Frequently Asked Questions About Hygrometry

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Hygrometry

 

What is hygrometry?

Hygrometry is a branch of physics concerned with the measurement of moisture in air and other gases.

It is generally accepted that moisture measurement refers to water and water vapour in solids, liquids, and gases, whereas humidity refers to purely gas measurements.  This includes low-level water vapour at low dew points at ppm and ppb levels, as well as relative humidity, mixing ratios & dew point at much wetter levels which may be close to saturation.

IMA offers a complete range of hygrometers, to measure from trace levels to saturation.

 

What happens at the dew point of air or other gases?

At the dew point of air and other gases, the water vapour contained will start to condense out of vapour phase.

The dew point temperature of a gas is the temperature at which liquid water will begin to condense out of vapour phase.

Normal atmospheric dew point in the UK varies from day to day, and from winter to summer, but it is generally around +10 °C dewpoint.  This equates to 12000 ppm or 1.2 % of the total air volume.

As gases that contain less water vapour need cooling further before the dew point is reached, the dew point temperature of a gas is used to indicate the water vapour content of a gas.

Note that if a gas is cooled beyond its dew point, more liquid water will condense out.

When cooling below freezing, the water vapour condenses as frost.  The terms frost point and dew point are ofter interchanged, and -60 °Cdp will in fact be a frost point.

 

Why does dew point change with pressure?

Answer to follow shortly!

 

What is relative humidity?

Answer to follow shortly!