The Basics of Infrared Thermometry

Oct 1, 2000 12:00 PM, By Kate McGuire

You can gather important temperature information quickly and safely using infrared thermometers. But, you should know some key principles first.

Although infrared technology is sophisticated, using a portable infrared (IR) thermometer is simple. To get a reading, you point the device at the target, pull the trigger, and read the LCD. But, how can you be sure that measurement is accurate or meaningful?

IR thermometer facts. First, IR thermometers measure only surface temperatures. They can't read through panels, covers, or glass. To measure shiny or polished metal surfaces accurately, you must cover a target spot with masking tape or black paint. Also, airborne particles (steam, dust, and smoke) and aerosols reduce accuracy.

Second, consider the ratio (D:S) of distance from the spot you are measuring to the size of that spot. What is the D:S ratio of an IR thermometer? A higher D:S means better resolution - the ability to measure smaller spots from greater distances.

Third, the range of temperatures you can measure with an IR thermometer depends on the model. As the Table shows, this varies among performance categories. So, it's important to know if your IR thermometer matches the application.

Basic techniques. There are three ways to take IR readings:

• Spot measuring. Determines the absolute surface temperature of an object.

• Temperature differentials. Compares two separate spot measurements either in different locations or at the same location over time.

• Scanning. Identifies hot spots when you hold the trigger while moving the unit along the target.

Spot measuring is useful for identifying bad connections or improper process temperatures. Temperature differentials allow you to identify voltage imbalances. They reveal problems with motors, transformers, and grounding systems. You can scan a distribution system or panel to find hot spots, the cause of a breaker trip, or harmonics problems. Scanning reveals clogged air filters, overheated motors, and bus in need of preventive maintenance.

Monitoring and trending equipment temperatures is critical to preventive maintenance efforts. Because of its ease of use, low cost, and short training curve, IR thermometry is a complement to IR thermography and other predictive maintenance practices - especially as new accessories and features (such as data logging) make IR thermometers even more versatile.