The calculator
Runs in your browser. Nothing is stored. Switch between °C and °F at the top.
How the check works
Dew point is the temperature at which the water vapour in the air saturates. A surface at or below it collects condensation. The calculator gets it from the World Meteorological Organization's formula for saturation vapour pressure over water, e = 6.112 exp(17.62 t / (243.12 + t)) hPa, valid from −45 °C to +60 °C. Solved for the dew point: γ = ln(RH/100) + 17.62 T / (243.12 + T), then Td = 243.12 γ / (17.62 − γ).
The margin is the surface temperature minus the dew point. Condensation does not wait for the exact dew point: a part that is a degree above it, in air that is cooling, crosses it during the job. Coating practice asks for the surface to stay at least 3 °C (5 °F) above the dew point, the figure in ISO 8502-4. Your product's data sheet may ask for more, and it governs.
Measure the part, not the room. A wing skin that spent the night in an unheated hangar can sit several degrees below the air around it the next morning. That gap is the usual cause of amine blush on epoxy and of primer that peels in sheets.
Worked example
Morning in the hangar: air 25 °C, relative humidity 70 %, the panel reads 22 °C on a contact thermometer.
- γ = ln(0.70) + 17.62 × 25 / (243.12 + 25) = −0.357 + 1.643 = 1.286.
- Td = 243.12 × 1.286 / (17.62 − 1.286) = 19.1 °C.
- Margin = 22 − 19.1 = 2.9 °C, under the 3 °C required.
- Surface floor = 19.1 + 3 = 22.1 °C. The panel is 0.1 °C short.
Not yet. Warm the panel or dry the air, then measure again. A tenth of a degree is inside the error of most thermometers; that is the point of a margin.
Limits — when NOT to use this
- The product data sheet governs. Many primers, sealants and resins set their own minimum surface temperature, maximum humidity and margin. Use those figures in the margin field.
- Below 0 °C the formula gives the dew point over water. Frost forms over ice at a slightly different temperature; coating work below freezing is outside what this checks.
- Your inputs set the accuracy. A hygrometer within ±3 % RH moves the dew point by about half a degree at room temperature. Keep the margin for that.
- Conditions change during the job. Check again before each coat and through the cure, not only before you open the can.
Questions people actually ask
Why 3 °C and not 3 °F?
The margin comes from coating standards written in Celsius: 3 °C is 5.4 °F, which US data sheets round to 5 °F. The calculator converts the difference without an offset, so 3 °C shows as 5.4 °F.
The humidity is fine, why does it still fail?
Because the surface is cold. Humidity sets the dew point; the part's temperature decides whether it reaches it. At 50 % RH and 20 °C air, the dew point is 9.3 °C, so a part straight out of a cold store can still be below it.
Does this matter for wet layup epoxy?
Yes. Moisture on the surface, and humid air during the cure, cause amine blush: a waxy film on the cured resin that the next layer will not bond to.
Sources
- WMO-No. 8, Guide to Instruments and Methods of Observation, Volume I, Annex 4.B — saturation vapour pressure over water, e = 6.112 exp(17.62 t / (243.12 + t)) hPa, −45 to +60 °C. The dew point here is that formula solved for temperature.
- ISO 8502-4, Preparation of steel substrates before application of paints — Guidance on the estimation of the probability of condensation — the source most coating data sheets give for the surface-above-dew-point margin of 3 °C. We cite it from those data sheets; the standard itself is paywalled.