How to Choose the Best SPOT Pyrometer for your Required Application?

AMETEK Land’s SPOT pyrometers / thermometers fall into three broad types:

  • Mono
  • Ratio
  • Application 

The best one to choose will depend on the product surface you want to measure, and the process environment.

Understanding the Range of SPOT Pyrometers Available for Non-Contact Infrared Temperature Measurement
A single wavelength Mono pyrometer is the simplest and cheapest solution, but for accurate temperature measurements you must have a clear view of the product and know the emissivity of the surface

The advantage of a dual wavelength Ratio pyrometer is that it can correct for dust, dirty windows, and for unknown or changing emissivity - as long as both wavelength channels are affected proportionally.

A more complex Application specific pyrometer is required for certain surfaces such as aluminium and galvanised / galvannealed steel where emissivity changes significantly with wavelength.

Emissivity is the key parameter to understand for accurate infrared temperature measurement.  

The emissivity of a surface is the proportion of radiation that surface emits, compared to the maximum theoretically possible according to the laws of physics (the ‘blackbody radiation’).  Emissivity is the opposite of reflectivity, so a highly reflective smooth mirror-like surface has emissivity close to zero.  Rough, dark matt surfaces have high emissivity.  If you have two objects of the same temperature, one bright and shiny, one dark and matt, the dark one will feel hotter if you hold your hand in front of it as it radiates a higher proportion of its internal heat.  An infrared pyrometer measures the same effect that you feel.  With a Mono thermometer, you need to enter the correct emissivity value for the surface so that the pyrometer can correct for the reduced emission level, whereas Ratio and Application pyrometers can automatically fit and even calculate the emissivity.  

Dirt and dust that obscure the view of a pyrometer have the same effect on the measurement as reduced emissivity – both reduce the amount of radiation collected.  If the level of obscuration remains constant, it would be possible to enter a correction factor as for emissivity and continue to use a Mono thermometer.  However, in many cases clouds of dust come and go, viewing windows become gradually dirtier between cleaning cycles.  A Ratio pyrometer is the device to choose in this case, which can automatically adjust for the changing obscuration level. 

Ratio and Application pyrometers measure the infrared radiation received at two or more wavelengths – so they collect more data than a single wavelength Mono pyrometer.  If the surface emissivity or transmission level of the optical path is changing, there are effectively two unknowns to calculate: the proportion of radiation arriving at the instrument, and the temperature.  Given two measurement wavebands, it is possible to eliminate the reduced signal level from the equations and measure temperature accurately despite changing emissivity or obscuration.  In the SPOT range of Ratio pyrometers, the signal level is also provided as an output, and can be configured with an alarm as a ‘dirty window alert’.

An Application pyrometer is a more complex variant of the Ratio pyrometer, for materials whose emissivity changes with emissivity in a non-linear way.  In this case there are multiple unknowns: the temperature, the emissivity at each wavelength, and the complex relationship between the emissivity change at those different wavelengths.  For each temperature measured by an Application pyrometer, the instrument is calling on a huge ‘3D Dataset’ of historically measured values of temperature, emissivity and radiance at each waveband of the instrument.  For this reason, Application thermometers are only available for specific industrial process materials that we have thoroughly researched, such as the SPOT AL EQS for aluminium extrusion, quench and strip, and the SPOT GS for galvanised and galvannealed steel strip.

ABOUT THE AUTHOR
AMETEK Land Fiona Turner - Physics Section ManagerDr. Fiona Turner - Physics Section Manager at AMETEK Land is a Chartered Physicist specialising in instrumentation and metrology. Fiona joined AMETEK Land in 2010 as Physics Section Manager, where her main project has been the development of the SPOT pyrometer range and subsequently application specific SPOT models for aluminium and steel industries.

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