Dew point before coating: checking Delta T correctly
A surface can look dry while being close to condensation. The key is therefore more than air temperature: it is the difference between surface temperature and dew point. This practical guide explains how to assess the actual coating location and prepare a traceable release decision. The coating manufacturer's limits and project specification remain decisive.
Products and methods at a glance
| Method / parameter | Meaning / application | Practical decision |
|---|---|---|
| Ta and RH | Air temperature and relative humidity | Measure locally; use them to determine Td. |
| Ts | Substrate temperature | Measure where the coating will be applied. |
| Delta T | Ts − Td | Compare with the specified minimum margin. |
Settings and assessment framework
A minimum dew point margin of 3 °C is a common guideline, not a universal release criterion. Also check humidity, permitted temperatures and curing conditions. Example: Ts = 18 °C and Td = 14 °C give a margin of 4 °C.
Professional workflow
- Prepare the coating data sheet and project limits.
- Measure air temperature and humidity close to the surface.
- Read the stabilised surface temperature.
- Determine Td and calculate Delta T as Ts − Td.
- Assess all limits together; pause if any requirement is not met.
- Record time, location and readings; repeat when weather changes.
Troubleshooting
Small margin
Pause application; repeat local measurements later.
Fluctuating values
Allow sensors to stabilise; avoid heat sources.
Different component sides
Add measurement locations; include the coldest areas.
Moisture despite acceptable conditions
Inspect the surface; check material moisture separately.
Maintenance and quality assurance
- Keep probes clean.
- Check sensor contact.
- Store the instrument dry.
- Verify against references following the manual.
- Record inspection and service intervals.
FAQ
Is Ts the same as Ta?
No. A component can be colder or warmer than the surrounding air.
What is Delta T?
Here it means Ts minus Td: the margin above the condensation temperature.
Is 3 °C always enough?
No. It is a common guideline; stricter product or project requirements take priority.
Is one morning reading enough?
No. Weather changes and local temperature differences require further checks.
Does this measure substrate moisture?
No. Porous substrates need a suitable additional moisture test.
Conclusion
A sound release decision combines local climate readings with coating system requirements. Beschicht.com can help you select an appropriate dew point meter and suitable probes.
Explore inspection equipment at Beschicht.comSources and sections used
German - Elcometer Guide to Climatic Testing_corrections (1).docx
- Why control dew point?
- Calculating dew point temperature
- Measuring relative humidity and dew point
