Dew Point Calculator for Surface Preparation and Painting (3K Rule)
Formula
γ = ln(RH/100) + (17.62 × T) / (243.12 + T); Td = (243.12 × γ) / (17.62 − γ)
Governing Standard
ISO 8502-17 and SSPC-SP1 — Surface Preparation Standards
Required Inputs
- Air Temperature (T) in °C
- Relative Humidity (RH) in %
- Surface Temperature (Ts) in °C (if measured)
Output
Dew Point Temperature (Td) in °C, Delta (Ts - Td), and Safe-to-Paint Status
Notes
The 3K rule: surface temperature must be at least 3°C above the dew point before painting. At 25°C air temp and 70% RH: γ = ln(0.70) + (17.62 × 25)/(243.12+25) = -0.357 + 1.639 = 1.282; Td = (243.12 × 1.282)/(17.62 - 1.282) = 19.1°C. If surface temp is 23°C, delta = 3.9°C — SAFE to paint. If surface temp is 21°C, delta = 1.9°C — DO NOT PAINT.
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