How a Michell chilled mirror hygrometer measures relative humidity

What is relative humidity?

Relative humidity is the ratio of the amount of water vapor present in a gas, versus the maximum amount of water vapor the gas can support before condensation occurs at a given temperature. This means that changes in temperature, pressure or the amount of moisture present will all affect the %rh.

The formula for calculating relative humidity is as follows:

%rh = (water vapor pressure/saturation water vapor pressure) * 100.0

Why use a chilled mirror hygrometer to measure %rh?

Chilled mirror hygrometers are usually used for relative humidity calculations in industrial settings. They are highly reliable and have virtually no drift, when compared to polymer rh sensors. In power generation, a rugged industrial chilled mirror hygrometer – such as the Michell Optidew 501 – would be a good choice because of these features. One such application for the Optidew in this field is to protect the intake systems of gas turbines by helping predict the onset of frost-forming conditions – improving efficiency and preventing damage to turbine internals. Its new design chilled mirror sensor gives it a speed of response which is comparable to a polymer sensor in many applications including environmental monitoring. Chilled mirror hygrometers are also relied on in many meterological applications for their accurate and repeatable measurements.

Calculations used to determine %rh

The Optidew and S8000 chilled mirror ranges calculate %rh automatically, however at the very low %rh conditions that are often found in industrial settings, it’s helpful for you to understand how these calculations are performed to ensure you get the best accuracy from the instrument. Chilled mirror instruments primarily measure the dew point or – in very dry conditions – frost point and there is a subtle effect on %rh measurements depending on whether the dew point or frost point is measured.

In Michell chilled mirror products, the calculation for water vapor pressure (wvp) and saturation water vapor pressure (swvp) are both calculated using the Sonntag vapor pressure formula.

The Sonntag formula exists as an “over water” formula, and as a distinct “over ice” formula. Normally the “over water” formula is selected when the temperature (or dew-point temperature) is > 0 °C, otherwise the “over ice” formula would be used.

The WMO Standard Method (force swvp over water)

However, when it comes to calculating relative humidity for meteorological purposes there is another consideration. In the atmosphere, it is possible to have condensation of vapor into the liquid state (rather than ice) at temperatures below freezing, through the phenomenon known as super-cooling.

Since the super-cooling phenomenon can occur in clouds, and the upper atmosphere, it was decided nearly half a century ago by the World Meteorological Organisation (WMO) that rh be calculating by computing the saturation water vapor pressure over water regardless of the temperature. This decision was to prevent the occasional possibility of %rh values exceeding 100% when the atmosphere was in a super-cooled state. It is important to note that using this calculation method only gives different results when the temperature is below 0 °C.

It is possible to use this calculation method in both the S8000 series and the Optidew 401/501. If you would like help, advice or have any questions about any of the topics raised in this post, please contact us.