The humidity measurement expert

What is your humidity meter measuring?
banner-hm-trendOnline — The obvious answer is “humidity”, but delve a bit further into the technology of the humidity meter. Vaisala’s Rule #6 for Humidity Good Measurement Practice says, ???Know what psychrometric parameter your humidity sensor is measuring???.

For example::

  1. chilled mirror hygrometers measure dewpoint
  2. polymer sensors measure relative humidity
  3. aluminum oxide sensors measure vapor pressure
  4. psychrometers measure wet-bulb temperature

The reason you should know what type of sensor your instrument is using is that, depending on the parameter, there may or may not be adverse effects due to temperature or pressure changes.

A sensor measuring relative humidity can vary significantly when there is a difference in temperature between the air being measured and the instrument probe.

For example, if the probe temperature is 1 ??C lower than the air temperature, you would increase your error by about 3% RH!

With this lack of temperature stability, if your “true” RH were 50%, and you have this 1 ??C temperature difference, your RH measurement instrument would be reading 53-54%.

Temperature equilibrium is critical if your sensor is measuring relative humidity.

Dewpoint temperature measurement will vary significantly with changes in pressure.

If you measure the dewpoint of a compressed air system at a point where the air has been adjusted to a lower pressure but you want to know the dewpoint of the air under system pressure, you will not have an accurate reading.

In fact, dewpoint temperature rises as the air pressure increases. If you measure dewpoint at atmospheric pressure when the point of interest is under pressure, you will have a false low dewpoint temperature.

Check out the Vaisala Humidity Information website:
??? live humidity seminar recording
??? seminar training course guide
??? calibration handbook
??? links to humidity resource tools
– humidity calculator
– product adviser
??? and more great humidity measurement resources

Humidity information at your fingertips. Visit the Vaisala Humidity Information website.

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