Infrared thermometers (also called radiation thermometers) measure temperature by reading the infrared heat rays given off by objects. This allows them to work at a distance, without needing to touch the object. They can be used for:
- Cooking. Checking the temperature of soup, candy, ovens, grills, bbqs, frying pans.
- Thermal leak detection. Checking wall insulation and air conditioning; detecting air leaks in houses, refrigerators and freezers. No need to spend thousands of dollars on a thermal imaging camera.
- Troubleshooting engines, motors, high voltage systems and electronics.
They cannot be used to measure ambient air temperature and can have trouble with shiny objects (unless the thermometer is properly calibrated for the object). While capable of being used as a forehead thermometer, general purpose consumer infrared thermometers aren't accurate enough for medical use. Dedicated clinical infrared thermometers (example: Thermofocus, Extech) are needed.
Most, but not all, have a built-in laser pointer to help aim the thermometer (causing them to be called laser thermometers). Many look like guns, complete with a trigger to activate the sensor, and are small enough to fit into a large pocket. All infrared thermometers are electronic and digital. Prices start at $40 for consumer models and can reach a few hundred dollars for industrial or professional models. Popular brands include Raytek (MiniTemp), Fluke, Actron (PocketTherm), ThermoTech, Mastercool, Extech, Taylor, CDN and Cen-Tech.
Infrared Thermometer Accuracy and Measurement Range
Aside from their no-contact convenience and safety, infrared thermometers are popular because of their wide measurement range. Some examples from actual models:
- -4 to 500 degrees Fahrenheit (-20 to 260 degrees Celcius)
- -20 to 932 degrees Fahrenheit (-30 to 500 degrees Celcius)
- -58 to 1832 degrees Fahrenheit (-50 to 1000 degrees Celcius)
The thermometer itself needs to be kept reasonably warm in order to work properly, usually between 30 and 120 degrees Fahrenheit. Resolution can range from 0.1 to 1 degree Fahrenheit. However accuracy is typically plus or minus a few degrees.
For accurate readings, the size of the measuring spot needs to be known. This changes with distance. More advanced models shine a cone of light to indicate the size of the spot. The distance-to-spot ratio determines the width of the spot at different distances. At 12 feet (distance between the thermometer and the surface being measured), a 12:1 thermometer will measure the temperature of a spot one foot wide.
Distance-to-spot ratios range from 1:1 to 50:1, with 6:1 to 12:1 being average for consumer models. A larger ratio is generally better, allowing measurement of small items (pipes, wires, electronic components) without including objects in the background.
Infrared Thermometer Features and Usage
Most thermometers are switched on with a pull of the trigger:
- Some will then take the reading and freeze the display for a few seconds to allow the temperature to be easily read.
- Others will continuously take readings, automatically updating the display as the laser dot is moved across the surface being measured. Releasing the trigger freezes the display.
Useful features include:
- Switchable Celcius and Fahrenheit display.
- Separate display of maximum temperature measured.
- Backlit display.
- Wrist strap.
- AA or AAA batteries. Button cell and 9 volt batteries are less convenient.
- Automatic power off to conserve the batteries.
For testing HVAC systems and house insulation, special "thermal leak detector" models are available:
- The temperature of a reference point is measured.
- The thermometer is swept over the surface. On areas hotter than the reference point, the color of the beam of light changes from green to red. On colder areas the light changes to blue.
- The color change sensitivity can be set (1 to 10 degrees Fahrenheit).
The maximum temperature might be lower compared to other models, perhaps 300 degrees Fahrenheit. The thermometer also functions as a standard infrared thermometer, giving a digital readout of the temperature.
Infrared Thermometer Calibration and Emissitivity Adjustment
Industrial thermometers can be calibrated for higher accuracy. Calibration is done by adjusting for the emissitivity (related to reflectivity) of the object being measured. This is why uncalibrated units don't measure the temperature of shiny objects accurately.
For normal home use, calibration isn't necessary. Consumer models assume a fixed 0.95 emmisivity.
The Best Infrared Thermometer
While not necessarily the most accurate thermometer, the no-contact and wide temperature versatility of a handheld infrared thermometer is very appealing.
Cooks, home owners and DIY mechanics mainly need to choose the model that fits their budget, and range and accuracy requirements. For most home use, a wide measuring range is more important than accuracy.
The Omega Engineering and All QA Products websites have more information on infrared thermometers, calibration and emissitivity. Parts of this article are based on information from these two sources.
Yuen Kit Mun - Kit Mun is a self-confessed information junkie, reading an average of a book a week over the past two decades. His growing Internet ...
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