NO TOUCHING!
This Hubble Telescope picture of 10,000 galaxies did not make sense until the Ultra Violet spectrum of light was included. The light emitted from each body is related to their temperature.
Images of broken light,
Which dance before me like a million eyes,
They call me on and on
Across the universe.
("Across the Universe," by John Lennon, 1968-70)
PYROMETRY: NO-TOUCHING TEMPERATURE MEASUREMENT
Unlike any of the temperature-measuring technologies focussed on thus far in the PTOA ...
Pyrometry technology makes it possible to measure the temperature of a surface without having the temperature-measuring instrument touching that surface.
The temperature of the galaxies captured in the above Hubble telescope photo can be inferred from the colors of emitted light that are radiated from their respective surfaces.
The same pyrometry technology that is used to determine the temperatures of celestial bodies is put to good use on earth determining surface temperatures ...
with no touching!
Across great distances ... even across the universe ... temperatures can be measured using pyrometry technology.
RADIATION REVIEW
PTOA Readers and Students that are reading the PTOA Segments in the intended sequential order were already introduced to radiation heat transfer in PTOA Segment #66 entitled "The Mother of All Heat Transfer."
The surface labelled "A" lies in the path of heat radiated from a hot surface labelled T-Hot.
PTOA Readers and Students will remember the nearby photo and recall that any surface standing in the way of beams of radiated heat will absorb some of the heat.
PTOA Readers and Students will also recall that the surface will reflect some of the radiation as shown in the nearby photo. The reflected radiation is labelled "reflected irradiation".
The reflected radiation is emitted in the form of light.
A dude named Wien figure out that the temperature of the surface that emits the radiation can be inferred from the wavelength and intensity of the emitted light.
So the operating principle of a pyrometer links the intensity of the emitted radiation to a surface temperature.
NOW YOU SEE ME ... NOW YOU DON'T
The wavelengths of radiated energy are eensy-weensy.
They are measured in microns, a very small unit of length.
In the olden days, the symbol for a micron was µ which is the small case for Greek letter "mu" (pronounced me-yew ... like the sound a kitty cat makes).
There are 1000 microns in a single millimeter. There are a million microns in one meter.
Just to make life confusing, nowadays the micron may be interchangeably called a "micrometer" and labelled "µm" or even "mm" instead of just the singular µ.
The PTOA is going to stick with "µ" to mean "micron."
Regardless ...
a generalization about microns emerges:
Whatever the label for micron ... stand alone µ or µm or mm ... anything measured in microns is super dinky!
The nearby graphic compares the diameter of a large circle that represents the cross section of a single human hair (100 microns) to objects like a strand of the E.coli virus (1 micron in length). That means the E.coli virus is 1/100 th the diameter of a human hair!
The VISIBLE SPECTRUM of light that makes it possible for humans to see things is just a very small part of the entire electromagnetic spectrum.
A human being can see light that is radiated in wavelengths that are 0.40 to 0.70 µ long.
This 0.40 to 0.70 µ range is called the "visible spectrum of light."
The visible spectrum that you and I can see is just a small smidge of the entire electromagnetic wave spectrum that includes all the wavelengths and frequencies over which electromagnetic radiation extends.
The below table summarizes the same information shown in the above graphic; in order of increasing wavelength on the right side column, the table conveniently identifies the separate regions of the electromagnetic spectrum on the left side column.
| gamma ray | <0.03 nanometers |
| X-ray | 0.03 - 3 nanometers |
| ultraviolet | 3 nanometers - 0.4 micrometers |
| visible | 0.4 - 0.7 micrometers |
| near infrared | 0.7 - 1.3 micrometers |
| mid-infrared | 1.3 - 3.0 micrometers |
| thermal (far) infrared | 3.0 - 5.0 micrometers AND 8 - 14 micrometers |
| microwave | 0.3 - 300 centimeters |
The energy of short wavelengths outside of the visible spectrum are used to shoot x rays that help identify cancer and other ailments without cutting a person's body open.
Longer wavelengths outside of the visible spectrum are used for microwave cooking, radio and television broadcasting.
The graphic and table show that the visible spectrum is sandwiched between the shorter "ultra-violet" wavelength and the longer "infrared" wavelength family which includes the near, mid, and far infrared wavelengths.
Most emitted radiation from a surface occurs in the visible and infrared portions of the spectrum.
Ergo, pyrometer technology detects radiation in just this limited wavelength range of the electromagnetic spectrum.
It is possible to feel but you can't see radiant energy in the form of radiated light until the temperature reaches 550°C (1022 °F); at that temperature the wavelength of the radiated energy will be attaining the visible spectral region.
TWO KINDS OF PYROMETERS USED TO MEASURE SURFACE TEMPERATURES
This Process Operator is holding a portable pyrometer.
The information above is essential to understanding how a pyrometer can measure the temperature of a surface in the absence of any direct contact that would enable conduction or convection heat transfer.
Manual and Automatic Optical Pyrometers
In the world of science and physics, the word "optical" means "operating in or employing the visible part of the electromagnetic spectrum."
Who would be surprised to learn that manual and automatic optical pyrometers operate only within the visible spectral range?
Because optical pyrometers only work within the visible spectral range (.35 to .70 µ) they are also referred to as "narrowband pyrometers."
The Optical Pyrometer compares the intensity of a platinum filament (lamp in schematic) to radiated light from a surface to determine a surface temperature.
Optical pyrometers compare the intensity of emitted radiation to the intensity of a glowing reference filament made out of platinum that is located within the instrument case.
The platinum filament is heated by an electric current that is adjusted by the pyrometer's operator.
The manual optical pyrometer is at the correct measured temperature when the filament appears to disappear.
When the intensity of the filament matches the intensity of the emitted light from the surface of interest, the surface temperature is inferred from a scale on the pyrometer.
The filament plays an important role in accurate pyrometry temperature measurement; human beings that guesstimate a temperature from the color of radiated light can easily be in error by ±200°C (± 392 °F).
Manual optical pyrometers are very portable and used by Outside Process Technicians and Instrument Technicians to check stationary and moving surface temperatures above 800 °C (1500 °F).
Automatic optical pyrometers incorporate an internal feedback loop that automatically matches the intensity of the filament to the detected radiation intensity emitted from the surface.
Automatic optical pyrometers can both measure and record temperatures.
The Optical Pyrometer must be focussed closely on the surface so that the brightness of the filament matches just the light being radiated from the surface.
The accuracy of the temperature measurement is highly dependent upon the positioning of the optical pyrometer.
The optical pyrometer must be pointed so that the surface of interest is clearly visible; the filament brightness must be easy to match to just the light emitted from the surface.
Infrared Pyrometers
Who is surprised to learn that infrared pyrometers operate in the visible and the infrared spectrum ... so from 0.35 µ all the way through 14 µ?
Because they have a broader wavelength band, infrared pyrometers are also called "broadband pyrometers."
Infrared pyrometers (aka IR pyrometers) determine a surface temperature after adding up all the radiation detected in the broader band of wavelengths.
Instead of a filament, a lens in the IR pyrometer focuses all of the detected radiation on the measuring junction of an actual thermal element like those PTOA Readers and Students are already familiar with (a thermistor, RTD, or set of thermocouples, or perhaps a photo cell).
The thermal element has both a measuring-temperature junction and reference-temperature junction.
When the thermal element is an electrical measuring device, the difference in temperature sensed between the measuring and reference junctions creates a voltage that can be correlated to the temperature of the surface.This electrical output can be transmitted to a temperature recording device.
IR pyrometers can measure lower temperatures than optical pyrometers; one manufacturer declares a measurement range from -50 °C to 4000 deg °C (-58 °F to 7232 °F).
The IR pyrometer will not be able to distinguish between radiation generated from the surface of interest and other sources of radiation ... like dust or random particles and all the other things in the vicinity that are heated by the environmental temperature.
Otherwise stated, IR pyrometers are much more susceptible to environmental sources that cause 'emittance errors' than optical pyrometers are.
IR Pyrometers are much more susceptible to 'emittance error' than Optical Pyrometers. Positioning of the pyrometer is crucial to determine an accurate surface temperature.
To generate an accurate temperature measurement, the surface of interest must be in direct line of sight of the pyrometer and the instrument must be pointed so that the lens is focussed on and filled by the surface that is having its temperature measured.
The laser beam helps to correctly position the IR pyrometer for temperature measurement.
Attaching scopes and using laser-pointers helps to reduce environmental emittance errors.
Bandpass Pyrometers
The lens, filters, and detector of an IR pyrometer can be tweaked to create a bandpass pyrometer that operates within a much tighter, specific wavelength range.
The bandpass pyrometer only incorporates radiation emitted in the specific, specialized wavelength band of the instrument.
The bandwidth selectivity provides a much more accurate temperature measurement.
Bandpass pyrometers are so specialized that measurement error caused by dust and particles is eliminated.
Glass making uses bandpass pyrometry.
Glass making requires accurate temperature measurement within the 4 to 8 µ range wavelength. Glass is transparent below 2.5µ, and becomes increasingly opaque until 4µ whereupon it becomes totally opaque.
TAKE HOME MESSAGES: Pyrometry technology makes it possible to measure a surface temperature without any direct contact between the pyrometer and the surface that is having its temperature measured.
Pyrometry works on an operating principle that links the intensity of emitted radiation to a surface temperature.
Optical pyrometers operate in the visible spectral range. They are also called 'narrowband' pyrometers.
Optical pyrometers match the intensity of an electrically energized platinum filament to the intensity of emitted radiation from the surface that is having its temperature measured.
Automatic optical pyrometers can measure and record temperatures.
Infrared pyrometers operate in both the visible and infrared spectrum and are thus called "broadband pyrometers."
IR pyrometers focus all the radiation in their bandwidth onto a thermal detector; the difference of temperature between the thermal detector's measurement and reference junctions creates an electrical output that can be correlated to the surface temperature.
IR pyrometers are susceptible to emittance error caused by the heat absorbed by stuff in the environment and must be accurately oriented to the surface to obtain an accurate temperature measurement.
©2016 PTOA Segment 00118
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area
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