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Will a 1 µm and 3.9 µm Pyrometer or Thermal Imager Read the Same Tube Wall Temperatures (TWTs)? If Not, Which Value is Correct?

To answer this question, we first need to discuss the infrared temperature measurement behaviour of pyrometers, line-scanners and thermal imagers working at different spectral responses.

The infrared radiation emitted by a steel tube depends on the temperature of the surface to be measured, material, surface condition and – of course – the spectral emissivity of the target.  The radiation detected by infrared measurement equipment will also include reflected radiation from other hot tubes and the walls of the furnace.

While the radiation emitted by the target surface varies with temperature, material and surface condition, emissivity itself equals the ratio of emitted radiation of a real object to the emitted radiation of a black body at the same temperature. 

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Fig. 1:
Typical emissivity values for different materials

An ideal black body has an emissivity of ε = 1, which is constant at a given spectral range (Fig. 1, red line). A material whose emissivity remains constant with wavelength is known as a gray body (Fig. 1, blue line). Real objects/materials, versus the theoretical black/gray bodies, have different emissivities at different wavelengths, as shown as an example in Fig. 1 for iron or any other material.

A 1 µm instrument, compared to a 3.9 µm instrument, measuring the same target at the same position and orientation on a typical iron surface, will require different emissivity factors (e.g., 61 % and 21 %). This object emits 61 % of its heat energy at 1 µm and only 21 % at 3.9 µm. If the same emissivity (e.g., 61 %) is used, each pyrometer will show different temperature readings. To get the correct temperature reading, the instrument emissivities must be adjusted to the correct object emissivities for the instrument wavelengths.

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Fig. 2: Relationship between absorption/emission, reflection and transmission

The general principle of the absorption, reflection and transmission of radiation is illustrated in Fig. 2. For any given spectral range or wavelength, the sum of absorption, reflection and transmission for an object will always result in a value of 1 or, in some cases, referred to as 100 % (1) = Absorption + Reflection + Transmission.

Most objects do not transmit infrared radiation, resulting into a simpler equation: 100 % (1) = Absorption + Reflection. Meaning if an object emits at 61 % at a certain wavelength, it will reflect at 39 %.

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Fig. 3: Exponential increase of radiation power with the increase of the object/surface temperature, following the Stefan-Boltzmann Law

Following the Stefan-Boltzmann Law (Fig. 3), the radiation power increases with the 4th power of the temperature. If the background temperatures reflected on the target surface are much lower than the object temperatures, the reflected background radiation can be ignored in most applications. The object temperature can be measured using the object surface emissivity setting with the measuring instrument. In such applications, measuring at the shortest possible wavelength is recommended to minimize the reading error in case of target emissivity changes.

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Fig. 4:
Thermal Imaging view into a tube furnace – NIR-B-640-EX

Hotter background temperatures (e.g., in tube furnace reformer) at longer wavelengths (e.g., 3.9 μm), the background radiation influence is lower compared to shorter wavelength instruments (e.g., 1.0 μm).

However, hotter background temperatures (e.g., in many petrochemical fired-heater applications such as steam methane reformers and ethene crackers) result in the reverse. Background radiation influence is lower at 3.9 μm compared to shorter wavelength instruments (e.g., 1.0 μm). 

When measuring object temperatures with higher surrounding temperatures, the insulation/background temperatures must be considered and corrected for to obtain an accurate temperature measurement of the object. Fig. 38 shows a (much) higher radiance influence for the 1 μm shorter wavelength instrument, compared to a lower influence for the longer wavelength 3.9 μm instrument.

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Fig. 5:
Radiation intensity with hotter background radiation

The target surface emissivities are still different at different instrument spectral responses (wavelengths) when the background is hotter, so to get a correct tube surface temperature reading, the tube surface emissivity and the instrument spectral response (wavelength) need to be known. Together with the background correction integrated into all AMETEK Land instruments, the correct tube wall temperatures (TWT) can be accurately measured by using a pyrometer or a furnace thermal imager (Fig. 6 shows the NIR-B-640-EX furnace thermal imager).

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Fig. 6:
NIR-B-640-EX furnace thermal imager – e.g., used for TWT continuous measurement

In summary, if the emissivity and background temperatures are known and configured correctly in each instrument, both 1um and 3.9um instruments will read correctly.  If background temperatures are lower than target temperatures, the shorter wavelength instrument is less sensitive to emissivity changes or small errors in the compensation value for emissivity.  However, in a tube furnace with high background temperatures, a 3.9um may be less sensitive to those changes and errors.

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