Optical Pyrometer

Optical Pyrometer

By: Klaas van Ditzhuyzen


Units are expressed in metric and prices in US dollars, unless otherwise stated.

An optical pyrometer is a device which allows contactless temperature measuring by using the incandescense color. It is based upon the fact that all black bodies do have the same incandescense color at a given temperature. It is very straightforward and allows any temperature from which a hot object emits light ( > 500 deg C). It is made from a small magnifying optical device (like a monocular or very small telescope) in which a small incandescent bulb is placed which image is sharp when the user views through the eyepiece (the lens(es) on the eye end of the optical device). The background is the hot object to be gauged. The electrical current flowing through the filaments in the bulb is an indication of their temperature. This current is controlled by a potentiometer which is put between the power source (a battery) and the bulb. An ammeter is used to display the temperature. Its range is from 500 C (== 900F lower limit when an object incandesces) to 1600 C (3000 F), which is suitable for most applications.


The stuff needed is available in a local hardware or electronics shop unless stated otherwise.

This all should not cost more than $40 all together. The binocular can be obtained at a photo repair shop where they probably have an abandoned half of a pair of binoculars.

Tools needed:


Here is the electric scheme:
                           6 V 60 mA bulb
  R=47 Ohms            ------(X)------------------
              \        |                         |
 + |--XXX---- X\X      |                         |
   -            \--------- O ------- XXX --------|
  ___                                            |
 - |                                             |
5..6 V      470 Ohms       10 mA     200 Ohms 1%
battery     potmeter      ammeter     resistor


When it is all assembled, then the heat goes on....
The readout takes place the following way: Point the pyrometer to the object to be measured, preferably a nonlustrous object (blank metal surface) and not at flames, because the latter are gases or scattered soot particles. The glowing filaments in the bulb in the pyrometer should be in the image of the object. Turn the potmeter so far till you hardly see the top (hottest part) of the bulb filaments. Then the temperature of the filaments is correct and you should read the current on the ammeter Determine which calibration temperatures you are going to use. If you have a thermocouple and a furnace with a stable temperature and the display of the thermocouple is correct, just adjust the furnace to a few round numbers (e.g. 600 C, 1000 C or 1000 F , 1800 F, etc.) and read the current for these values when you point the pyrometer at the thermcouple and the filaments of the bulb glow the same color.
When you don't have a thermocouple, melting (or freezing) metals is another option. The best are:

pure platimum                           1773 C / 3222 F
pure nickel                             1453 C / 2647 F
pure copper (use a reducing flame !)    1083 C / 1981 F
pure aluminum (calibrate with low light) 660 C / 1220 F
Point the pyrometer on a thin oxide layer, as the lustrous metal reflects too much and does not show exactly the proper color. Repeat each calibration once or twice to be sure about the correct temperature.

Write down all currents matching the temperatures and make an interpolation to make a new scale. Do this on a piece of paper with the same size at the ammeter scale.
When putting the scale on the ammeter, remove carefully the plastic cover of the ammeter and (don't touch the needle hand!) paste it carefully over the ampere scale.


Now your pyrometer is ready for use. When batteries get low, it will not affect the accuracy, because the display is just the voltage over the bulb and low batteries will just limit the range. When using high temperatures (over 1300 C) placing a welding filter between the light bulb and the eyepiece or in front of the front lens (objective) is a strong recommendation to protect your eyes. But not between the object and the light bulb, because then you filter the hot object, but not the light bulb.

Alternate sources for manufactured pyrometer equipment:
1.) The Pyrometer Instrument Company, Inc.
2.) Spectrodyne, Inc. Providers of the latest in disappearing filament Optical Pyrometers and Calibration Services.

Klaas van Ditzhuyzen

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Posted: 05/05/96
Rev: 08/01/2000