Teledyne 7300a - NDIR analyzer - Manual (Page 31)

Infrared Gas Analyzer Start-up and Operation
3-5 Teledyne Analytical Instruments
3.4 Circuit Description
The Teledyne Analytical Instruments IR bench is a multiple wave-
length, single beam design. It uses a quadruple detector that consists of a
specially designed, patented thermopile with small IR filters mounted in front
of it to produce independent voltages. These correspond to the transmission
of energy at each filter’s wavelength. They are amplified by special high
stability operational amplifiers to produce the output of the device.
IR energy is generated by a filament (IR source) operating at a moder-
ately elevated temperature.
The cell cavity allows the sample gases to pass between the IR source
and the detector. Sapphire cell windows allow passage of this IR energy
with minimal absorption. The gases that pass through the cell absorb energy
of the IR band at different wavelengths within the spectrum.
An optical filter is mounted on the face of each detector. Each detector
responds to a particular wavelength, producing a voltage proportional to the
energy that has passed through the gas and filter. If the gas concentration
changes, the energy at the measuring wavelength changes, changing only the
measuring voltage output. The reference voltage output is not affected by the
gas concentration changes. This “measurement” voltage change is electroni-
cally processed as the instrument’s output signal
IR Bench Block Diagram