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Preparing outdoor surfaces for painting – crucial weather factors

Why Delta T measurement is so important

Climactic testing

By far the most important parameter in surface preparedness evaluation of all is the Delta T, as it tells you the difference between the surface temperature and the dewpoint temperature. The smaller the difference, the more likely that moisture (or dew) will have condensed on the surface.

It is generally accepted within the industry that the Delta T should be at least 3°C (5°F) or higher for the coating to be applied.

How do you actually measure and calculate the key climatic parameters? There are two main methods you can follow.

The traditional method requires multiple pieces of equipment to complete.

The first piece required is the whirling hygrometer, also known as a sling psychrometer, and it is used to measure the wet bulb and dry bulb temperatures. These temperatures are then used to work out the dewpoint and relative humidity. Elcometer provides two types of hygrometers, but they work in pretty much the same way.

How does a hygrometer work?

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).

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: https://youtu.be/HLbuapAnoqA

Calculation Delta T with a digital hygrometer

However manual Hygrometers are still missing the all-important Delta T measurement, and to calculate this, you still need a surface temperature measurement – something a hygrometer can’t provide, meaning you’ll need a separate surface thermometer. Once you have your surface temperature, subtract your dewpoint temperature from it, and you have the Delta T.

A digital hygrometer makes this process very easy. It also continuously monitor and log the conditions as you paint, and instantly alarm if one of the parameters falls outside of the specified range.

it can continuously monitor and log the conditions as you paint, and instantly alarm if one of the parameters falls outside of the specified range.

Elcometer gauges for dewpoint temperature

Dewpoint Temperature is calculated by recording the surface temp (Ts) vs air temp (Ta) and its relative humidity (%RH). This difference (TΔ) is the determining factor in surface preparation for painting. The Elcometer 319 dew point meter can be used as a hand-held gauge or a stand-alone data logger – ideal for monitoring climatic conditions over a period of time. The Elcometer 114 Dew Point Calculator provides accurate values of Dewpoint and Relative Humidity (RH) from the wet and dry bulb temperatures measured by Whirling or Sling Hygrometers / relative humidity meters.

The Elcometer 308 digital hygrometer has been specifically designed for use in very hot climates where the surface temperature of the substrate can exceed the paint manufacturer’s recommended limits for successful painting Surface Temperature (Ts).

Observing climatic conditions like temperature, relative stickiness, dew point, and dampness is imperative for the successful adhesion of a coating.

Adhesion testers for ConcreteElcometer 308_instruction_manual.pdf


Elcometer 319 Dew Point Meter

Elcometer 309 Delta T Hygrometer

Elcometer 116C Sling Hygrometer

Elcometer 116A Whirling Hygrometer | Metric (øC) |


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Read more on tools and procedures for establishing dewpoint