General Information About Infrared Thermometers
When to Use an Infrared Thermometer
Infrared thermometers allow users to measure temperature in applications where conventional sensors cannot be employed. Specifically, in cases dealing with moving objects ( i.e., rollers, moving machinery, or a conveyor belt), or where non-contact measurements are required because of contamination or hazardous reasons (such as high voltage), where distances are too great, or where the temperatures to be measured are too high for thermocouples or other contact sensors.
What should I consider about my application when selecting an infrared thermometer?
The critical considerations for any infrared thermometer include
- field of view (target size and distance),
- type of surface being measured (emissivity considerations),
- spectral response (for atmospheric effects or transmission through surfaces),
- temperature range and mounting (handheld portable or fixed mount).
Other considerations include
- response time,
- mounting limitations,
- viewing port or window applications, and desired signal processing.
What is meant by Field of View, and why is it important?
The field of view is the angle of vision at which the instrument operates, and is determined by the optics of the unit. To obtain an accurate temperature reading, the target being measured should completely fill the field of view of the instrument. Since the infrared device determines the average temperature of all surfaces within the field of view, if the background temperature is different from the object temperature, a measurement error can occur. Many infrared thermometers feature laser circle or dot. A built-in laser sighting clearly indicates the target area being measured.
How To Choose an Infrared Thermometer?
1. Determine the field of view (target size and distance)
2. Consider the type of surface being measured and its emissivity
3. Analyze the spectral response for atmospheric effects or transmission through surfaces
4. Specify the temperature range and the mounting needs
5. Don’t forget: response time, environment, mounting limitations, viewing port or window applications, and desired signal processing.
What is emissivity, and how is it related to infrared temperature measurements?
Emissivity is defined as the ratio of the energy radiated by an object at a given temperature to the energy emitted by a perfect radiator, or blackbody, at the same temperature. The emissivity of a blackbody is 1.0. All values of emissivity fall between 0.0 and 1.0. Most infrared thermometers have the ability to compensate for different emissivity values, for different materials. In general, the higher the emissivity of an object, the easier it is to obtain an accurate temperature measurement using infrared. Objects with very low emissivities (below 0.2) can be difficult applications. Some polished, shiny metallic surfaces, such as aluminum, are so reflective in the infrared that accurate temperature measurements are not always possible.
Five Ways to Determine Emissivity
There are five ways to determine the emissivity of the material, to ensure accurate temperature measurements:
- Heat a sample of the material to a known temperature, using a precise sensor, and measure the temperature using the IR instrument. Then adjust the emissivity value to force the indicator to display the correct temperature.
- For relatively low temperatures (up to 500°F), a piece of masking tape, with an emissivity of 0.95, can be measured. Then adjust the emissivity value to force the indicator to display the correct temperature of the material.For high temperature measurements, a hole (depth of which is at least 6 times the diameter) can be drilled into the object.
- This hole acts as a blackbody with emissivity of 1.0. Measure the temperature in the hole, then adjust the emissivity to force the indicator to display the correct temperature of the material
- If the material, or a portion of it, can be coated, a dull black paint will have an emissivity of approx. 1.0. Measure the temperature of the paint, then adjust the emissivity to force the indicator to display the correct temperature.
- Standardized emissivity values for most materials are available. These can be entered into the instrument to estimate the material’s emissivity value.
What is fixed emissivity?
Fixed emissivity is a setting in some infrared thermometers (usually of 0.95 or 0.97) that attempts to simplify their operation while leaving them suitable for most material surfaces, including almost all foods. Other infrared thermometers come with adjustable emissivity settings, so you can more accurately prepare your thermometer for the type of surface being measured, particularly when measuring non-organic surfaces.
Common misconceptions about infrared thermometers
1 – An infrared thermometer will tell you the internal temperature
An infrared thermometer is a surface temperature tool – period. If you’re grilling, baking, smoking, or roasting you’re going to need a penetration probe to tell you the internal temperature of the food you’re cooking. An infrared will only give you the surface temperature of the food, and depending on your optical range, the temp of the surrounding grill, skillet, oven, etc.
Ideally you would use infrared thermometers to temp the surface of hot oil, a cast iron skillet, a saute pan, even chocolate and soup. However, whipping out your infrared “laser gun” to temp burgers on the grill may have you explaining to your guests why they’re undercooked.
2- The laser reads the temperature
This is just plain wrong. The laser pointer in an infrared thermometer is a guide that indicates where you’re pointing the instrument. When measuring the heat coming from an A/C duct (for example), the laser helps to steady your aim and ensure that you’re close to the area you’re trying to temp.
Depending on the make and model, an infrared thermometer is actually reading the temperature above, below or around where you see the laser. Some infrared thermometers are equipped with two lasers. They provide an indication of the infrared radiation being measured between the laser points. And depending on your infrared’s optics, the diameter of the area being measured will change as you get farther away from your target. This is called the optical range.
3 – All surfaces are created equal
As a matter of fact, just the opposite is true. Not all surfaces are created equal. Depending on what you’re pointing your infrared gun at you’re likely to get variations in emitted infrared energy. This variation is called emissivity. Emissivity is a measure of a materials ability to emit infrared energy. It is measured on a scale from just about 0.00 to just below 1.00.
Generally, the closer a material’s emissivity rating is to 1.00, the more that material tends to absorb reflected or ambient infrared energy and emit only its own infrared radiation. Most organic materials, including the byproducts of plants and animals, have an emissivity rating of 0.95. These are ideal surfaces for accurate temperature readings.
Substances with very low emissivity ratings, like highly-polished metals, tend to be very reflective of ambient infrared energy and less effective at emitting their own electromagnetic waves. If you were to point an infrared thermometer with fixed emissivity at the side of a stainless steel pot filled with boiling water, for example, you might get a reading closer to 100°F (38°C) than 212°F (100°C). That’s because the shiny metal is better at reflecting the ambient radiation of the room than it is at emitting its own infrared radiation.
source: www.omega.com, www.thermoworks.com
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