ISO 8502-4
Dew point and surface temperature calculator
Enter the air temperature, relative humidity and steel surface temperature. The calculator returns the dew point, the margin between surface and dew point, and whether the surface is clear to blast or coat under the 3 °C (5 °F) rule.
Dew point
18.2°C
Surface minus dew point (ΔT)
2.8°C
Verdict
hold: too close to dew point
Minimum surface temperature
21.2°C
Advisory result. Provisional until a qualified inspector or engineer verifies it against the governing code edition.
How it works
- 01Dew point from the Magnus formula with the Sonntag constants, accurate to a few tenths of a degree in the coating range.
- 02ΔT = surface temperature minus dew point.
- 03Clear to coat when ΔT is at least 3 °C (5 °F), the ISO 8502-4 and SSPC guidance; otherwise hold.
- 04Minimum surface temperature = dew point + 3 °C, the number to watch as the day cools.
- 05Second rule: the surface must also stay below the coating's maximum application temperature from the data sheet; the calculator holds when it does not, and refuses implausible readings.
Formulas
Td = b γ / (a - γ), γ = aT/(b+T) + ln(RH/100), a = 17.62, b = 243.12Ts - Td ≥ 3 °C (5 °F)Read before you rely on it
- Measure the surface, not the air, and measure at the coldest point of the item: undersides, near ground, shaded steel.
- Record before blasting, before each coat, and whenever the weather turns; the record is what the inspector checks.
- Some specifications require a larger margin; the specification governs.
Answers
Questions about this calculator
Why must the surface be 3 °C above the dew point?
Below that margin, moisture can condense on freshly blasted steel as a film too thin to see, causing flash rust and adhesion failure under the coating. ISO 8502-4 and SSPC both use the 3 °C (5 °F) margin.
How is dew point calculated from humidity?
From air temperature and relative humidity with the Magnus formula, the same relationship a dew point meter uses. At 24 °C and 70 percent humidity the dew point is close to 18 °C.
When should I take the readings?
Before surface preparation, before every coat, and during the cure, at the coldest surfaces on the job. Repeat whenever conditions change.
Does intelliSPEC record these readings?
Yes. intelliPAINT's environmental readings form captures air temperature, humidity, dew point and surface temperature against the hold point, so the coating record shows the conditions at each stage.