Like any other measuring instrument, infrared imagers require periodic calibration. One of the most often asked questions about imager calibration is how often to have it performed. The answer you get will largely depend on who you ask.
Ask the representative who sold you the camera and he will advise you to have it done annually without fail. Is there benefit to this practice? Well sure, the greatest of which is peace of mind. The accuracy and reliability of your measurements (assuming high emissivity and all of those factors) should rarely come in to question if your instrument is calibrated on an annual basis. Is this practice necessary? Again, that answer depends on who you ask.
Under perfect conditions, the most accurate any handheld thermal imager is going to be is +/- 2% or 2°C, whichever is greater. If you’re inspecting electrical apparatus, for example, and you detect an anomaly in the 250°F neighborhood, that’s a +/- 5°F swing, if you’re viewing a high emissivity surface. Under ideal conditions, with a perfectly calibrated imager, the measurements you attempt aren't exactly precise. That’s one of the limitations to radiometric temperature measurement. If your calibration slips slightly, what’s the impact on the quality of the inspection you perform? In many applications, qualitative analysis is the goal, and that can be accomplished without the benefit of accurate temperature measurements.
Still, calibration remains important to most thermographers. Aside from sending your imager back to the manufacturer, there are other options for calibration. In Level I – Thermographic Applications, we cover the Camera Functionality Test that utilizes the tear duct of an individual to give the thermographer an idea of how reliable measurements around 100°F are. For lower ranges, you can mix ice and water in a container, stir it for a few moments and then measure the surface of the water. You should get a measurement approximating 32°F. If you have a thermocouple and a coffee maker, you can turn on the coffee maker, allow it to stabilize thermally, measure the warmer surface with the thermocouple, and then measure it with your camera. The coffee maker warmer should be fairly high emissivity (they’re usually coated with a non-stick material or painted) so if you measure within 10°F or so of what the thermocouple reads, you’re in the ball park there too. That gives you three measurement points across a spectrum close to what most of us encounter in the field.
At the end of the day, your calibration interval should be something that is addressed in the written policy of your inspection program. If the cost associated with calibration is an issue, Thinking Thermally can help you find work arounds. Also some exciting news for those who need to have their imager calibrated, keep an eye out in the next coming months as The Snell Group will officially be offering calibration services to the public. For more information you can always contact or call them: 1.802.479.7100 or 1.800.636.9820.