How to measure Dewpoint & Relative Humidity using a Whirling Hygrometer or Sling Psychrometer

Applying a coating in the correct climatic conditions is vital, as if conditions are not suitable - say the surface is too hot, the atmosphere is too humid, or moisture has condensed on the surface to be painted for example - this could cause the coating to fail. That’s why you should monitor a range of climatic parameters before and during the application process, and one method that has been long used in the industry to do this is the whirling hygrometer, also known as a sling psychrometer.

Whirling hygrometers are designed to measure:
- Dry bulb temperature (Tdb), the ambient temperature that is usually thought of as air temperature;
- and Wet bulb temperature (Twb), the temperature as read by a thermometer that’s covered in water-soaked fabric over which air is passed.

The results of these two measurements are then used to determine:
- % Relative Humidity (RH), the amount of water vapour present in the air compared to the maximum possible, expressed as a percentage;
- and the Dewpoint (Td), the temperature at which moisture condenses on a surface, calculated from the air temperature and % Relative Humidity.

Elcometer provides two types of hygrometers, but they work in pretty much the same way.

Hygrometers consist of two liquid filled thermometers positioned side-by-side in a rotating body. One thermometer is covered with a fabric “sock” or “wick” connected to a reservoir (this measures your wet bulb temperature), while the other is uncovered (this measures your dry bulb temperature).

While the dry bulb thermometer is measuring what is thought of as ambient air temperature, the wet bulb thermometer is used to calculate relative humidity.

To prepare the wet bulb thermometer, distilled water is added to the reservoir of the hygrometer, and this saturates the wick. In the case of the Elcometer 116C Sling Hygrometer, you are additionally instructed to fully submerge the thermometer bulbs in distilled water first to ensure the wick is saturated, and then fill up the reservoir to keep the wick saturated as you test. Regardless of which hygrometer you are using, it’s important you ensure before testing that the wick is not only fully saturated with water, but also clean, tight fitting, and fully covers the thermometer bulb.

Once the wick is fully saturated with distilled water, you then whirl (or spin around) the hygrometer above your head for between 20 to 40 seconds; at approximately 180 revolutions per minute, or 3 revolutions per second, or faster.

After the 20 to 40 second whirl, the thermometers are quickly read, noted, and then the hygrometer is spun again. After another 20 to 40 seconds, once again read the thermometers. If both results are exactly the same, these are your wet and dry bulb temperatures. If they aren’t the same, you need to repeat the process until you get two matching, stable results in a row for both your wet and dry bulb temperatures.

Once you have your results, typically conversion tables are then used to determine the relative humidity and dewpoint temperature, like the ones supplied with the Elcometer 116 Hygrometers. Alternatively, the Elcometer 114 Dewpoint Calculator provides a quick and easy way to determine these values, and we show you how it works in our Elcometer 114 video.

Whilst the whirling hygrometer has given us all of these climatic conditions, there’s one climatic parameter that we haven’t measured yet, and it’s regarded as the most important one in the coatings industry – the Delta T.

Since you require the surface temperature to calculate the Delta T, we need another piece of equipment, as whirling hygrometers can’t provide this measurement. So, you’ll need a separate surface thermometer.

This could be a magnetic thermometer for working in intrinsically safe environments (such as the Elcometer 113), a digital thermometer with a surface temperature probe (such as the Elcometer 212 or 213), or an IR digital laser thermometer for measuring substrates that are on a moving production line or are extremely high temperatures (such as the Elcometer 214).

Once you have your surface temperature, subtract your dewpoint temperature from it, and you have the Delta T.

Click here to download the script in English