Measuring Relative Humidity and Dew Point in the Field

This was published in the CIP Inspect This News Letter in Summer 2005.

Many coating failures have been attributed to applying coatings when climatic conditions were not within specifications. When trying to determine Relative Humidity and Dew Point temperatures, an understanding of the wet bulb, dry bulb, relative humidity and dew point is useful in getting accurate values.

Typically, most project requirements are a Relative Humidity below 85% and 5^oF between the surface temperature and the dew point. When Relative Humidity is around 50% and the Dew Point spread is 10^oF to 15^oF, accuracy in the tests are not critical. However, when the Humidity is close to 85% (or whatever the requirement is) and the dew point/surface temperature spread is about 5^oF, it is important that readings be accurate.

There are two basic methods of measuring Relative Humidity and Dew Point Temperatures in the field. These are addressed in ASTM E 337, Standard Method for Measuring Humidity with a Psychrometer (The measurement of Wet and Dry Bulb Temperatures). One is with a sling psychrometer and the second is with the newer electronic meters. It is generally assumed that the most accurate method of determining Relative Humidity and Dew Point are the Sling Psychrometer. This assumption is only correct if it is being used properly. Sling psychrometers used by meteorologists are “laboratory grade” and have much greater accuracy than sling psychrometers typically used by inspectors and contractors. The sling psychrometer measures two parameters, Dry Bulb (ambient temperature) and Wet Bulb.

The dry bulb temperature (DBT) or ambient temperature is the temperature of the air. This is the temperature that you would get in the shade and not the temperature in direct sun.

The wet bulb temperature (WBT) measures the temperature that results from evaporation. It is directly related relative humidity. When moisture evaporates, it cools the environment, reducing the temperature slightly. The WBT will vary with Relative Humidity (RH). When the relative humidity is high, evaporation is low and there is less of a cooling effect. When relative humidity is low (air is dry) evaporation increases and the cooling effect is greater. The difference between the wet bulb and dry bulb temperature therefore gives a measure of atmospheric humidity.

Relative Humidity (RH) is the measure of how much moisture is in the air divided by the amount of moisture the air can hold times 100. The amount of moisture the air can hold is dependent on the atmospheric pressure. When the air is 100% saturated, evaporation will stop and the Dry Bulb Temperature will be equal to the Wet Bulb Temperature.

When DBT – WBT = 0 then RH = 100%

Now that we understand the terms, how do we improve the accuracy of the test methods.

SLING PSYCHROMETER.

- The first item is to make sure the thermometers are reading correctly.

1) Inspect the thermometers. Today, most inspectors are using the red spirit thermometers because they do not contain mercury which is considered to be a very toxic heavy metal. Make sure that the column is not separated. Often, especially when left in the heat or after shipping, the red column will separate. This will result in an inaccurate reading. Sometimes by putting the thermometer in ice water followed by hot (not boiling) water, the column can be fixed. If it cannot be fixed, replace it.

2) Calibrate the thermometers – sometimes, thermometers do not read exactly how they are marked.

a. Always field check thermometers. With the wick removed and in the shade, both thermometers should read the same.

b. By definition, ice water (not ice) is always 32^oF. Put ice cubes in a glass of water. Allow time for the ice and water to reach equilibrium. 30 minutes should be safe. Place thermometers in the water and they should read 32^oF. Since the scale is linear, if it is off you can add or subtract the difference to get the thermometer to read accurately. Example: If the thermometer reads 33^oF in the water it is reading 1^oF high. This needs to be subtracted from each reading to get the correct value. If the temperature between thermometers is off by more than 2 ^oF, replace the thermometer.

- Store the Thermometers properly between uses:

If the psychrometer is stored in the sun or heat, the plastic case will become heated. When you go to take your readings, the temperature being radiated by the sling psychrometer case will affect your reading introducing an error. Keep the sling stored in the shade near the temperature you will be testing in. NOTE: Thermometers stored in a trunk or car can get hot enough to pop the top off the thermometers.

- Check the Wick:

The wick should be white, not yellow, brown or black. The wet bulb thermometer measures the rate of evaporation of water between the surface of the wick and the bulb of the thermometer. The wick should be clean and flexible. Often, when onsite, you use whatever water is available. This water often has dissolved solids and impurities that get left behind as the water evaporates. Eventually the wick becomes non-porous and while the wick may feel wet, evaporation is effected giving erroneous readings for wet bulb temperature.

When possible use distilled water and when the wick becomes discolored or hard, cut it off and put a fresh part of the wick on the bulb. (Caution, if you continuously use hard water in the reservoir, the unused part of the wick can also become discolored and hard. It is always recommend to use distilled or demineralized water to maximize the life of the wick)

- Check the Water

When using a new wick, make sure it is soaked thoroughly. If you put a drop of water on the wick, it should not bead up. Let the wick sit for a few minutes to make sure it is saturated.

- How to Check Readings

It is important to take readings in the same area that is to be painted. Face the wind if there is any and rotate the sling psychrometer at about 2 revolutions per second for about 90 seconds. Read the wet bulb temperature first. Be careful to keep you fingers off both thermometers. Continue to rotate for 20 to 30 seconds and take a second reading. If the wet bulb has not changed you are finished. If it is decreasing, continue until you get two readings the same.

Accuracy of the method

The accuracy of the method depends on the accuracy of the thermometers as well as the operators’ procedures. Let’s assume your reading is off by one degree in opposite directions on the wet bulb and the dry bulb.

Example:

Measured WBT = 58^oF DBT = 72^oF therefore Wet Bulb Depression (WBD) = 14^oF

RH = 42% DB = 47^oF

Actual WBT = 57^oF DBT = 73^oF therefore Wet Bulb Depression (WBD) = 16^oF

RH = 35% DB = 44^oF

A 2^oF error makes a 16% difference in Relative Humidity and a 6% difference in Dew Point.

Electronic Meters:

Electronic meters come is several varieties from meters that just provide Wet Bulb and Dry Bulb Temperatures to meters such as the Elcometer 319 Dewmeter, that measure, Wet Bulb, Dry Bulb, Relative Humidity, Dew Point, Surface Temperature, Calculate the ∆T between the surface temperature and the dew point., electronic time and date stamp data and can download information to a computer.

As for all electronics, the quality of the sensors is key to how well the meter works. Some of the early meters as well as some still manufactured today, suffer from using low cost sensors that can give erroneous readings and have given electronic meters a bad reputation. Be careful that the great price you got on an electronic meter doesn’t affect the accuracy of the meter.

Electronic meters have some distinct advantages over sling psychrometers. Because there are no moving parts, you can take readings close to where you will be doing the work. Atmospheric conditions at or near the surface of steel can be considerably different inches or feet from the surfaces. Because a sling psychrometer requires room to “sling” it, you can never get readings near the surface.

Electronic meters also minimize operator error. When multiple inspectors or quality control personal use an electronic meter, they should all get the same readings.

Some meters have calibration kits you can use to verify accuracy. Check before purchasing that this can be done in the field.

As with the rest of industry, keeping electronic records of projects will not only become the norm, it will be required by many owners. Make sure the electronic meter has the capability of time and date stamping data as well as sending the data to a computer.

The typical costs for a sling psychrometer are $60.00 to $100.00 based on the model. Replacement thermometers are generally $25.00 to $35.00 and replacement wicks are about $1.00 to $2.00 each. While may sling psychrometers come with a slide rule calculator on their side, for better accuracy, it is generally recommended to use a psychometric table which you can get for under $10.00 or you can get the information for free at http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml.

The cost for electronic meters varies from $150.00 to $600.00. The higher priced models generally have higher quality sensors, measure surface temperature, record readings and will interface to a printer or computer.

Good quality electronic meters and sling psychrometers can both supply accurate information when used properly. It is important to make sure that your readings are accurate, especially if they are going to restrain the contractor from painting. It is probably a good idea to keep both on site incase your batteries die in your meter or you break a thermometer and don’t have a replacement.