1-4 Chapter 1: Description of functions
SDE_U14
Physical principles, Fortsetzung
IR radiation
• Generated by broad-band emitter
• Emitted as a beam package alternately in the form of a sample and reference
beam through the sample and reference chambers of the sample cell and is
partially absorbed by the sample component molecules
• Counterphase modulation by means of a motorized aperture wheel
• Both modulated beam packages appear alternately at the infrared detector
Apertures • Created by applicable regulation of the sample and reference beam balance
Sample cell • Depending on the application, the sample chamber receives a sample, zero-point
or end-point gas flow so that a part of the infrared radiation is absorbed in a
concentration-dependent manner
˝
• The radiation passes unhindered if the reference chamber is filled with a gas that
does not absorb infrared (N
2
).
Infrared detector • A two-part transmission detector with front and rear chambers filled with the gas
components to be measured, with selectivity being determined by the infrared
detector. The two chambers are separated by an infrared-transparent window.
Additionally, the two chambers are separated by a stressed metal membrane with
counterelectrodes. This unit is known as the diaphragm capacitor.
•It reacts in the following manner in the presence of the sample component:
• IR radiation is weakened in the sample cell's sample chamber and enters the
receiver's front chamber.
• The equilibrium between the sample and reference beams initially established
by calibration and the aperture is now disturbed.
• There is an energy difference (temperature change) in the form of reduced
pressure in the front chamber.
• This pressure reduction is transformed into a capacitance change in the
membrane capacitor by deflecting the metal diaphragm.
• Since the diaphragm capacitor is connected to a high-impedance DC voltage, a
corresponding periodic AC signal is generated.
Fortsetzung auf der folgenden Seite
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