Information from Fluke –
The accuracy of any reading is only as true as the instrument measuring it. With proper setup and planning, infrared thermometer calibrations can be accurate. The steps outlined below should be followed to perform accurate infrared thermometer calibrations.
The sources of uncertainty listed below may affect an infrared thermometer calibration:
- Emissivity estimation of the calibration source
- Field-of-view of the infrared thermometer
- Temperature gradients on the radiation source
- Improper alignment of the infrared thermometer
- Calibration temperature of the radiation source
- Ambient temperature
- Reflected temperature
The following equipment is mandatory for any infrared thermometer calibration:
- Thermal radiation source: The thermal radiation source is a calibrated temperature source that provides radiation. The strength of the radiation is dependent on the source’s temperature; this radiation is what the infrared thermometer uses to determine temperature.
- Transfer standard
- Ambient temperature thermometer
- Mounting device
- Distance measuring device
Before the calibration, the infrared thermometer should be allowed enough time to reach the temperature of the laboratory, typically 15 minutes. This is an especially important consideration when bringing a thermometer in from the outside.
For most calibrations, cleaning the infrared thermometer’s lens is not recommended. Any lens cleaning that is done should be done with the permission of the customer and according to the infrared thermometer’s manufacturer’s recommendations.
The radiation source should be set to the desired calibration temperature and should be allowed to stabilise. If the calibration is to be done with a dry gas purge, the purge shall be set up before the radiation source stabilises.
The customer should determine the calibration points used. They should be based on the customers’ wants and needs. If the customer does not know what calibration points they want, the calibration laboratory may offer advice. If the infrared thermometer is used over a narrow range of temperature, one calibration point may be enough.
For an infrared thermometer used over a wide range of temperatures, at least three points should be used. These points should represent at least the minimum, maximum and mid-range of the infrared thermometer’s usage range. The order of the calibration points may be chosen in an arbitrary fashion. However, due to the phenomenon of thermal shock, it is best practice to perform the lower temperature calibration points first and the higher calibration points last.
The following steps should be repeated for each calibration point. If the infrared thermometer has a reflected temperature setting, it should be set to the radiation source’s reflected temperature. The reflected temperature setting may be called background temperature. It should be noted that Fluke infrared thermometers do not have a reflected temperature setting. The emissivity setting of the infrared thermometer should be the same as the source’s calibrated emissivity. Some infrared thermometers have a fixed emissivity. In these cases, a mathematical correction may be made.
If a Fluke 4180 or 4181 is being used, this correction can be made automatically by the instrument. The next step is to align the infrared thermometer. To do this, first set the measuring distance. For Fluke infrared thermometers, the measuring distance is set from the flat plate surface to the front housing of the infrared thermometer.
The Fluke Calibration 4180 and 4181 provide a measurement point so that the calibrator surface does not have to be touched. The concave portion at the top of the display panel is within 1 mm of the calibrator surface.
Measurements should be taken from this point as shown in Figure 5. Once the distance is set, the infrared thermometer should be centered on the calibrator surface. This can be done by using the laser provided with the infrared thermometer, or by maximising the signal by moving the infrared thermometer up and down and side to side as shown in Figure 6.
When alignment is completed, the line from the infrared thermometer to the calibrator surface should be no more than five degrees from normal (perpendicular). At this point, a measurement is ready to be made. The measurement should be initiated. The measurement time should be ten times longer that the infrared thermometer’s response time, typically five seconds for Fluke infrared thermometers.
For Fluke infrared thermometers, the measurement is made by holding the trigger for five seconds. The resulting final readout temperature should be recorded as the readout temperature for the calibration. In spite of the complex sounding nature of this method, the procedure is actually quite simple. For one measurement, it should take the calibration technician no longer than 15 s.
Uncertainty analysis is necessary for any calibration. This type of analysis is beyond the scope of this document. For a complete look at uncertainty analysis for infrared thermometer calibrations, consult ASTM E2847, “Standard Practice for Calibration and Accuracy Verification of Wideband Infra-red Thermometers”. An example uncertainty budget is listed in Table 1.
Reporting your results
The report of calibration is a communications tool for you and your customer. The report should be in a standardized form and meet the requirements of your laboratory’s accrediting body. The results of the calibration should be reported. This is best represented by a table of source temperatures versus infrared thermometer readout values. An indication of “pass” or “fail” may be made in this table as well.
The report should include the following items:
- Unique identification of the calibrated infrared thermometer
- Record of the person who performed the calibration
- Date of calibration
- Source temperature versus infrared thermometer readout temperature
- Measuring distance
- Emissivity setting of the infrared thermometer
- Diameter of the source
- Ambient temperature
- Description of the aperture including aperture distance (if used)
- Measurement uncertainties
Other supplementary information such as a description of the calibration procedure, a list of reference instruments used, a statement regarding the traceability of the calibration, and a description of the uncertainty budget may be included in the report as well.
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