Even in applications where qualitative analysis is utilized, most reports include temperatures. For comparison purposes it’s advisable to measure a point of comparison, or reference, for the anomalous finding. In electrical applications, we generally advise comparing an adjacent phase of the same apparatus under similar load and environmental conditions. For mechanical applications it may be a nearby similar asset under similar load/cycling in a similar environment. Another commonly used point of comparison is ambient temperature. Sadly, many thermographers, both novice and experienced, are using ambient temperature in different ways.
Ambient temperature is defined by McGraw-Hill Science and Technology Dictionary as “… the temperature of the surrounding medium, such as gas or liquid, which comes into contact with the apparatus.” Depending upon the application, ambient temperature for the object being inspected can vary quite widely from what the thermographer may consider as ambient. Thermographers often cite different sources for ambient– anywhere from the air temperature where they’re standing during the inspection, the temperature of the nearest measuring device (such as a thermostat), a nearby component, etc. So, where should you measure it?
In a substation for example the air temperature at ground level could be much different than a component 30 feet in the air around the corner of the structure. In distribution equipment, the ambient temperature where the thermographer is standing might be different than the inside of an electrical enclosure. The interior of the enclosure might be warmer, due to inherent temperature from operating components, or it could be cooler if there is forced convention from cooling fans or cabinet air conditioners. If panel covers are taken off a long time before the inspection occurs, ambient temperature could have changed drastically. In mechanical applications there can be variances too. Rotating equipment might be at, or near, air temperature because of convection, but there may also be boundary layers on surfaces where the air movement is reduced due to the geometry of the surface.
Be cognizant of the true operating environment of the devices or systems you are inspecting. If you’re reporting an electrical anomaly, measure ambient on a high emissive surface inside of the enclosure or in close physical proximity to your object of interest. Note if the environmental conditions are substantially similar or different. Carrying a pocket weather station on inspection routes is a good idea, which in addition to air temperature can provide you with relative humidity and air velocity, if those are factors that need consideration in your application. It can also be beneficial to document the measurement, the method used to take the data and where it was taken. This information is especially valuable when obtaining a baseline or trending over time.
Ambient may seem like a simple figure, but in some cases it can be anything but simple.
Think Thermally, www.thesnellgroup.com The Snell Group, a Fluke Thermal Imaging Blog content partner