We get a lot of questions here at Williamson about pyrometers reporting different temperature values than they did the day before. While a different temperature value could be the result of a number of different factors, there are 3 basic troubleshooting steps that apply to most all pyrometer applications. If you have seen a difference in temperature reading from your pyrometers, it is best to first check these 3 things to see if that resolves the issue. Better yet, if these three steps can be incorporated into maintenance routines, you can extend the performance life and accuracy of your pyrometer.
1) Check that Pyrometer is NOT Overheated
Like any electronic device, a pyrometer cannot operate properly over a certain temperature limit. For Williamson sensors we recommend operating under 140°F/60°C. Above this ambient temperature limit, the circuit boards inside the pyrometer start to act funny. Pyrometers exposed to ambient temperatures above this limit will start to drift out of calibration and can produce errors in temperature reading. Keeping pyrometers at a cool temperature can extend the life of a pyrometer and decrease the intervals between calibrations. Williamson pyrometers include a temperature strip that indicates if the pyrometer has been exposed to high ambient temperatures. We generally recommend that if the ambient temperature is above 120°F/50°C, then active cooling should be applied to a camera style pyrometer, or consider using a fiber optic pyrometer (400°F/200°C ambient limit)
2) Make Sure the Pyrometer Lens is Clean
Just like eyeglasses, a pyrometer works best when the lens is clean. Dirty lenses on your eyeglasses will leave you with blurry or unclear images. Similarly, a dirty pyrometer lens on a single-wavelength pyrometer will lead to some fuzzy temperature readings. A dirty pyrometer lens blocks infrared energy from reaching the pyrometer and will lead to lower measured temperatures. For accurate temperature readings, it is essential to check the pyrometer lens and to clean it if there is any dirt/debris/dust/oil/crud built up on the lens. Similarly, if you are viewing a target through a window, you want to make sure the window is also clean as this will have the same effect as a dirty lens. Cleaning a pyrometer lens is rather simple: take a soft cloth or a Q-tip and wipe it clean using any alcohol based cleaner. One simple way to help prevent lens contamination is by using an Air Purge connected to a clean/filtered air supply.
For real nasty environments and applications where dirty optics are unavoidable, dual-wavelength pyrometers might be worth considering. Dual-wavelength pyrometers report a temperature based on the ratio of infrared energy at two separate wavelengths. Assuming the dirt and debris affects both wavelengths equally, the ratio between the two wavelengths stays the same and therefore the temperature stays the same – essentially unaffected by the dirty optics. Obviously, if there is an inch thick layer of crud on the lens no infrared energy can get through and the pyrometer won’t be able to make a reading. But, dual-wavelength pyrometers can better tolerate dirty optics compared with single-wavelength sensors.
3) Verify the Alignment of the Pyrometer
While it may be an easy step to overlook, it is always important to verify that the pyrometer is aimed at your target. Pyrometers can be accidentally bumped, moved, misaligned, or reinstalled improperly so it is always a good practice to verify alignment. A pyrometer is an optical device so it only can see what is in its field of view. If you can see a physical obstruction between the pyrometer and the target, you can be sure that the pyrometer will see that too if it is in its field of view. For single-wavelength sensors, alignment is critical as the pyrometer takes an average temperature of whatever it sees in its field of view. Therefore a single-wavelength pyrometer needs a full field of view of the target to make an accurate measurement, so as not to average in other non-target temperatures. A dual-wavelength pyrometer can tolerate a partially filled field of view so dual-wavelength pyrometers are ideal for smaller or wandering targets as they can better tolerate misalignment.
Williamson has a few different methods of aiming our pyrometers: line-of-sight, through the lens visual aiming, laser aiming, and aim light. With the line of sight pyrometer you simply point and shoot the pyrometer in the direction of your target and eyeball it to get an idea of where the pyrometer is aimed. Through the lens aiming provides you with a “bulls-eye” target to show you where the pyrometer is aimed. Laser aiming provides you with a laser dot in the center of the sensor’s field of view. The aim light is for fiber optic pyrometers and outlines the field of view of the fiber optic unit.
If you incorporate these 3 simple steps into your maintenance routine, you truly can extend the performance life and accuracy of your pyrometers. For a free pyrometer maintenance checklist that outlines these 3 steps click on the link below: