Dewcheck

A dewcheck is a device used to measure air temperature, surface temperature and relative humidity. Out of the combination of these three parameters the dewpoint and difference between dewpoint and surface temperature is determined. These parameters are commonly required to be monitored during coating applications. The dewcheck is the coating industry's short for dewpoint gauge. Dewpoint is not only relevant for human comfort but also influences the curing of a coating. Coating defects due to application of coating near dewpoint can vary from no adhesion to early corrosion. The word Dewcheck is now worldwide used for dewpoint gauges of any brand.

Importance

Dewpoint is of influence to coatings of multiple types. Two component coatings and water or solvent based coatings are the main groups of coatings. The water or solvent based coating need to evaporate one or some of their components to harden. To do this the air must be able to absorb this water vapour. If this evaporation go’s to fast or to slow it will influence the curing of the coating. Deformation, colour change and bad adhesion are all possible. The standardised minimum temperature difference is 3 or 5 °C depending on the coating specifications and the ISO or ASTM standard that has to be adhered. The temperature difference is also called Delta T.

Prior to the dewcheck

In the past coating inspectors used to work with a psychrometer, surface temperature meter, calculator, dewpoint table and notebook. These inspections took up to 10 minutes per set of measurements. Calculations errors and measurement errors where a big problem. A coating inspector would take the measurements as specified varying from one measurement per day up to continuous measurement.

The first dewcheck

The first dewcheck was launched by Thermimport Quality Control TQC in 1994. This became possible by the development of the microprocessor and the capacitive RG sensor. The development of the dewpoint gauge let to an increase of the amount of possible Delta T measurements during an inspection. The gauges showed the value directly on the LCD screen making the measurement also possible for the people on the work floor. Thus lowering the threshold to raise the quality specifications.

Developments

Trough the years several manufacturers started producing dewpoint gauges. Different designs, probe layouts, and accuracies all made their appearance. Most of them where fashion statements. However one option to the devices became permanent in the later versions. Data logging became the hot item for the new devices. Surveying standards as well as regulations started to demand strict control of climate data. Even more measurements where required to get approvals.

What can be seen on a dewcheck

*Ta - Air Temperature
*Ts - Surface Temperature
*RH - Relative Humidity
*Td - Dew Point Temperature
*ΔT- the difference between surface and dew point temperatures

tandardization

Many standards cover the measurement of environmental conditions during coating applications. Worldwide these standards and the usage of dewpoint gauges fall under the responsibility of ISO TC 35 SC 12. Many countries have national committees reporting to this international committee.

References

1. ASTM D3276-00, “Standard Guide for Painting Inspectors (Metal Substrates)”

2. ISO 8502-4, “Preparation of steel substrates before application of paints and related products -- Tests for the assessment of surface cleanliness -- Part 4: Guidance on the estimation of the probability of condensation prior to paint application”

3. ASTM E337-02, “Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)”

4. Dewcheck site by TQC [http://www.dewcheck.nl]