Question

In optical pyrometers temperature is measured by

• A. Thermocouple effect
• B. Photocell principle
• C. Comparison of brightness of the source with that of a standard source
• D. PV cell effect

Hint:

Optical pyrometers are best for temperatures above 700°C because they rely on the object being "red hot" (emitting visible light) to perform the brightness comparison.

Answer 1

The Correct Option is C

1. Operating Concept: The device works on the principle that the intensity of light (brightness) emitted by a hot object depends on its temperature. It utilizes a "disappearing filament" technique.

2. The Comparison Process:

• The user looks through the pyrometer at the target hot source. Inside the device, there is a calibrated tungsten filament (the standard source).

• The current through this filament is adjusted until its brightness matches the brightness of the target source.

• When the brightnesses match, the filament "disappears" against the background of the target source.