Hi Supakorn,

Thank you for reading our Blog! I have forwarded your question to our Application Engineer and here is the reply:

Justin writes:

The HMT120 and HMT130 have a +/-1.5 %RH accuracy spec with a stability of +/-2 %RH over two years. Drift in the readings that exceed these levels would be considered unacceptable, or ‘out of tolerance’. This level can be verified with a calibrated reference such as your HM70(with MI70 Indicator), keeping in mind the accuracy of that unit as well. Calibration procedures are published in the User Guides for each instrument, which can be downloaded for free on our website. Please search by product name: I.E. HM70, MI70, HMT120/130.The frequency of calibration is user dependent, and does not have a strict interval. For example, the HMT120 and HMT130 have a +/-1.5 %RH accuracy with a maximum drift of +/-2 %RH for two years.

If your process requires an instrument that has an accuracy of +/-5 %RH, then you would be able to confidently employ a 2-3 year calibration interval. Should you choose to maintain the maximum potential accuracy of your instrument, an annual calibration interval is recommended.

Please keep in mind that the probes for the HMT120 and HMT130 can be interchanged, and so having a spare probe for each instrument would allow the unit to remain in operation while your other probe is being calibrated/adjusted.

You can find some information here https://www.vaisala.com/en/products/instruments-sensors-and-other-measurement-devices/instruments-industrial-measurements/hmp110

Thank you for contacting us!

Best regards,

Justin Walsh | Application Sales Engineer