Free tool
Dew point calculator
Check whether it is safe to blast or paint. Enter your site readings to see the dew point and whether the steel is at least 3 °C above it.
Result
- Dew point
- 7.7 °C
- Steel above dew point
- +3.3 °C
The steel is at least 3 °C above the dew point. Condensation risk is low for this rule.
Calculated with the Magnus formula. A guide for site checks, not a substitute for the specification, product data sheet or a calibrated instrument.
Why must steel be 3 °C above the dew point?
At or near the dew point, an invisible film of condensation forms on the steel. Blasting or painting onto it causes flash rust, poor adhesion and blistering. The 3 °C margin allows for measurement error and changing conditions, and it is written into most coating specifications.
How do I measure the readings?
Use a calibrated hygrometer or electronic climate gauge for air temperature and relative humidity, and a contact thermometer on the steel surface itself. Take readings where the work is happening, and repeat them through the shift, because conditions change.
What about relative humidity limits?
Many products and specifications limit relative humidity, commonly to 85%, and some coatings need a minimum humidity to cure. Always check the product data sheet and the specification.
Is this calculator accurate enough for inspection records?
It uses the Magnus formula, which is accurate to about 0.1 °C across normal site conditions. For formal records, use calibrated instruments and record the readings as the specification requires.
Need climate monitoring on site?
We log conditions throughout application as part of every inspection, so the records are there if anyone asks.
Prefer to talk? Call 07810 022184