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How to measure dewpoint before coating any surface – professional climatic surface testing

What climatic conditions do we measure?



Key climatic parameters for Delta T include:

Surface temperature (Ts)
Air temperature (Ta)

% Relative Humidity (RH), the amount of vapour in the air compared to
maximum possible.

Wet bulb temperature (Twb), the air temperature around water-soaked fabric over which air is passed;

Dry bulb temperature (Tdb), the temperature that is usually thought of as air temperature;

Dewpoint (Td), temperature when which moisture condenses on a surface (calculated from
Air Temperature and % Relative Humidity)

Finally, Delta T (TΔ), the difference between the surface temperature (Ts) and the dewpoint
temperature (Td).

 Delta T is the most important variable, indicating direct presence of moisture on the surface (results in coating failure). If the surface is unevenly sunlit and shaded, delta T will be affected as per given location.

What is dewpoint?

What tools to use for Delta T measurement?

1. Manual Measurement: Elcometer 116C Whirling Hygrometer, also known as a Sling Psychrometer

It shows wet bulb and dry bulb temperatures. They are then used to work out the dewpoint and relative humidity.

As the hygrometer whirls in the air, the water on its fabric-covered part evaporates. The rate at which it evaporates and rate of resulting cooling on the bulb, depends on the level of humidity in the air (the lower the humidity, the greater the cooling). The dry bulb thermometer is measuring the ambient air temperature.

Hold the hygrometer facing the wind to avoid interferance of your body temperature. On a windless day, walk slowly forward while whirling. Spin the hygrometer at least twice with 40 sec interval to make sure the measurements are repeatable.

*There’s an even easier method to convert your wet and dry bulb readings into dewpoint and relative humidity, using the free ElcoCalc App for Android and Apple mobile devices. Simply select the climate tab, choose your temperature units, enter your dry and wet bulb measurements, and ElcoCalc does the rest.

Dew point calculaiton

Image above: Elcometer Whirling Hygrometer

2. Digital Thermometer

For this, a digital thermometer with a surface temperature probe i need (such as the Elcometer 212 or 213), or an IR digital laser thermometer for measuring substrates. Once you have your surface temperature, subtract your dewpoint temperature from it, and you have the Delta T.

3. All-in-one solution: Elcometer 319 Dewpoint Meter

Why use several pieces of equipment, when you only need one? The Elcometer 319 Dewpoint Meter is a digital gauge that can monitor and record all of the key climatic parameters instantaneously.

How to use Elcometer 319

Allow the sensors some time to adapt to the environment (if exiting from an air conditioned office for ex.). Keep fingers away from sensors. Hold
securely against the surface until the Ts reading has stabilized
. If any of the parameters go up or down, the gauge shows it  using arrows next to Ts.

The basic principle: the Delta T should be at least 3°C (5°F), the relative humidity should not exceed 85%, and the air temperature should be at least 10°C (50°F).

Elcometer 319 Model T offers an advantage to continuously monitor and record the conditions at rate of your choice. You can now access climatic conditions of the localised area and accessing difficult-to-reach areas.

Every reading of every climatic parameter can be transferred via USB / Bluetooth to ElcoMaster®, Elcometer’s free data management software. It is possible to remotely monitor the Elcometer 319 live, while it’s logging the conditions, creating a mobile climate station.


Elcometer 319 Dew Point Meter

Elcometer 116C Sling Hygrometer

Elcometer 116A Whirling Hygrometer | Metric (øC) |

Related article:

Calculating dew point from relative humidity and temperature