Electronic Ambient Meters or Sling Psychrometers, what should I use?

It is strongly suggested that electronic meters are used instead of sling psychrometers. This ensures the highest accuracy. Nevertheless, sling psychrometer are still widely used in the field. We will guide you to properly use the sling Psychrometers and electronic meters. It is highly recommended to follow this procedure to minimize any errors.


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

1) Inspect the thermometers

Today, most inspectors are using red spirit thermometers because they do
not contain mercury which is 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
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

In general, thermometers do not read exactly how they are marked.

  • Always check thermometers in the field. With the wick removed and, in the shade, both thermometers should read the same.
  • By definition, ice water (not ice) is always 32oF (0 oC). Fill glass 2/3 full of ice cubes. Allow time for the ice and water to reach an equilibrium. 15 minutes should be safe. Place the thermometers in the water and they should read 32oF (0 oC) . 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 33oF (1 oC) in the water it is reading 1oF high (1 oC) . This needs to be subtracted from each reading to get the correct value. If the temperature between thermometers is off by more than 2oF (2 oC) , you should 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

How to Check Readings?

Prior to starting record the Dry Bulb Temperature. 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 (about 1 and a half minutes). Read the wet bulb temperature first. Be
careful to keep your 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

Read the Dry Bulb Temperature before you start “Slinging” the psychrometer. Read the dry bulb at the
end of the procedure. They should be the same. If they are not, the accuracy of the readings should be
questions. 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.

Electronic Meters

Electronic meters come is several varieties from meters that just provide Wet Bulb and Dry Bulb Temperatures to meters such as the PosiTector DPM and TQC Dewcheck, that measure, Wet Bulb, Dry Bulb, Relative Humidity, Dew Point, Surface Temperature, Calculate the ∆T between the surface temperature and the dew point. You can download the Electronic time and date 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 does not 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 personnel use an electronic meter, they should all get close to the same readings. A 2% to 3% difference in RH (Relative Humidity) and a 2° to 3° difference in DP (Dew Point)should be expected between meters. Some meters have calibration kits you can use to verify accuracy. Check before buying that verification can be done in the field. In most cases, even the cheapest electronic meters will be more accurate than using a sling psychrometerin hot weather.

As with the rest of industry, keeping electronic records of projects will not only become the norm, but it will also 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.

To summarize

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 in case your batteries die in your meter or you break a thermometer and do not have a replacement.

For information on the accuracy and linearity of sling and digital psychometers, Tom Swan from MTest has written a detailed paper about the accurate use of meters. ACCURACY AND LINEARITY OF SLING AND DIGITAL PSYCHROMETERS

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