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It has become more common to validate humidity as well as temperature in controlled environments in order to meet GxP requirements. Typically we use the same methods for humidity mapping as for temperature, however, there are a few special considerations for mapping humidity.
Verify that calibration certificates are available for the humidity sensors, and that each sensor was within its calibration interval at the time of use.
2) Sensor Density & Placement
The simplest practice is to use the same number of humidity and temperature sensors. If fewer humidity sensors must be used, it is important to understand humidity measurement well enough to explain to an auditor what the expected humidity values would be (based on temperature) in the spots that had only temperature sensors.
When deciding the number of sensors, a good rule of thumb is a minimum 10 sensors for a space up to 2 cubic meters of volume, and 16 sensors for a space up to 20 cubic meters.
Tere is a regulatory expectation of regular revalidation, from yearly to once every three years, depending on the criticality of the products or processes involved. We also revalidate whenever a significant change to the system occurs. Revalidation does not need to be as thorough as the original mapping, but it must provide assurance that the space or unit is operating correctly and maintaining conditions that match the original acceptance criteria.