A Practical Guide To IR
24 Feb 2005
Infrared thermometers play a central role in the control of many processes. Fred Ritchie offers some guidance on how to look after them in the field.
Infrared (IR) thermometers use optical lenses to gather energy from a spot on the target object and focus it on a detector in the sensor.
The target must completely fill the measurement spot to produce accurate readings.
For most applications, the target objects are large, and simple optics will do. However, small objects require higher resolution optics — a parameter normally specified as distance-to-size ration, field-of-view, or minimum spot measurement.
Distance-to-size ratio (D:S) enables the user to calculate the size of the measurement spot at a given distance. For example, if a sensor has a D:S ratio of 10:1, then a target at a distance of 600mm must be at least 60mm in diameter in order to fill its measurement spot. Field-of-view describes the measurement area in terms of view angle in degrees from the sensor. Minimum spot refers to the diameter of the smallest object that can be measured at a given focal length.
Your IR sensor should be inspected regularly for dirt and dust on the lens since any contamination of the lens surface, including scratches, will affect its calibration and, as a result, the accurate measurement of temperature. Do not be tempted to use a cloth or handkerchief to wipe the lens as this could cause more scratches and make the problem worse.
Instead, use a 'puffer' bottle of clean, dry, oil-free air to blow off the dust. More stubborn particles and contamination can be tackled with de-natured alcohol and a fine camelhair brush.
The energy detected by an IR sensor can be emitted, transmitted, or reflected by the target. However, only the emitted energy relates to the target's temperature, so it is essential to eliminate reflected and transmitted energy to achieve accurate temperature measurements.
Reflected energy is eliminated by positioning the IR thermometer so that energy radiated from heaters, sunlight, or other energy sources does not reach the sensor. Transmitted energy can be eliminated in a similar manner.
Some materials, such as thin-film plastics, are transparent to IR radiation and, again, the IR sensor should be mounted so that it is not looking through the target at a heat source. In some cases, it may be possible to choose the operating wavelength of the sensor so that, in effect, the interfering radiation is filtered out.
IR sensors are often required to operate in environments that are less than ideal. For example, the sensor must operate reliably in conditions where high ambient temperatures, dust and steam may be present. To ensure a longer system life, it is essential to provide sensor cooling when the ambient temperature is greater than 50ºC. Cooling is generally achieved by circulating water around a cooling jacket, although air can be used if the conditions demand it.
Dust and steam are particularly detrimental to the efficient operation of an IR sensor. In such cases, the sensor should be fitted with an air-purge collar or protecting tube. Wherever possible, it is better if the sensor points downwards. A light pressure of clean, oil-free air is usually enough to keep the dust or steam from interfering with the measurement.
The above are just some of the reasons why you should not simply 'fit and forget' your IR sensor. However, Calex Electronics can offer an inexpensive piece of equipment that allows you regularly to check the sensor's calibration. This is the Calex PTK portable calibration checker, which does not require the sensor to be disconnected or removed from service.
Dr. Fred Ritchie is a non-executive director with Calex Electronics.
