Dew point and spread table
The dew point is the temperature at which moisture condenses. The values below use the Magnus formula (a = 17.625, b = 243.04), the same calculation the CoatSpec tool runs on your inputs.
Common field practice, reflected in many specifications, is to require the surface temperature to be at least 3 degrees Celsius (5 degrees Fahrenheit) above the dew point during application and initial cure, with relative humidity at or below 85 percent. Your project specification governs - some coatings carry different limits.
| Air (°C) | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH |
|---|---|---|---|---|---|---|---|
| 0 | -15.5 | -12 | -9.2 | -6.8 | -4.8 | -3 | -1.4 |
| 5 | -11.2 | -7.5 | -4.6 | -2.1 | 0 | 1.8 | 3.5 |
| 10 | -6.8 | -3 | 0 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15 | -2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 20 | 1.9 | 6 | 9.3 | 12 | 14.4 | 16.4 | 18.3 |
| 25 | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30 | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
| 35 | 14.8 | 19.4 | 23 | 26.1 | 28.7 | 31 | 33.1 |
| 40 | 19.1 | 23.8 | 27.6 | 30.8 | 33.5 | 35.9 | 38 |
Frequently asked questions
Why must the surface be 3 degrees Celsius above the dew point?
As the surface approaches the dew point, a molecular film of moisture can form even before visible condensation, risking flash rusting on steel and adhesion loss for most coatings. The 3 C / 5 F spread is a widely used safety margin - verify what your specification requires, since some products have their own limits.
When during the shift should conditions be measured?
As commonly applied, at least before start-up and every four hours thereafter, and more often when conditions are changing - rising humidity, falling temperature, or work moving between sun and shade. Log every set of readings with the instrument used.
Does the table replace a dew point meter?
No. It shows the arithmetic relationship so you can sanity-check readings. Field acceptance should rest on calibrated instruments measured at the work location.