To make sure coatings last and perform well, it is important to apply them under the right environmental conditions. Factors like temperature, humidity, and surface moisture can affect how coatings stick and hold up over time—especially on steel structures. That is why painters, inspectors, and project owners rely on tools that measure these conditions, helping avoid problems and ensure a quality finish.
Why Monitoring the Environment Matters
Monitoring environmental conditions is vital to prevent rework and premature coating failure. Key environmental factors to monitor:
1. Temperature : Both air and surface temperatures affect the quality of the coating. Incorrect temperatures can cause defects like blistering or cracking. Surfaces may also be cooler or warmer than the air, which can impact curing rates and moisture formation. The minimum recommended temperature for most coatings is 40°F (5°C), but cold-curing systems may allow for lower temperatures.
2. Relative Humidity : High humidity slows solvent evaporation and affects coating application. The maximum allowable humidity for applying coatings is typically 85%.
3. Dew Point Temperature : The dew point is the temperature at which moisture forms on a surface, which can lead to rust on steel or premature coating failure. To prevent this, the surface temperature must be at least 5°F (3°C) above the dew point during coating preparation, application, and curing.
Properly measuring and controlling these environmental factors helps avoid moisture-related issues, such as rust and coating failure, and ensures coatings perform as expected. By using specialized instruments to measure these conditions, contractors can optimize the coating process and prevent rework.
How Environmental Conditions Affect Coating and Paint Performance
1. Air and Surface Temperatures
To avoid moisture issues on a coated surface, both the surface and air temperatures should be measured. Steel surfaces often cool down at night and warm up during the day, making the surface temperature different from the air temperature. It’s important to check both to prevent problems when applying the coating.
If the temperature is too high or too low, defects like blistering or cracking can occur. Coating manufacturers provide recommended temperature ranges for application. According to ASTM D3276, the minimum temperature for most coatings is 40°F (5°C), and as low as 0°F (-18°C) for cold-curing systems. The maximum surface temperature is usually 125°F (50°C). Surfaces that are too hot can cause the coating to dry too quickly, leading to poor results.
2. Relative Humidity (RH)
Relative humidity (the amount of moisture in the air) affects how quickly the coating dries. High humidity slows down the evaporation of solvents, which can delay curing. Generally, coatings should be applied when the RH is below 85%, but some coatings may need moisture to cure, so it's important to check the product’s instructions.
3. Dew Point Temperature
The dew point is the temperature at which moisture starts to form on a surface. It depends on air temperature and humidity. If the surface temperature is close to or lower than the dew point, moisture can form on the surface and interfere with the coating.
4. Ts-Td (Delta) Difference Between Dew Point and Surface Temperatures
For a successful coating, the surface temperature should be at least 5°F (3°C) higher than the dew point. This helps prevent moisture from forming on the surface. This gap is crucial during the preparation, application, and curing stages to ensure the coating is applied properly and dries as intended.
Instruments for Measuring Dew Point and Other Environmental Conditions
Environmental conditions during surface preparation and coating application are critical to the long-term performance of coatings on steel structures. Dew point meters are used to monitor these conditions before, during, and after painting, helping ensure proper coating application. These meters are essential for determining the likelihood of dew formation, as required by ISO 8502-4.
DeFelsko offers 3 types of instruments to measure environmental conditions:
- Dew point meters.
- Wind speed sensors.
- Infrared thermometers.
1. Digital Dew Point Meters – PosiTector DPM Dew Point Meter
The PosiTector DPM measures and records several environmental factors including: relative humidity, air temperature, surface temperature, dew point temperature, the difference between surface and dew point temperatures (Delta), wind speed, and wet bulb temperature.
Available options include:
- Integral Probe with Direct Contact Surface Temperature Sensor
- Integral Probe with Infrared Surface Temperature Sensor
- Integral Probe with K-type Cabled Surface Temperature Sensor
- Integral Probe with Wind Speed Sensor
- Cabled Probe with 1/2" NPT Threads (for compressed air)
2. Autonomous Dew Point Meter Loggers – PosiTector DPM L Dew Point Meter Logger Series
The PosiTector DPM L and DPM L+ are autonomous loggers that attach to steel structures to measure and record environmental conditions independently for up to 200 days (DPM L) or 600 days (DPM L+). Data can be downloaded using the PosiTector app (iOS/Android).
3. Non-contact Infrared Surface Temperature Thermometer – PosiTector IRT Infrared Thermometer
The PosiTector IRT measures surface temperatures in areas that are hard to reach, such as moving parts, freshly sprayed coatings, or areas too hot for direct contact sensors.
What environmental parameters are measured with dew point meters?
Depending on which PosiTector DPM model is selected, a dataset can consist of five or more of the environmental parameters below.
- Standard models feature storage of 2,500 datasets.
- Advanced models can store up to 250,000 datasets.
- The PosiTector DPM L can store up to 10,000 datasets.
| Dataset* | |
| Measures | Calculates |
| RH — Relative Humidity: the amount of moisture in the air expressed as a percent of overall volume | Td — Dew Point Temperature: the temperature at which moisture will begin to form on a surface |
| Ta — Air Temperature: the temperature of the air | Ts–Td —Delta: the difference between surface and dew point temperatures |
| Ts — Surface Temperature: the temperature of a surface | Tw — Wet Bulb Temperature |
| V — Wind Velocity: the speed at which the air is moving (PosiTector DPM A only) | |
*Dependent on model
Advantages of Digital Instruments
Digital instruments like the PosiTector DPM Dew Point Meter provide higher accuracy, greater ease of use, and faster response times compared to mechanical methods. They are especially convenient for one-handed operation, such as when working from a ladder or scaffold. The display provides fast, continuous readings, and many models have built-in alarms that alert users when the surface temperature is close to the dew point, signaling high moisture risk.
Some models, like the PosiTector DPM L Logger, feature sealed containers and can measure environmental conditions for up to 200 days without battery replacement. Additionally, these instruments take much of the guesswork out of measurements, with features like stabilizing surface temperature readings before displaying them. This is particularly useful for measuring hard-to-reach areas where the display is difficult to see.
DeFelsko Distributor in Singapore
LFC PTE LTD is the authorized distributor of DeFelsko products in Singapore. Our expert team can assist you in selecting the appropriate solutions for your needs.
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