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​To Sling or not to Sling - That is the Question

Apr 5th 2020


Choose the right meters when testing for humidity.


There are a wide range of instruments used to measure Relative Humidity and Dewpoint. Old school uses Sling Psychrometers to determine these measurements while new school uses electronic meters. There are advantages and disadvantages to both. Everything being equal, to skip the conclusion of this blog, I recommend going electronic. Now, lets explore why.

Sling psychrometers vs. electronic meters

While both can be valid tools for humidity checks, it's important to understand some basic facts about the psychrometer. First of all, it's used to measure two different types of temperature: the ambient temperature and the temperature from evaporation. These separate readings are known as dry bulb and wet bulb, respectively. Both are necessary to calculate relative humidity and Dew point. Mercury sling psychrometers used by Meteorologists that conform to ASTM E337 are generally accurate. Members of the original standards committee estimated that errors in measurement using these slings is in the 5% to 7% range. The Sling Psychrometers used in our industry do not meet these standards and the people using these are not usually properly trained. When used properly, when the temperature is less than 85oF,reasonalbe accuracy can be achieved. 

Pros of sling psychrometers: Provide wet and dry bulb measurements, are considered standard tools, can supplement other instruments, do not require batteries can be cheaper than electronic meters.

Cons of sling psychrometers: Thermometers can be hard to read, red column can separate, needs to be stored in correct temperature. At high temperatures (above 85oF) they can be highly inaccurate. (SEE: ACCURACY AND LINEARITY OF SLING AND DIGITAL PSYCHROMETERS in this Blog)

Electronic meters range from $30.00 to over $1000.00. If you buy a midrange psychrometer, while it is more expensive, it Is less prone to operator error when used properly. Some psychrometers offer a calibration kit to verify accuracy. The calibration/verification test generally takes 60 to 90 minutes. Many electronic meters have the added capability of being able to measure surface temperature via a built in thermocouple or a built in IR gun. Since ambient conditions can vary in a few feet, electronic meters make it easier to test where you are doing the work. Conditions may be good to paint on one side of a tank but not on the other side.. it's necessary to gather as much specific data as possible. Finally, the data from these sources can be ported electronically for storage and analysis. The main detriment comes when users don't maintain the tools successfully, as older models may be more prone to error.

Pros of electronic meters: Better accuracy than a sling, electronic data, time stamping for better details.

Cons of electronic meters: May cost more for better units, batteries can go dead. Lower cost unit sensors may not hold their accuracy.

Whether using a sling psychrometer or an electronic meter, it is important to let the temperature of the gauge stabilize prior to taking measurements. This can be 5 to 20 minutes for a sling or 15 to 30 minutes for an electronic meter

Choosing and preparing the right methods
The ASTM International Active Standard ASTM E337 explains the proper use of a Sling Psychrometer. There is more than one way to use a Sling. For example, inspectors may decide to ventilate their psychrometers in a different way depending on the environment. Ventilated psychrometers that use a fan will give consistently more accurate readings than using a sling psychrometer especially in hotter weather.

Before you use an electronic meter, it's wise to check the battery. I have seen low batteries give incorrect readings on electronic meters. With sling psychrometers, users should examine the wick to make sure it's clean and white, as well as soaked with distilled or demineralized water. The thermometers should agree with each other prior to doing the sling,

It is generally best to make sure everyone on the project is on the same page when it comes to Relative / Dewpoint measurements. Slings will not always agree with electronic meters, especially in hot weather (Above 85oF). If using a sling in hot weather – make sure you use the ventilation method and not by using the sling method.

Automatic and manual calculations
If using a manual psychrometer, many of the psychrometers have a slide rule calculator built into them. You can buy a set of psychrometic tables or you can look up the tables up on line. Our handbook has the formulas to create your own spreadsheet to calculate RH and Dew Point and if you co under technical information on the web site there is a Java Script calculator that will do the math for you.

You can learn more about digital and sling psychrometers in this paper from MTestco. We offer the paint testing equipment needed to perform a safe and thorough inspection, as well as important guidance documents for out in the field. Contact us to learn more about our product lines.


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