Temperature Measurement · Glossary

What is Dew Point?

°C at which water condensesTemperature Measurement

Dew point is the temperature to which air must be cooled for its water vapour to start condensing.

Dew Point Explained

A low dew point means dry air. Instrument air is specified by its pressure dew point, which must stay below the lowest ambient temperature so no water condenses in the lines.

Dew point is measured with chilled mirror or capacitive humidity sensors, or calculated from temperature and relative humidity.

Dew Point Formula

T_d = b γ ÷ (a − γ), γ = ln(RH ÷ 100) + a t ÷ (b + t)

a, b	17.62 and 243.12 °C (Magnus constants over water)
t	air temperature (°C)
RH	relative humidity (%)

Example: 25 °C at 60 % RH gives a dew point of about 16.7 °C.

Calculators for Dew Point

CalculatorDew Point CalculatorFree online calculator with formula and worked example.CalculatorWet Bulb Humidity CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Dew Point?

Dew point is the temperature to which air must be cooled for its water vapour to start condensing.

What dew point should instrument air have?

ISA 7.0.01 asks for a pressure dew point at least 10 °C below the minimum ambient temperature at the site.

What is pressure dew point?

The dew point at line pressure, which is higher than the dew point of the same air expanded to atmosphere.

Learn more on the blog

Thermocouple and RTD installation precautions

A practical guide on instrumentationblog.in with more detail on dew point and how it is applied on real plants.

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