An Argument for Unpowered IR Thermocouple

Table of Content

Introduction
Black Body vs Gray Body vs Real Body
Conventional IR vs IR Thermocouples
Ambient Temperature Coefficient Specification
Compensation for Emissivity Variations
High Degree of Accuracy is Possible
Repeatability and Interchangeability
IR Thermocouple Pluses

Introduction

“This infrared technology is ideally suited for applications in which temperature swings are less than ± 30 °C, and high accuracy coupled with precision and repeatability are critical.”

Conventional infrared (IR) temperature sensors provide an ideal measurement solution for industrial applications in which high accuracy is not entirely critical and wide temperature swings (more than ± 50 °C) are the norm. However, IR thermocouple (t/c) technology can be the best way to go for processes in which temperature swings are not so great (less than ± 30 °C) and high accuracy coupled with repeatability and precision are needed.

Conventional powered IR devices that develop a simulated t/c signal are also classified as IRt/c's, but the type discussed in this article is unpowered, generating signal from the Seebek effect. This sensor develops a signal proportional to the difference between cold junction temperatures and hot junction (target), using standard t/c extension wire and t/c readout devices.

This article begins with a look at the effects of variable emissivity and variable ambient temperatures on both conventional IR techniques and IR thermocouple sensors. It then describes a few of the applications for which the latter technology is considered to be best suited.

Black Body vs Gray Body vs Real Body

Emissivity refers to a measure of what proportion of an object’s surface emits radiation instead of reflecting it. For instance, a perfect mirror would reflect all light and infrared radiation that reached it and emitted none; thus, it would have an emissivity of zero. Certainly, the fact that one can see a “real” mirror explains that it is not perfectly reflective. If 90% of the mirror’s surface reflected light, the remaining 10% would emit light; the mirror would thus have an emissivity of 0.1. For any non-transparent material, emissivity (ε) plus reflectivity (ρ) always equals one:

     ε + ρ = 1

A black body is an object that has an emissivity of 1 and reflects no radiation (ρ = 0). The concept of a black body is an extremely useful and vital mathematical construction in the application of infrared radiation physics and has had firm theoretical support from the time of Max Planck.

However, IR devices do not measure black bodies in the real-world temperature control applications. They measure objects that have an emissivity of less than 1 and reflectivity of more than 0. For instance, shiny metals that behave like mirrors have emissivities in the 0.05 to 0.2 range, and are more complex to measure with IR technology. Non-metals, organic materials and coated metals, on the other hand, have emissivities in the 0.8 to 0.95 range and can be measured with IR devices with much greater precision.

A useful body that is an approximation of real world targets is the gray body that is an object with an emissivity of less than 1.0. Gray bodies also have the added characteristic that their emissivity is constant at all wavelengths of interest and, therefore, is constant at all temperatures of interest. Accordingly, for gray bodies:

     qgb = qbbε

At all wave lengths, where qgb and qbb are radiated energy from a gray body and black body respectively. Figures 1 and 2 illustrate gray body properties.

Figure 1. Radiated energy vs wavelength for black bodies and gray bodies.

Figure 2. Equations for gray bodies. Ta = ambient temperature; Ts = surface temperature.

A real body comprises of the additional property, that its emissivity is not constant as temperature changes. Mathematically, the gray-body approximation proves to be useful in handling reflective errors and the real-body property must be used to deal with target temperatures.

Conventional IR vs IR Thermocouples

Conventional IR devices experience more difficultly in coping with emissivity shifts, since these devices are designed and calibrated to theoretical black body conditions at single ambient temperature points. However, IR thermocouple devices are calibrated with multiple ambient temperatures and are constructed in order to minimize ambient reflective errors and the effect of emissivity shift errors.

Figure 3. Comparison of errors caused by shifts in ambient temperature coefficient between IRt/c's (left) and conventional IR systems (right).

Figure 4. In waterless printing, an IR sensor is used to monitor the temperature of the ink platen roll. Conventional IR error is 3 °F (1.7 °C), IRt/c error is 0.2 °F (0.1°C) in this example.

Figure 5. Comparison of errors caused by emissivity variations between IRt/c (left) and conventional IR devices (right).

Mathematically, the signal output of an IR thermocouple is a complex function of ambient temperature, target temperature, reflected energy, thermocouple type, target emissivity and so on. These specifications can be clarified by representing the change in signal with respect to a variable of interest, while holding all other variables constant, as a partial derivative.

Ambient Temperature Coefficient Specification

The output signal of an IR device changes according to its ambient temperature coefficient. Figure 3 displays this effect and the resulting error for conventional IR sensors and IR thermocouple devices.

In Figure 3, S is the output signal and Ta is the ambient temperature. The equation explains the change in output of the IR thermocouple with respect to ambient temperature, assuming the sensor itself is at the same temperature as the environment, and that emissivity = 0.9 (gray body assumption).

The practical implication of this is that an IR thermocouple, once installed and calibrated in place, tends to change temperature with the ambient background and then internally applies the correction needed to reduce errors. Lacking multiple ambient temperature calibration, a conventional IR device typically will generate a bigger error, which, in turn, will lead to unwanted shifts in process control temperatures, even though the black body calibration of the device may be perfect.

For instance, in waterless printing processes, the temperature of the ink application roll must be controlled in order to maintain high-quality output (Figure 4). If the temperature is to be maintained at 80 °F (26.7 °C) and temperatures inside the press enclosure can range from 70 to 100 °F (21.1 to 37.8 °C) because of warm-up, building air ventilation, weather and other factors a conventional IR device can generate an error of about 3 °F (1.7 °C). An IR thermocouple should produce an error of about only 0.2 °F (0.1 °C) under the same conditions.

To estimate the improvement in control accuracy produced by the IR thermocouple for a specific application, the following approximation can be applied:

Error with conventional IR

     ≈ (1 - ε)(ΔTa)(Ts - Ta)

Error with IR t/c

     ≈ (0.9 - ε)(ΔTa/10)(Ts - Ta))

Compensation for Emissivity Variations

The standard assumption for conventional IR thermometry is that emissivity is constant with variations in target surface temperature. However, real target materials do not have this characteristic. The average value for non-metals, for which the change in emissivity with respect to surface temperature has been reported, is about -2% per 100 °F target temperature change (-3% per 100 °C).1 Emissivity can vary by much more for some materials.

Figure 5 displays the emissivity variation for IRt/c’s and conventional IR devices when the partial derivative mathematical formulation is applied. Conventional IR devices overlook this effect by not taking into account that emissivity variations can be brought about by temperature changes and the resulting error can lead to process control errors.

The signal generated is proportional to the radiation emitted by the surface:

     S = εqbb

The variation in signal with respect to target surface temperature is shown in Figure 6. Note that, with respect to surface temperature, the conventional IR device loses one term of the signal change.

Since real-world emissivity for almost all non-metal materials decreases with temperature, the steady emissivity assumption of conventional IR devices generates errors in readings over wide temperature excursions that are not clear to the typical IR user. These errors can cause inaccurate process control over a wide temperature range. However, over a narrow range, these devices can be used with reasonable assurance that measurements will be repeatable, precise and accurate.

To obtain the measurement benefits from IR thermocouples, it is essential for Engineers to specify these devices for a specific useable temperature range, in which the effect of emissivity change is considered for in the optimum range specification of the device. This will enable the user to be confident that process control will be correct over the temperature range. Note that testing an IR thermocouple with a black body will not provide the same results as a test with a real body.

A second effect on useable temperature range refers to that of target surface temperature on ambient temperature and, thus, on the reflected component of radiation to the sensor. The increased radiation heat transfer to the surroundings will cause the target ambient radiant background to increase in temperature as target temperature increases within a process.

For instance, consider a laminating process that has a number of temperature control settings, each of which relies on the material and feed speeds (Figure 7). These target operation temperatures may differ by as much as 100 °F (56 °C). As the temperature of the material differs, the temperature of the background radiation in the vicinity of the measurement will also differ and influence the IR reading. In this situation, the difference in signal with target temperature comprises of an additional component, as shown in Figure 8.

High Degree of Accuracy is Possible

The calibration of IR thermocouple is set up over different temperature ranges. It is possible to predict the optimum temperature ranges for particular thermocouple models when the numbers are done on the combined effects of ambient reflection variations and emissivity variations (Figure 9). This guarantees a high degree of accuracy for a particular device over its rated temperature range.

Figure 6. Change in signal with respect to target surface temperature for IRt/c (left) and conventional IR (right).

Figure 7. A laminating process with several temperature control settings.

Figure 8. Error caused by the effect of target temperature on background temperature for IRt/c (top) and conventional IR (bottom).

Accuracy is based on the width of the temperature span that has to be measured. For instance, if it is necessary to measure and control a target temperature at 200 °F (90 °C), the error is 0.0% after preliminary calibration at that point.

As the target temperature differs from the 200 °F (90 °C) calibration point, the error increases slowly, as demonstrated in the accuracy table. The error at the temperature extremes would be ± 0.4% or 1.0 °F (0.6 °C) for a temperature span of 190 to 210 °F (87 to 99 °C); this is shown in the table as the error for a span of ± 10 °F (± 6 °C) from the calibration point.

OEM’s and users of programmable controllers and computers who need higher accuracy over wider temperature ranges, should keep in mind that multiple point calibration will bring about a reduction in errors caused by changing reflections, changing emissivities etc.

Repeatability and Interchangeability

The IR thermocouple’s repeatability error, defined as the potential to reproduce a reading under identical conditions, is extremely small. No source of spurious signals until the resolution limit which is 0.0001 °C (due to Johnson noise) is reached as there are no active electronics that can shift. The result is a repeatability error of less than 0.02 °F (0.01 °C).

Interchangeability error, which is defined as the difference in reading between any two IR thermocouples of the same model making identical measurements, is equal to or less than 1% or 1 °F (0.5 °C). This is of particular importance when a temperature sensor must be replaced.

IR Thermocouple Pluses

The key points to remember about IR thermocouple technology are provided below:

  1. In-place device calibration is always recommended, due to uncertainties in ambient temperature and emissivities.
  2. Once an initial system has been calibrated and qualified, IR thermocouples of the same model can be substituted without the requirement for recalibration.
  3. An additional benefit of the IR thermocouple is its specified optimum range per model. A user is not misled into believing that the measurement is accurate over a wide temperature range.
  4. It is not possible to accurately check the performance of an IR thermocouple with a black body. Standard laboratory black bodies can be used only for pass-fail or reproducibility testing.
  5. The performance of an IR thermocouple cannot be accurately checked by using a standard handheld portable IR device, since the reflected component and emissivity are almost never known with any precision. IR instruments that are certifiably accurate with unknown emissivities should be used to accurately check IR thermocouple performance and installation.

IR thermocouple devices, like standard IR sensors, are manufactured for use in several different process control, OEM and factory automation applications. Prices for these devices range from $99 to $699.

Regular IR device is considered to be ideal where wide temperature ranges need to be measured and tight accuracy is not that important. For applications with more narrow temperature swings that also need a high degree of repeatability and accuracy, the IR thermocouple (Figure 10) may be the solution of choice.

Figure 9. Infrared thermocouple selection chart shows company's various model IR thermocouples as they are optimized for specific temperature ranges. In the accuracy table, error is shown for various target temperature spans.

Figure 10. Infrared thermocouples, such as the IRt/c from Exergen, can provide a high accuracy temperature measurement system where relatively narrow temperature ranges are involved.

This information has been sourced, reviewed and adapted from materials provided by Exergen Global.

For more information on this source, please visit Exergen Global.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit