Selection of a hygrometer
Consider:
- what you want to measure
- is it relative humidity, dew point or something else? Many instruments display results converted into several alternative units but the measurement will be intrinsically just one of these. - what humidity range, and what temperature range?
- must it survive extreme conditions - very dry, hot or wet? - instrument performance needed
- resolution, short-term stability, long-term drift, speed of response, non-linearity, hysteresis, temperature coefficient, uncertainty of calibration. - configuration
- a hygrometer probe measuring in free space, or will humid gas be supplied through tubing to flow through the instrument? - hand-held, bench-top, or surface mounted
- any space constraint? - power
- battery, mains electricity or even unpowered - data output format
- display, analogue voltage or current, digitally-read output (RS232 ...) or data logged and stored, to be downloaded later. Some instruments can record on paper charts. An instrument that gives an electrical output (analogue or digital) but no displayed result is sometimes called a 'transmitter'. - are any 'alarms' needed if humidity passes any limit?
- is sensor output to be used to automatically control something?
- compatibility with any unusual gases
- contamination such as dust or chemicals
- use in hazardous areas (may require special design and certification)
- robustness
- cost and upkeep
- calibration
- versatility
- interchangeability
- ease of use or level of skill required
... and possibly others!
Once you have your hygrometer, some other actions follow:
- operating procedures, and staff training if needed
- maintenance (if any)
- initial calibration, and repeat calibrations at intervals
- other checks in between calibrations
- this is particularly important when a hygrometer has been exposed to extreme conditions, or mistreated, or whenever there is doubt about whether it is reading correctly.
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