AND DEW POINT

# GENERAL

This method describes equipment and methods used to determine the wind speed, relative humidity, and ambient temperature used to determine the theoretical evaporation rate when placing concrete bridge decks or bridge deck overlays.

The same equipment may be used to determine the dew point. When painting structural steel, specifications may require the dew point to be measured, to ensure that no moisture is condensed on the surface to be painted.

# APPARATUS

·         Kestrel 3000 Weather Meter (Figure 1.)

·         ExTech 45158 Mini Hygro Thermo-Anemometer (Figure 2.)

·         Equivalent meter meeting the following:

 Measurement Range Resolution Accuracy (±) Wind Speed (mph) 1.5 to 60 mph 0.2 mph 3% Relative Humidity, (%) 10 to 95% 1% 5% Temperature, (°F) 0 to 120 °F 0.1 °F 1.8 °F Dew Point, ((°F) 0 to 120 °F 0.1 °F 3.6 °F

PROCEDURE – Theoretical Evaporation Rate

1.    Determine the wind speed, relative humidity, and ambient temperature at a location within 10 feet of the deck and at approximately shoulder height above the deck.

2.    Determine the concrete temperature in accordance with IM 385.

3.    Determine the theoretical evaporation rate, using the Theoretical Rate of Evaporation Chart in Article 2412 or Iowa DOT Form E122.

PROCEDURE – Dew Point

Determine the dew point in accordance with the meter instructions. The dew point should be at least 5°F less than the surface temperature of the steel being painted.

1.   The surface and air temperature will vary depending on the location of the temperature measurements. For instance, a beam exposed to direct sunlight will be much warmer than a beam in the shade. Therefore, if painting work is done in the shade, temperature measurements should also be done in the shade.

2.   Moisture problems are most likely to be encountered in the morning, so temperature measurements should be made in the morning before painting begins.

3.   Report the dew point and surface temperature.

 Figure 1. Kestrel 3000 Figure 2. ExTech 45158