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Wie funktionieren Quotientenpyrometer?

Es gibt einige Anwendungen, bei denen ein Standard-Einkanalthermometer die Temperatur nicht korrekt misst. Dazu gehören Anwendungen bei denen:

  1. Das Messobjekt kleiner als der Messfleck ist.
  2. Die Sicht auf das Messobjekt durch Staub, Rauch oder Dampf beeinträchtigt ist.
  3. Die Fenster schmutzig bzw. schwer sauber zu haltende sind.
  4. Der Emissionsgrad sich während der Messung ändert.

Quotientenpyrometer (auch Zweifarben- oder Verhältnispyrometer genannt), können die Temperatur auch unter solchen problematischen Bedingungen korrekt messen.

Was ist der Unterschied zwischen einem Zweifarben- und einem Einfarbenpyrometer? Bei einem Zweifarbenpyrometer werden zwei Detektoren verwendet, die beide auf verschiedenen - aber dicht zusammenliegenden - Wellenlängen arbeiten, und auf dasselbe Ziel ausgerichtet sind.

Implementation scheme for a ratio pyrometer using 2 detectors in a sandwich structure

Implementation scheme for a ratio pyrometer using 2 detectors in a sandwich structure

 

No Signal Attenuation

Let’s first consider the blue graph in the example below. The two-color thermometer is looking at a blackbody with an emissivity value of 1.0 and a blackbody temperature of 1500°C. Based on the Planck law, the two detectors are providing the following energy units in accordance to the blue curve at an emissivity value of 1.0:

Detector #1 at wavelength λ1 will give an output of 500 units.

Detector #2 at different wavelength λ2 will output a signal of 1000 units.

Since this is a ratio thermometer, we divide 1000 by 500 and get a ratio of 2.  The instrument is calibrated in a way to read 1500°C when it sees a ratio of 2.

Planck curves for the ratio thermometer looking with two detectors at a blackbody at a temperature of 1500°C

Planck curves for the ratio thermometer looking with two detectors at a blackbody at a temperature of 1500°C

 

Signal Attenuation

Now what happens if somehow the signal from the hot target is reduced or prevented from getting to the detector?  This could be caused by a dirty window, object too small to fill the cone-of-vision, or maybe there is smoke in the line of sight.  The brown graph depicts an example with a 90% signal loss, but the target temperature is still 1500°C.  This is the same as having the apparent emissivity drop from 1.0 to 0.1.

Detector #1 will output a signal of 50 units.

Detector #2 will output a signal of 100 units.

Both signals have been reduced by 90% as compared to the upper curve (E=1.0). Note that 100 divided by 50 is again a ratio of 2, or the instrument will read 1500°C even though we have lost 90% of the signal. Every two-color thermometer has a limit as to how much signal can be reduced.  This is called the attenuation which can vary from 0% to as high as 95% of the signal and still read an accurate temperature.

 

E-Slope

Basically, a two-color thermometer works properly as long as whatever affects one wavelength, must affect the other wavelength the same amount.  Unfortunately, there are applications where the object emissivity is different for the two wavelengths for example measuring molten metals.  When the two-color thermometer looks at the molten metal, the signal ratio (or slope) will be incorrect and an error will occur in the temperature reading.

How can this be corrected?  All two-color thermometers have an adjustment called E-Slope.  When viewing the molten metal, the E-Slope adjustment is turned until the instrument reads the correct metal temperature.  The correct temperature may be obtained by using a disposable thermocouple.  This E-Slope adjustment simply corrects the ratio by a constant which corrects the instrument indication for the unequal spectral emissivities of the target.  Once the E-Slope is adjusted, then the problems of smoke, steam, dust, small targets, etc., are handled correctly by the instrument.

 

Next Article: Accurate Noncontact Infrared Temperature Measurement >>