 transmitter.
When calibrating in the field, be sure to check and adjust a fixed transmitter’s reading against a calibrated reference probe connected to the HM70. One or two-point calibrations can be performed on industrial transmitters and modules (HMP230/240 series, HMT320/330 series, and HMM210 modules) using a Vaisala MI70 indicator and Vaisala’s HMK15 humidity calibrator.
In the video below we go through how to perform one-point in-situ calibration and adjustment for the Vaisala HMT330 using a Vaisala HM70. See other HMT330 calibration alternatives here.