Most shops have more than one tool to read temperatures. Typically, a popular one is the infrared thermometer, which everyone likes because it permits aiming at a target and taking the temperature, even if the target is physically difficult to reach (or too hot to chance touching). However, the infrared type doesn’t read air temperature, and its greatest accuracy is based on aiming at a minimally-reflective surface, perhaps by wrapping a shiny refrigerant line with a strip of flat black or magenta-colored (deep red) tape, even plain masking tape. However, you have to apply the tape so it adheres uniformly. That way it comes up very close to the surface temperature of the metal surface. If a metal surface is painted black or even a very dull gray, you can take a direct reading. How do we know the readings were a bit more accurate with use of tape? We compared coolant temperature readings at the coolant temperature sensor with those from a scan tool.
Keep in mind that infrared thermometers cannot be absolutely accurate when you try to do something like measure fluid temperature in a line, because of the heat exchange from fluid through the line to the line’s surface. A drop of 5-10 degrees F. through a line or hose is to be expected.
The important thing is to get the IR thermometer as close as possible to the surface whose temperature you want to measure. The greater the distance, the more likely the thermometer also will take readings from adjacent parts, which may be hotter or cooler. A laser-sighting feature on an IR thermometer can help you get more accurate readings.
Because an infrared doesn’t read air temperature, measuring air from the A/C or floor heat registers is better done with a conventional thermometer. Certainly, don’t expect the plastic of the A/C outlet grill to get to the same temperature of the air that’s passing through it. Of course, even a conventional thermometer may take a couple of minutes to reach a stable temperature reading, so don’t just insert, read and remove, or you’ll get a reading that could be way off. Some infrareds come with an ambient temperature sensor – a part that is designed to come up to ambient temperature quickly and display the reading on the infrared.
There are other limits to infrared. One obvious test for accuracy – aiming at a pot of boiling water— should convince you of possible shortcomings of infrared thermometers.
Using the two different brands of IR thermometers, tests produced readings that were just barely at 200 degrees F. while aiming at a dark pan of boiling water, but were in the 208-214 degrees F. range aiming at a white pan. Although there was a six degree F. difference between the two with boiling water, the thermometers were a lot closer (typically within two degrees F. of each other) with temperatures in the 150-190 degrees F. range, taken as the water heated up.
The same two IR thermometers were also aimed at a white wall vs. a dark brown panel. Both produced the same 70-71 degrees F. readings. Readings from these two thermometers were also compared with the reading from a thermocouple attached to a digital multimeter (infrareds aimed directly at the thermocouple), and also with a probe thermometer. All produced virtually the same readings, with one infrared slowly switching to one degree lower and then back again (meaningless), and the probe type a degree higher.
So when you’re comparing pressure gauge readings vs. ambient temperature around the engine compartment, you want to be able to measure temperatures and have a confidence level within a few degrees F., and pressure gauge readings within 2-3 psi as a result. It’s certainly a reasonable expectation if you have the right testers. (By the way, there are no in-shop calibration adjustments possible with an infrared thermometer. If its readings are off, send it back to its manufacturer for service, or replace it.)
When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
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The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.