Dewcheck

A dewcheck is a dewpoint gauge intended for the coating industry, a device used to measure air temperature, surface temperature and relative humidity. Dewpoint is calculated from air temperature and relative humidity, and the difference between dewpoint and surface temperature is determined and can be displayed. These parameters are sometimes required to be monitored during coating applications for proper drying or curing of the coating and to prevent condensation on the surface which would interfere with the coating. Although the brand name of a gauge first marketed in 1994, the word "dewcheck" is not claimed as a registered trademark by that company and is now used generically for any brand of dewpoint gauge.
Importance
Dewpoint and relative humidity are significant for multiple types of coatings of which 100% solids, water, and solvent based are the main groups. The evaporation of solvent or water in an applied coating will lower its temperature. If the surface temperature approaches the ambient dewpoint water could condense underneath a solvent-based coating, causing defective adhesion or corrosion-producing trapped water. This temperature difference, called Delta T, is required to be a minimum of 3 or 5 °C depending on the coating specifications and the ISO or ASTM standard being followed. The maximum relative humidity (RH), defined as the percentage of actual water vapor relative to saturation for a given temperature, is generally specified as 85%. Moisture-containing air cannot hold as much solvent as dry air and so high RH can retard solvent vaporation of solvent-based coatings as well as water evaporation of water-based coatings. Water-based coatings can also evaporate too quickly at very low RH. These problems can cause deformation, colour change and bad adhesion of the coating. 100% solids coatings, whether single or two-component, frequently use atmospheric water vapor as part of their curing reactions, and thus may require a minimum specification for RH or dewpoint.
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 were 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
Through the years several manufacturers started producing dewpoint gauges. Different designs, probe layouts, and accuracies all made their appearance. Many units had only cosmetic differences. However, data logging as an option came to be a common feature, as standards as well as regulations required stricter documentation.
Timeline
The Original 1994 Dewcheck was a great marked success and Elcometer asked to be able to have a branded version as their first dewpoint meter this meant the dewcheck now had an orange coloured brother called the Elcometer 219, which continued for the entire Dewcheck 1 and 2 production period. Later in 2002 the data communication capable Dewcheck NG or also known series 3 got a branded housing for Elcometer, but this time with the name Elcometer 319. The in 2007 launched Dewcheck 4 also was sold as a branded version for Elcometer with the extinction 319/2. During all years names as dewpoint gauge, dewmeter and many others where also applied to the Dewcheck.
Dewcheck timeline
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
Standardization
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.

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