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How to measure relative humidity with chilled mirror hygrometers

Precise relative humidity measurement with chilled mirror hygrometers from Michell


The most commonly used measurement technology for monitoring relative humidity is based on polymer capacitive sensors. They are fast responding and generally very cost-effective in the range of 10-98% RV. However, for accurate measurements of relative humidity and dew point, it is preferable and recommended to use chilled mirror hygrometers. From our product range we present chilled mirror relative humidity meters our partner Michell Instruments, part of the PST Group.

The following article discusses relative humidity and the purpose of chilled mirror humidifiers.

What is relative humidity?

Relative humidity (RH) is the ratio of the amount of water vapour present in a gas compared to the maximum amount of water vapour that the gas can hold before condensation occurs at a given temperature. This means that changes in temperature, pressure or humidity will affect the % RH.

Formula for calculating relative humidity:

RV = (water vapour pressure/water vapour saturation pressure)*100

Why use chilled mirror embeddings?

They are commonly used to calculate the % RH in industry. Compared to polymer sensors, chilled mirror hygrometers are very reliable and virtually drift-free. For power generation, the robust (EX) industrial chilled mirror hygrometer Optidew 501 is preferred . The latter is used to protect gas turbine systems as it helps to predict the conditions that cause frost. This improves efficiency and prevents irregularities or damage inside the turbine. Its design allows the chilled mirror sensor to provide a fast response comparable to a polymer sensor in most applications, including environmental monitoring. Precise and repeatable measurements make chilled mirror hygrometers very useful and desirable in many meteorological applications.

How does a chilled mirror hygrometer calculate relative humidity?

Relative humidity meters Optidew 501 and S8000 automatically calculate the % RH. To ensure the best level of accuracy of the instrument, it is necessary to understand how these calculations work under industrial conditions where %RV values are very low. Chilled mirror instruments primarily measure the dew point, or in very dry conditions the frost point.

For Michell chilled mirror products, the calculation of the water vapour pressure and the saturation water vapour pressure are calculated using the vapour pressure formula.

Image: the S8000 and Optidew
WMO Standard Method

When calculating relative humidity in meteorology, it is important to understand that in the atmosphere below freezing, condensation of vapours occurs, turning them into a liquid state (instead of ice). This process occurs at temperatures below the freezing point, and thus Mpemba phenomenon (so-called ”supercooling”). In technical terms, this is the phenomenon where water with a higher initial temperature freezes faster than water with a lower initial temperature.

Because this effect can occur in clouds and the upper atmosphere, the World Meteorological Organisation(WMO) decided almost half a century ago that the % RH should be calculated by taking into account the saturation of water vapour above water, regardless of temperature. This decision was taken to avoid the possibility of relative humidity values exceeding 100% when the atmosphere is in a super-cooled state (Mpemba effect). Using this calculation method gives different results only when the temperature drops below 0 °C.

This method of calculating % RH is used by the S8000 and Optidew 401/501 chilled mirror relative humidity meters . You can see other chilled mirror hygrometers and their applications in our programme.

Author Katarina Žunko
May 2021

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