A transformer tank can have a cold shaded panel while the workshop air is warmer. Condensation risk follows the local steel surface and the gas moisture condition. One air-temperature reading at the start of painting cannot represent the entire application period. [1]
Record air temperature, relative humidity and substrate temperature at representative locations. Derive or measure dew point under the specified method. Define the margin as Tsteel − Tdew, using a common temperature scale; the difference is kelvin.
Choose the steel location deliberately
| Location/time | Why it matters | Evidence |
|---|---|---|
| Shaded or thick steel | May lag room warming | Local substrate temperature |
| Door-side or exposed panel | Air boundary can change | Air/humidity near the work |
| During spray application | Conditions can change after initial release | Time-linked environmental readings |
| Specified cure period | Film formation has its own requirements | Relevant coating conditions over time |
Use the product requirement for the margin
The approved coating data and application procedure establish the permitted temperature and dew-point conditions. This article supplies no universal margin. A favourable calculated margin is only meaningful within the measurement uncertainty and spatial coverage.
If the coldest steel location is unknown, inspect likely thermal lag points rather than taking another reading at the warmest accessible panel. Contact sensing and infrared sensing have their own surface and emissivity assumptions.
Combine standard uncertainties with covariance where effects are correlated; repeatability alone does not include all systematic contributions. [2]
Propagate the measured dew-point margin
Assumptions: local steel is 12 °C, dew point is 10 °C, and the approved product rule requires at least a 1 K margin throughout application. The nominal margin is 2 K. Independent standard uncertainties of 0.5 K and 0.7 K give u = √(0.5² + 0.7²) = 0.860 K. Under an assumed lower-bound decision rule with k = 2, the margin is only 2 − 2u = 0.280 K, below 1 K. Hold painting at that location until the thermal condition or measurement evidence satisfies the actual rule. [2]
References
- ISO, ISO 12944-7:2017, Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 7: Execution and supervision of paint work.
- B. N. Taylor and C. E. Kuyatt, NIST Technical Note 1297, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, 1994, sections 5 and 6.