Theory and Operation of
Condensation Chilled Mirror Hygrometers

The inherent accuracy and long term stability provides many advantages over other types of humidity measurement technologies, explains Ken Soleyn.

Introduction
Chilled Mirror Hygrometers are used in Standards and Metrology laboratories as well as in industrial applications where precise and repeatable humidity measurement and control is required. The inherent accuracy and long term stability provides many advantages over other types of humidity measurement technologies. Since chilled mirrors fundamentally measure the dew or frost point temperature directly by controlling a reflective surface to equilibrium between dew/frost formation and evapouration and precisely measuring the temperature of the mirror at this point, the methodology has been validated by standards labs worldwide.worldwide. Typically, chilled mirrors are capable of measuring dew/frost points (Td) from - 80°C to +85°C with an accuracy of ±0.15°C. An introduction of the principles and theory of operation of chilled mirror follows. Definitions of Humidity Terms Vapour Pressure Water exists in the gas phase known as humidity. The partial pressure exerted by water vapour is proportional to its mass. Saturation Water Vapour Pressure When water vapour in a mixture of other gases is at the saturation point,the space has reached the maximum capacity to hold water vapour in the gaseous phase. Any water vapour in excess of the saturation point will be converted to the liquid phase. As the temperature of a fixed volume of air or gas is increased the saturation water vapour pressure increases.

Relative Humidity (%RH)
It is the ratio of the partial pressure of water vapour to the saturation water vapour pressure at a given temperature multiplied by 100. (In a closed system if the temperature changes, the relative humidity will change in an inverse proportion. The water vapour pressure would not have changes but the saturation water vapour pressure changes proportional to temperature changes)

Dew Point Temperature (Td)
It is the temperature at which the water vapour pressure in the gas mixture equals the saturation water vapour pressure and is usually expressed in °F or °C. Any plane surface in a given environment, which is cooler than the dew point temperature of the air or gas surrounding it will accumulate a dew or frost layer.

Frost Point (Td)
Below 0°C frost will exist on plane surfaces. It should be noted that the saturation water vapour pressure is different over ice than water.

Supercooled Water
Below 0°C and typically as cold as - 20°C, water can exist on smooth plane surfaces in liquid form rather than ice crystals. This can lead to some ambiguity when utilizing chilled mirror hygrometers.

Absolute Humidity
It is the ratio of the mass of water vapour to volume. Usually expressed in g/M3 or grains/Ft3.

Mixing Ratio
It is the ratio mass of water vapour to the mass of dry air or gas. Usually expressed in mg/kg, g/kg or grains/lb.

PPMv (Parts per Million by Volume)
It is the ratio of partial pressure of
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