Infrared thermometers are a staple tool across many industries to test temperatures in various scenarios. It is recommended that a radiation thermometer (including visible and infrared pyrometers) should be calibrated as soon as the unit has been received. This initial test should always include sufficient points to confirm and demonstrate linearity. Once this initial calibration has been completed, a check of the technology should be conducted every 12 months, while calibration of the unit should take place every 24 months.
Preparation
There are a few elements and steps to take into consideration before calibration to ensure an accurate result. Firstly, you will need to find the specs of the unit. This is provided by the manufacturer. For instance, the Testo infrared thermometer units will always come with a manual that provides the following information that is used to authenticate calibration.
- Emissivity estimation of the calibration source
- Field-of-view of the infrared thermometer
This data is used as reference points when reporting the results to either external parties or as part of the internal audit for records that can be used to maintain consistency in future tasks. It is also important to know the specs of the radiation source that will be used to test the readings. A testable source for calibrating infrared thermometers is referred to as a black body. This usually comes as either a flat plate or an IR cavity. The IR cavity has the radiation sources inset whilst the flat plate does not.
Having knowledge on temperature gradients on the radiation source is vital, along with the calibration temperature. This can be found in the unit’s specs and is measured as a tolerance of plus or minus certain units.
Infrared thermometers will often utilise a laser pointer. This is used to assist the user in gauging their directional readings. This means that it is crucial to ensure that the alignment of the infrared thermometer is up to date, as an outdated alignment may throw off readings.
There are a few other unique aspects that need to be considered before calibrating a tool like an infrared thermometer. For instance, the ambient temperature of the environment of the testing facility or lab. The temperature of these facilities is often set to 23 degrees Celsius with a 5-degree margin of error. While this is common, there is still a chance that other facilities don't adhere to this same range and may have different temperature swings, but no matter what, knowing the ambient temperature around the testing area is crucial.
One element specific to radiated temperature readings when calibrating infrared thermometers is the concept of reflected temperature. This is because infrared optics are designed to give us the emitted energy that our human eye is unable to see. Therefore, radiation can be reflected by unknown elements in testing environments and thereby affect the reading results. To combat these elements of heat exchange, it is important to make sure that the radiation sources aren't set up against ceiling vents, fans or other gaps that can cause heat loss.
Additionally, consider allowing infrared thermometers time to reach room temperature before calibrating them. This should take a minimum of 15 minutes. During this time you can set up the testing site with the specific calibration equipment.
Necessary Equipment
Transfer Standards: In order to test for accuracy, you must start with an accurate tool. You'll require a probe tool or another highly calibrated IR gun that has been calibrated to a higher standard. Ideally, this tool will have been sent to and verified by a national independent lab.
Thermal radiation source: These units are usually referred to as black bodies. They are defined by an approximately zero reflected ambient radiation. Whether you chose to use a flare plate or IR cavity model, it is important to note the size of the radiation source. This will come in handy when you are calculating the distance between your tool and the radiation. If you are using more than one black-body unit, they mustn't face each other, as they may pass radiated energy backwards and forwards between each other and impede the reading. Furthermore, the units should under no circumstances be placed in the vicinity of air drafts caused by HVAC systems or similar ventilation sources or any other instruments that could emit radiation and throw off the results. If airflow is a problem, a diffuser may be a necessity.
Partition: A partition can be used to undermine additional radiation sources outside of the target that might be emitting heat. This can be used to limit how much the optics of an IR thermometer can be impacted and better ensure an accurate reading. If a thermal radiation source is out of budget or unavailable, an IR comparator cup can be a simple and inexpensive alternative to a black body. However, if neither of the alternatives is available, a properly made ice bath can be used. While measuring the boiling point with an infrared thermometer may be problematic due to factors like condensation as well as variance in air pressure and elevation, the surface of an ice bath is 0 degrees celsius.
Mounting device: This tool allows you to get a stable measurement setup and maintaining alignment and measuring distance. Consider using a tripod, fixture or hand.
Distance measuring device: The distance to spot size ratio that you need to maintain can be determined using a tape measure or measuring rod.
Ambient temperature thermometer: An accurate thermometer allows you to monitor the conditions of the testing site. Such factors include temperature and humidity levels. It can also act as an alert to any AC or other ambient temperature factors. It is important to set the temperature and then allow it to stabilise before testing begins.
Calibration Procedure
Once the prep work has been done and data points have been collected, it's time to begin. Firstly, calibration points should be tested based on the range of temperature for which you're calibrating the IR thermometer. If they are over a wide range of temperatures, a minimum of 3 calibration points should be used. On the flip side, if it is a narrow temperature range, you'll only require 1 or 2 points. A good rule of thumb is to go from the lowest calibration point to the highest.
Set the IR thermometer reflected temperature (if available) and align the unit to maintain a set distance and field of view so that everything is level. By doing this, you are enabling the entire optic to look at the source, which should be straight on in front of the device. The infrared thermometer should be no more than 5 degrees from normal to target the ideal surface area.
Once the thermometer is correctly set and aligned, it is time to take the measurement. A good standard to adhere to is to take multiple readings and base those measurements on the 10 times the infrared thermometer response time. Therefore, if the Infrared thermometer reading is achieved in .5 seconds, keep the device held for 5 seconds.
Report Findings
Whether for an external client or regulator or internal audit, a clear paper trail and record of calibration of equipment is crucial. These records should include.
- Name and address of laboratory as well as those running the test
- Unique ID of the calibrated infrared thermometer
- Method used
- Date of calibration
- Results (including any measurement uncertainties like source temperature versus IR thermometer readout temperature, adjustments etc.)
- Measuring distance
- Emissivity settings of the infrared thermometer and for the source if needed
- The diameter of the radiation source
- Ambient temperature and humidity of the testing environment
A well-calibrated IR thermometer is capable of maintaining accuracy in every facet of its use and can impact the process its readings rely on.
Because of this, regular maintenance and testing is key to keeping an IR thermometer working properly, accurately and efficiently. For more information on IR thermometer calibration and testing, get in contact with the Testo NZ team today.