Comparison of an infrared thermometer and thermal imager

Particularly with multi-point temperature measurement on large objects, IR measuring technology with an IR thermometer (pyrometer) is, also thanks to its comparably low price, seen as a simple non-contact measuring solution. For the measuring tasks described here, these instruments are more than adequate.

In contrast, however, the testo 870 thermal imager, for example, has 19,200 individual temperature values, which are used to create an IR image. In practice, this means the following advantages:

• Detection of critical temperatures even on extremely small objects, e.g. a hot cable. 
• Large surfaces or measurement objects (e.g. floors, buildings, switch cabinets, etc.) are shown on one image. There is no longer any need to carry out time-consuming “scanning” of a surface with an IR thermometer. 
• Documentation of measurement result as an IR image and real image at the touch of a button

These features may be decisive in industrial maintenance, for example, where faulty machinery parts such as an overheated motor have to be quickly detected to enable them to be replaced immediately in order to avoid downtime. Most available thermal imagers now also have a digital camera. This means that both a thermal image and a visual image of the measurement object are captured, ensuring better orientation during analysis. Tip: Essentially, if you have found the point at which you want to take a measurement, i.e. you know where to measure and are not dependent on the ambient temperature, and then a single-point pyrometer is adequate. For example, to measure fruit or the temperature in refrigerated displays, it is not necessary to use a thermal imager.
However, if you need to detect danger spots without knowing beforehand where they are, a thermal imager can bring considerable advantages.

Testing and calibration

To test and calibrate the display of radiation pyrometers, a black body radiator is required. During calibration, it is important to ensure that the respective measuring field of the radiation thermometer to be tested is smaller than the opening of the black body radiator.
In the case of a permanently set emission level (e.g. 0.95), the display must be converted to ε = 1.

Emission level
Even if the emission level is correctly set, measurement errors may occur!
With emission levels less than 1, the measuring value is extrapolated on the basis of instrument temperature   ambient temperature.
– If the instrument temperature does not correspond to the ambient temperature, the instrument's emission level correction will be incorrect. In other words: if the instrument temperature is lower, then the measurement result is too high, and if the instrument temperature is higher, then the measurement result is too low. 
– If individual heat or cold radiators (e.g. heating elements, lamps, refrigeration units, etc.) are reflected on the surface of the measurement object, then this radiation does not correspond to ambient temperature 

= instrument temperature. In this case too, the emission level correction performed by the instrument will be incorrect.

Remedy: Screen off such radiators, e.g. with a cardboard box. This will absorb any stray radiation and emit its own radiation ambient temperature.

Temperature check on ventilation ducts.

Temperature check on ventilation ducts.

Points to watch out for:
– The air is not measured, but rather the temperature of the grilles.
– Do not take measurement on bare metals.
– Do not take measurement too close to the measuring point.
Checking heat profiles or detecting critical points in buildings.

Points to watch out for:
– Materials such as wallpaper, wood, plaster, painted window frames and glass are easy to measure due to their high emission level of between 0.9 and 0.95.
– Either measure bare metal frames with a contact thermometer or apply a coating that increases the emission level.

Monitoring the thermal insulation in buildings.

Points to watch out for:
– Do not take measurement on bare metals.
– Note differences in emission levels
General applications of infrared measuring technology

Quick temperature measurement for road construction.
Points to watch out for:
– Permissible operating temperatures of measuring instrument
– Measuring spot/measuring distance
– Measuring instrument must have adjusted to ambient temperature.

– Only measure materials with
A high emission level, as “cold sky radiation” at -50 to -60 °C present a disturbance variable. If necessary, screen the sky e.g. with an umbrella over the measuring point.
Tip: Use a measuring instrument with a small measuring spot at a large distance.

In food inspection
Points to watch out for:
– Only surface temperature is determined contactless.
=> In the case of critical values, always verify with contact thermometer!
– Observe measuring spot/measuring distance
– Measuring instrument must have adjusted to ambient temperature.
– The ideal distance between measur-ing instrument and cooled product/ packaging is 1 to 2 cm. Outer card-board packaging should be opened and the measurement taken inside the packaging.
– With film-sealed food, only the temperature of the film is measured. Therefore only measure at points at which the film is in direct contact with the product.
– Do not measure at occlusions.
Tip: Use combi measuring instrument (e.g. testo 104 IR)

Infrared measuring technology Industrial applications

Detection of excessively high temperatures on switch cabinets, measurement on electrical circuits, such as resistors, transistors in printed circuits, etc.

Points to watch out for:
– Measuring spot/measuring distance
– Measurement not on bare surfaces (they reflect the ambient temperature), but on plastic with ε setting 0.95.
Tip: Use an IR measuring instrument with a small measuring spot (e.g. testo 845, testo 830-T3)

Temperature measurement on refrigeration unit

Points to watch out for:
– Measuring spot/measuring distance
– Measurement on surface with a high emission level (e.g. painted surface)
Tip: Use a measuring instrument that has a small measuring spot at a large distance and permits a comparison measurement using a contact thermometer (e.g. testo 845 or 835 set).

Checking and recording temperature values on generators and drives, on diesel units and on exhaust manifolds.
Points to watch out for:
– Measuring spot/measuring distance 
– Measurement on surface with a high emission level or moisten surface, e.g. with oil. 
Tip: Use a measuring instrument that has a small measuring spot at a large distance and permits a comparison measurement using a contact thermometer (testo 845 and 835).

Temperature check on output cables of an electricity generator.
Temperature check on rail vehicles, e.g. “hot box detection” on railway carriages through measurement of axle covers temperatures.
Points to watch out for:
– Use a measuring instrument that has a small measuring spot at a large distance. 
Heating, ventilation and air conditioning systems

History of infrared measuring technology

Up until 1960, radiation thermometers are primarily used to measure high temperatures. However, after that time, various types of radiation detectors that are also receptive to wavelengths larger than 8 µm are developed, enabling reliable and accurate temperature measurement right down to below the freezing point of water.

• 1800    Herschel discovers the IR spectrum through tests with a liquid thermometer with IR-absorbing ball
• 1900 Planckian radiation laws
• 1938    Book “Optical Pyrometer” (measuring technology application)

Guidelines for infrared measuring techniques

Infrared temperature measuring instruments are particularly suitable for...

• ...poor thermal conductors, such as ceramic, rubber, plastics, etc. A probe for contact measurement can only display the correct temperature if it can take on the temperature of the measurement body. With poor thermal conductors, this is generally not the case or response times are extremely long.
• ...for determining the surface temperature of rough surfaces (e.g. plaster, textured wallpaper, etc.). Measurement with probes can only sometimes be carried out due to the poor thermal contact.
• ...for moving parts, e.g. running paper webs, rotating tyres, running sheet metal webs, etc.
• ...for parts that must not be touched, e.g. food, painted parts, sterile parts or aggressive media.
• ...for live parts, e.g. electrical components, busbars, transformers, etc.
• ...for small and low-mass parts, e.g. components and all measurement objects where a contact probe draws too much heat, thereby causing incorrect measurements.
• ...for the measurement of extremely small or extremely large surfaces through the selection of various lenses.

Advantages of infrared measuring technology

There has been a huge increase in applications involving infrared measuring systems in recent years. The following factors undoubtedly play an important part in this trend.
Infrared measuring technology offers easy temperature capture and quick, dynamic processes. This is enhanced by the short response time of the sensors and systems. 
The systems offer sophisticated, modern technology with reliable sensors and modern microprocessor electronics. 
Their absence of interaction, i.e. they do not influence the measurement object, permits online measurements of sensitive surfaces and sterile products, as well as measurements in hazardous or inaccessible areas. 
Another factor in this trend that should not be ignored, over and above the technical advantages, is that these systems are attractively priced for customers as a result of cost-optimized production processes, which place the emphasis on high unit totals.