How To Calibrate A Laser Thermometer
With proper setup and planning, infrared thermometer calibrations can exist accurate. The steps outlined beneath should exist followed to perform accurate infrared thermometer calibrations. Much of the data presented here is independent in ASTM E2847, "Standard Practice for Calibration and Accurateness Verification of Wideband Infrared Thermometers."
Sources of uncertainty
There are several sources of uncertainty which contribute greatly to an infrared thermometer calibration. These sources are summarized below.
- Emissivity estimation of the scale 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
Mandatory scale equipment
The following equipment is a must for any infrared thermometer calibration.
- Thermal radiation source
- Transfer standard
- Ambient temperature thermometer
- Mounting device
- Distance measuring device
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 radiations is what the infrared thermometer uses to make up one's mind temperature.
One of the major concerns in choosing a radiations source is how big it should be. Much of the business concern is due to the infrared thermometer's field-of-view. For Fluke Calibration infrared thermometers, a source size of 5 inches (125 millimeters) in diameter is plenty for all models. For other manufacturers' models, this information should come from the infrared thermometer manufacturer or be determined by experimentation.
The transfer standard is used to calibrate the thermal radiation source. The transfer standard must be traceable to BIPM through a national metrological establish. The transfer standard tin can be a contact thermometer (a PRT, thermistor, or thermocouple) or a non-contact thermometer (a radiation thermometer). The transfer standard may be implemented internal to the laboratory performing the infrared thermometer calibrations, or may be implemented using a third party laboratory external to the lab performing the infrared thermometer calibrations.
The ambient temperature thermometer is used to monitor the temperature inside the laboratory. This is especially important because for some infrared thermometers, ambience temperature plays a large part in uncertainty, as the ambient temperature affects the reference temperature of the infrared thermometer.
The mounting device is what holds the infrared thermometer during a calibration. The mounting device maintains the measuring distance and alignment of the infrared thermometer during scale. The mounting device may exist a tripod, a fixture, or a hand.
A device should be used to determine measuring altitude. Measuring distance is the distance from the radiation source to the infrared thermometer. The distance measuring device will typically be a tape measure out or a measuring rod.
Non-mandatory equipment
Depending on the infrared thermometer being calibrated and the calibration temperature, boosted equipment may be required. Some infrared thermometers volition need to be calibrated with an aperture. If this is the case, the aperture size and the measuring distance should exist stated on the report of calibration. If a scale signal below the dew or frost bespeak is made, a provision should be made to prevent ice or moisture build-up on the calibrator surface. This tin be done via the use of a dry gas purge. In this example, the purge gas may be dried air, nitrogen, or argon.
Traceability schemes
There are two traceability schemes, Scheme I and Scheme Ii. The two schemes are classified past how the calibration source's true temperature is adamant. In Scheme I, the truthful temperature is determined by contact thermometry. In Scheme Ii, the true temperature of the source is determined by radiations thermometry. Scheme I would appear to be the best method to use; still, it is not. There are two generally large uncertainties that arise when using Scheme I: the emissivity uncertainty and the source rut substitution doubtfulness. The use of Scheme 2 accounts for these errors. The 4180 and 4181 Precision Infrared Calibrators come up from the factory with a Scheme Two calibration.
Laboratory setup
In order to perform infrared thermometer calibrations with minimized uncertainties and minimized errors, a proper laboratory setup should be observed.
The temperature in the laboratory should be maintained within reasonable limits. The ambience temperature during the scale or the laboratory'south temperature limits should be stated on the written report of calibration. This is important every bit it provides the customer information well-nigh the surroundings that the infrared thermometer was calibrated in.
The positioning of equipment within the laboratory is disquisitional. I of the biggest concerns is reflected temperature. This is especially a business concern when conducting calibrations at lower temperatures. To properly account for reflected temperature, the following provisions should be fabricated. First, never set up a laboratory so that a heat source is facing the radiation source. 2d, make sure that the temperature of the walls facing the radiation source is that of the laboratory. This is peculiarly a concern if the wall facing the radiations source is an exterior wall or an outside window. Tertiary, the position of the technician performing the scale should exist considered, since he/she influences the reflected temperature. For calibrations at temperature below 50 °C, a divider may need to be emplaced between the technician and the radiation source (see Figure vii).
Some other concern in setup of a laboratory is ambience air flow. In no example should any forced air be virtually the surface of the radiation source. This means intendance should be taken not to set the radiation source almost (or beneath) any HVAC vents and doorways.
Examples of correct and wrong laboratory set are shown in Effigy 6.
Scale process
Preparation
Before the calibration, the infrared thermometer should be immune enough fourth dimension to accomplish the temperature of the laboratory, typically 15 minutes. This is an especially of import consideration when bringing a thermometer in from the outside.
For nearly calibrations, cleaning the infrared thermometer'south lens is non recommended. Whatsoever lens cleaning that is done should be done with the permission of the client and according to the infrared thermometer's manufacturer's recommendations.
The radiation source should be fix to the desired calibration temperature and should be allowed to stabilize. If the calibration is to be done with a dry gas purge, the purge shall exist set up before the radiation source stabilizes.
Calibration points
The customer should determine the calibration points used. They should be based on the client's wants and needs. If the customer does not know what calibration points he/she wants, the calibration laboratory may offer advice. If the infrared thermometer is used over a narrow range of temperature, one calibration point may be plenty. For an infrared thermometer used over a wide range of temperatures, at least three points should exist used. These points should represent at to the lowest degree the minimum, maximum and mid-range of the infrared thermometer's usage range.
The guild of the calibration points may be called in an arbitrary way. 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.
Procedure
The following steps should exist repeated for each scale betoken.
If the infrared thermometer has a reflected temperature setting, it should be set to the radiation source'south reflected temperature. The reflected temperature setting may exist called groundwork temperature. It should be noted that Fluke infrared thermometers practise 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 take a fixed emissivity. In these cases a mathematical correction may be made. If a Fluke 4180 or 4181 is being used, this correction can exist 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 altitude is fix from the flat plate surface to the forepart housing of the infrared thermometer. The Fluke Calibration 4180 and 4181 provide a measurement point and then that the calibrator surface does non have to be touched. The concave portion at the peak of the display panel is within 1 mm of the calibrator surface. Measurements should be taken from this point equally shown in Effigy 8. Once the distance is set, the infrared thermometer should be centered on the calibrator surface. This tin can be done past using the laser provided with the infrared thermometer, or by maximizing the signal by moving the infrared thermometer upwards and downwards and side to side as shown in Figure 9. When alignment is completed, the line from the infrared thermometer to the calibrator surface should exist no more than 5 degrees from normal (perpendicular).
At this bespeak, a measurement is ready to be made. The measurement should exist initiated. The measurement time should be x times longer that the infrared thermometer's response fourth dimension, typically five seconds for Fluke infrared thermometers. For Fluke infrared thermometers, the measurement is made past property the trigger for 5 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 i measurement, information technology should take the scale technician no longer than fifteen seconds.
Dubiousness assay
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 Infrared Thermometers". An example uncertainty budget is listed in Table 1.
| Incertitude | Desig. | Type | U(100 °C) (°C) | |
|---|---|---|---|---|
| Source | Calibration temperature | U1 | B | 0.268 |
| Source emissivity | U2 | B | 0.128 | |
| Reflected ambient radiation | Uiii | South | 0.031 | |
| Source heat exchange | U4 | B | 0.012 | |
| Ambient conditions | U5 | B | 0.001 | |
| Source uniformity | U6 | A | 0.163 | |
| InfraredThermometer | Size-of-source effect | Useven | B | 0.019 |
| Ambient temperature | Uviii | A | 0.050 | |
| Atmospheric absorption | U9 | B | 0.020 | |
| Noise | U10 | A | 0.100 | |
| Brandish resolution | U11 | A | 0.058 | |
| Combined expanded dubiety (k=2) | 0.364 |
Reporting your results
The report of calibration is a communications tool for you and your customer. The report should exist in a standardized grade and come across the requirements of your laboratory's accrediting body. The results of the scale should be reported. This is best represented past a table of source temperatures versus infrared thermometer readout values. An indication of Laissez passer/FAIL may be fabricated in this table besides. The report should include the post-obit items:
- Title
- Unique identification of the calibrated infrared thermometer
- Record of the person who performed the calibration
- Appointment of calibration
- Source temperature versus infrared thermometer readout temperature
- Measuring distance
- Emissivity setting of the infrared thermometer
- Bore of the source
- Ambient temperature
- Description of the discontinuity including discontinuity distance (if used)
- Measurement uncertainties
Other supplementary information such as a description of the scale procedure, a list of reference instruments used, a statement regarding the traceability of the calibration, and a description of the dubiousness budget may be included in the report as well.
Additional learning resources
How to Calibrate an Infrared Thermometer on-demand webinar
Metrology 101: Infrared Thermometer Scale awarding note
Infrared Temperature Scale 101 application note
How to Create an Infrared Thermometer Incertitude Upkeep on-demand webinar
Related products
Infrared calibrators
Source: https://us.flukecal.com/literature/articles-and-education/temperature-calibration/application-notes/infrared-thermometer-cal
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