Installing and assuring that an infrared thermometer achieves accurate temperature results involves a number factors, including Aim, Focus & Lens Selection, Overcoming Obstructions, and Assuring Ideal Sensor Operating Conditions.

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Aim

One of the first questions IRCON product technicians typically get asked is: "Does the thermometer have to be aimed perpendicular to the surface?" The answer is NO.

For measurements of smooth surfaces such as plastic, glass and paper, the instrument can be aimed from a 45 to 90^o angle. If you exceed the 45^o angle, the surface becomes reflective, which in turn lowers the emissivity and gives the appearance of a lower temperature than it really is.

For rough surfaces like steel, textiles and food, the instrument can be aimed as low as 15^o from the horizontal and the temperature indication will be accurate. It often is advisable in a steel mill or paper mill not to look straight up or down at the target because of the debris that can fall into the lens, and heat and steam that can overheat the sensor. By placing the sensor off to the side and looking at an angle, the sensor will survive the environment more easily.



When measuring the temperature of objects with rough surfaces (as shown), the sensor can be aimed as low as 15^o from horizontal. Avoid placing it directly above or below processes with falling debris or obstructions such as smoke or steam.

When measuring smooth surfaces, the angle should be 90^o (perpendicular to surface) or angled not greater than 45^o from horizontal because reflective properties will distort the temperature reading.




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Focus and Lens Selection

All infrared thermometers measure temperature within a certain area. On infrared thermometers with a focusable lens, the sensor-to-target distance and the focal factor of the instrument determine the sensor's spot size. To define the target size requires a simple calculation -- d = D/F where:


d = the the diameter of cone (spot size) at the focal point

D = the distance from the sensor lens to the focal point

F = the resolution focal factor of the sensor lens.

The focal factor for any instrument is typically included in the product manual and usually varies from a low number of 20 for low temperatures such as 0 to 500^oF (-18 to 260^oC) to a high of 300 for high temperatures such as 1,500^oF (816^oC) and higher.

As an example, if the focal factor is 50 and the instrument is 100" away, then

           d = 100/50 = 2.0"

If the target is smaller than 2.0", then the sensor must be placed closer or work with an instrument that has a higher resolution factor. The focal factor (F) has no dimension so the formula will work in inches, feet, millimeters or centimeters, and the answer will be in the same dimension.

For instruments with a fixed focus, charts in the sensor's user manual typically show the spot size vs. the distance. The chart below shows a typical chart, and in this example, it shows that at 50" the spot is 5.7". This means that in order to measure the correct temperature, the target has to be larger than 5.7". Failure to fill the spot size will allow the instrument to measure anything else that is in the spot, and usually this will lead to an incorrect temperature.




Do you have to always be in focus? Not necessarily. Temperature indication is not typically affected in applications such as a paper or textile web where the target may continually move up and down.

As shown in this illustration, the No. 2 location is the ideal instrument focus and "spot" measurement area.

At locations 1 and 3, the instrument is not in focus, and the spot measurement area is wider, though acceptable. In these instances, the infrared thermometer simply averages and outputs the temperature of the larger area.

At location 4, the spot measurement area is unacceptable because areas outside of the target are being measured and incorporated as part of the averaged temperature output. In other words, the measurement area is contaminated, all but assuring that the temperature displayed will be an inaccurate representation of the target area. 

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Overcoming Obstructions

Ideally, the line of sight -- or "cone of vision" -- between the sensor and hot target should be a clear and direct. More often than not though, obstructions such as flames, dust, steam, smoke, and glass (windows) and metal barriers may cause problems.

Solid Obstructions.
Solid obstructions such as pipes and steel structure can be in the cone of vision. The ideal solution is to remove the obstruction, but often this is not a choice. The solution could be to look at the target at an angle or use an instrument that has a fiber optic cable and lens that goes around the obstruction.

Windows.
Some applications require windows to maintain a vacuum in a chamber or pressure in an oven. If a window is necessary, be sure that it is transparent for the wavelength of the instrument that is being used. The window must be kept clean. If it gets dirty, the instrument will measure the temperature of the dirt on the window. In addition, the window must be large enough so the cone of vision is not obstructed by too small of a window opening.

Intermittent Obstructions.
Intermittent targets and obstructions such as smoke, steam and dust can cause the instrument to provide erratic temperature indications. An electronic feature called a peak picker solves this problem. The electrical circuit allows the indication to rise as fast as the response time, but a delayed decay rate does not allow the temperature to go down when the interference in the line of sight.

The image at right shows an application with bottles at different temperatures. Without the peak picker, the instrument would indicate room temperature when there is no bottle present. With the peak picker, the spaces are ignored and only the product temperature is indicated. Now the user has to decide which decay rate he wants to use to provide the output for controlling or indication. Processes with intermittent targets can be measured with an infrared sensor outfitted with peak picker capabilities.

Flames.
Clean gas flames are transparent to an infrared thermometer; the infrared thermometer will see right through them and not measure the flame temperature. The same is true of inert gases such as argon, nitrogen or hydrogen -- infrared thermometers will not see the gases but instead measure the temperature of the target immersed in these gases. Dirty flames such as coal, oil or garbage flames are opaque, and the infrared thermometer will measure the actual flame temperature rather than see through it.

Maintain Proper Sensor Operating Temperature.
Most sensors can operate in ambient temperatures of 0 to 145^oF (-18 to 63^oC). If the ambient air surrounding the sensor is hotter or cooler than these temperatures, the sensor accuracy will drift, and/or the sensor may even fail entirely due to overheating. To keep the sensor cool, a water-cooling jacket accessory should be used to enclose the sensor. Water-cooling jackets provide insulation between the external heat and the sensor, circulating water through tubes enclosed within the jacket that surrounding the sensor. Concurrently, be sure not to overcool the sensor. Typically, it is best to operate the sensor within the cooling jacket at about 100 to 110^o, which will be above dewpoint temperature, thereby reducing the possibility of  condensation building up inside of the sensor enclosure.

Keep the Lens Clean.
If an infrared sensor's lens is dirty, it will indicate a temperature lower than actual. An air purge accessory should be used in smoke, steam or dust-filled environments to blow a constant stream of air over the sensor lens, thereby lowering the possibility of material collecting on the lens. The key is to use an air purge that will maintain enough constant air pressure to keep build-up from collecting. To clean a dirty lens, use isopropyl alcohol and dry with a soft cloth. On the instruments with focusable lens, be sure to clean both sides of the lens as well as the window behind the lens.

Calibrate the Device.
It is common practice to calibrate infrared thermometers once a year. Most sensors are rugged enough that they can operate several years without requiring calibration, but ISO 9000 procedures followed by many companies require annual calibration. To calibrate an infrared thermometer requires a blackbody source. This is a special oven that is a calibrated temperature source with a specific cavity design. But, before using a blackbody, this source itself should be calibrated annually by an authorized laboratory.

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Infrared thermometers can provide a valuable and accurate means for monitoring temperatures or critical processes -- either for simple indication or complex closed-loop control. When installing these instruments, all of the factors discussed in this article must be considered. If the instrument appears to indicate the incorrect temperature, take the time to review these factors and correct any errors. For detailed technical assistance contact the sensor manufacturer.