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What’s The Point Of An Isotherm?

EP Editorial Staff | November 15, 2018

By James Seffrin, Infraspection Institute

Originally designed for monochrome imagers of the 1970s, an isotherm is a user-definable, high-contrast overlay generated by an imager’s onboard computer or within image-processing software. Prior to the advent of imagers with multi-color displays, the isotherm feature was a necessity for defining areas exhibiting similar temperatures. For other imagers, it was essential for measuring temperature.

With most modern thermal imagers, capable of providing multi-color imagery and direct temperature measurement, it would seem that the isotherm feature could be destined for extinction. There are, however, several instances where it may still be useful, including:

• defining areas operating within a defined temperature range

• highlighting color that defines hot/cold areas on monochrome images

• functioning as a preset alarm that automatically appears when an object exceeds user-defined temperature limits.

For example, to detect temperatures above a specified alarm value, select the isotherm palette and set your lowest value to your alarm temperature and the highest value to the top of the temperature range displayed on your imager. While red is good color choice, any high-contrast overlay will work equally well.

With your imager’s emittance value set to that of the items you will be inspecting, pan the device across those items. Objects operating at temperatures above the alarm threshold will now be highlighted on the screen. This technique allows rapid inspection of a wide range of mechanical components, including motors, bearings, gearboxes, and process ovens. It’s particularly useful for inspecting photovoltaic panels (see photo). As always, when using an isotherm, be sure to practice proper measurement techniques, giving particular consideration to viewing angle, spot-measurement size, and emissivity settings. EP

Also Use Isotherms To Detect Condensation

Your thermal imager’s isotherm feature can also help detect potential condensation sites within buildings. For building envelopes, chronic condensation on interior drywall surfaces can cause unsightly staining by trapping dust or smoke particulates in these areas. Chronic condensation on organic building components is conducive to mold growth. Thermal imaging is capable of detecting such problems before they become serious.

The first step in the procedure is to identify the dew-point temperature for the room or areas to be inspected. Next, set your imager’s isotherm function to appear at, and for several degrees below, the dew-point temperature. As you inspect high-emittance building surfaces from the interior of the structure, note any components that cause the isotherm to appear. These areas should then be further investigated for cause and appropriate action taken.

A frequent contributor to Efficient Plant, Jim Seffrin, is the director of Infraspection Institute and a practicing thermographer with more than 30 years of experience in the field. This article is based on two of his “Tip of the Week” posts for For information on a wide range of workplace topics and infrared-related issues, as well as upcoming training and certification opportunities, email or visit


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