Introduction to Infrared Temperature Sensors

What are infrared sensors

At the core of every infrared sensor is a thermopile. A thermopile is multiple thermocouples connected in series that generate an electrical signal based upon a thermal gradient across them. They measure temperature by having infrared energy heat one side of the thermopile and the electrical signal generated by the difference in temperature between the hot side and the cold side directly corresponds to the hot side temperature.

Infrared sensors are used to make non-contact temperature measurements. They are available in many configurations to cover a broad range of applications. They are available for temperatures from below freezing up to the temperature range of heat treating ovens. They can be used for close or far distances, for small or large objects, some have very fast response times, and some can even see through certain windows.

Limiting Factors of Infrared Sensors

There are some limitations you need to be aware of when using an infrared sensor. Sensors tend to be application specific. You need to be aware of the emissivity setting, the temperature limitations of the infrared Spectral Range, and the field of view of the sensor.

Emissivity

Emissivity is a property of all matter. It is the amount of infrared energy the matter emits at a temperature. Think of it as the opposite of reflectivity. A black object reflects very little but has a very high emissivity. A polished piece of metal has high reflectivity but very low emissivity. To make an accurate infrared temperature measurement, you need to set the emissivity value of the sensor to match the emissivity of the object you are measuring. This is most often done by making a contact measurement (once) and then setting the emissivity so that the infrared sensor temperature matches the contact measurement.

Spectral Range

Most infrared sensors have an infrared spectral range on them that optimizes performance for certain temperature ranges and applications. An 8 to 14 micron range is the most common and used on most general purpose sensors. This range can see through water vapor in the atmosphere and is used for room temperature up to several hundred degrees. A 2 to 3 micron spectral range optimizes the sensor for high temperatures and can also see through a sapphire window in a high temperature oven. These sensors are used for measuring high temperature measurements such as objects in ovens and hot metals.

Field of View

Although most infrared sensors do not have lenses, they do have key internal geometries built into them that create a fixed field of view. The field of view is like a spreading flashlight beam. The area measured gets larger as the distance from the instrument increases. The target area that the instrument measures is a combination of the field of view (FOV) and the distance to the target. The field of view is stated as a ratio i.e. 10:1, 24:1, 50:1 etc. The size of the target area is a circle and the ratio define the size of the target circle at a distance. An infrared sensor with a 10:1 FOV will have a 1 foot diameter target area at a distance from the target of 10 feet. A 24:1 FOV will have a target area of 1 foot at 24 feet, or to compare the two examples, a 0.42 foot (5 inch) diameter target circle 10 feet from the target. The 24:1 FOV as a smaller target circle at the same distance than a 10:1 FOV. A 50:1 FOV is referred to as a Narrow FOV instrument and has a 1 ft diameter target area at a distance from the target of 50 ft, or 2.9 inches at 10 feet. The infrared instrument will average all the infrared energy in the target area so it should be sized to match your application. The target you are measuring should completely fill the field of view circle otherwise errors may occur.

Infrared temperature measuring devices make non-contact measurements and they are well suited for variety of applications such as:

Measuring the temperature of moving surfaces, i.e. heated rollers or objects moving on a conveyor belt

Objects that are out of reach such as a ceiling light fixture,

Electrically “live” items such as a bus bar, electrical panel, or a transformer on a utility pole.

Infrared is also used to measure the temperature of food samples that cannot be touched due to the possibility of contamination.

Some models have close focus optics that also make them good choices for measuring very small targets that are close in proximity, such as components on a PC board.

Infrared measurements are also immune to most electrical noise and can be used around engines to measure various components.

Pitfalls of using an infrared sensor

Infrared measurements are limited to measuring the temperature of the surface of an object. Even when measuring a liquid, they will only measure the surface temperature of the liquid, not the interior temperature. Thick vapors above the liquid may also affect the measurement.

The emissivity must be set properly to match the material of the object being measured. Be aware that the emissivity of hot metal (especially iron) will change with temperature so the emissivity must be set at the measurement temperature, not room temperature for metals.

The sensor “averages” the temperature of everything inside its field of view so be sure to match the field of view to the size of your target and the distance the measurement will be taken from.

Surface coating such as oil on a skillet or a wet surface can also affect emissivity. To avoid errors, you should set the emissivity with a contact measurement under the conditions of use.

Measurement of polished metal that is reflective or other reflective surfaces can be made using infrared, but it requires specific sensors designed for that application.

A general purpose infrared sensor with an 8 to 14 micron spectral range cannot “see” through a window, it will, in fact, measure the surface temperature of the window. To see through a window a sensor with a 2-3 micron spectral range is typically needed.

Types of Infrared Temperature Measurement Devices

Infrared sensors are available in many forms to meet measurement applications

Infrared Thermocouples

An “Infrared thermocouple” is a small sensor designed to replace a thermocouple in an application where a non-contact measurement is needed. (Give link to our models) They are generally self-powered, can be very economical but tend to require reading instruments with a very high input impedance and low leakage current. Their output simulates the output voltage of a thermocouple.

IRT/C Infrared Thermocouple

Fixed Mount Infrared Temperature Sensors

“Fixed Mount” Infrared sensors are designed to be permanently installed into an application. They are available with different FOV’s and outputs such as 0-5 Vdc or 4-20 mA. Some also have digital outputs like USB or RS485 to match the applications needs. Water cooled jackets are also available for high temperature applications. These are the sensor commonly used in automated machines in industrial applications.

PyroNFC Compact Infrared Temperature Sensor



Handheld Infrared Measurement Instruments(Guns)

“Handheld” or “Infrared Guns” cover a broad range of applications from general purpose, high temperatures or close focus situations. These are often used to replace a dial thermometer or as portable thermometers. They are available with various FOV and spectral ranges.

Infrared Cameras

Infrared Cameras or thermal imagers have fixed optics to form an image and are used to quickly scan an area to see hot (or cold) spots in large area applications such as building insulation or a large process application.

Overall, once you understand their usage requirements such as setting the emissivity, infrared sensors are reliable and excellent choices for a wide range of industrial and process applications.

Feel free to contact IOThrifty should you require any assistance finding an infrared measurement device to work for your application.