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3 Start-up and Theory of Operation Model 7300A
3-4
Teledyne Analytical Instruments
These lamps are typically rated for 20,000 hours continuous operation when
run in the DC mode (+5VDC). This collimated energy is directed through
parallel infrared beams. The radiant energy passes through tubes containing a
continuously flowing sample gas. As the beam passes through the reference
tube, the energy of the beam is unattenuated and balanced which provides a
standard of source energy output for comparison. The gas of interest is
present in the measurement tube, however, the energy of the beam will be
attenuated at the specific absorption wavelength.
After passing through the two tubes, the radiant beams are reflected and
imaged by a second concave mirror onto a photon detector. A spectral filter,
located just in front of the detector, represents the precise “window” of the
absorption band for the specific gas of interest. Energy outside the band is
eliminated.
The detector converts the optical energy from the radiant beams into an
electrical signal which is modulated in proportion to the gas concentration.
Thermistor
Heater
Temperature
Control
Fine
Adjustment
Coarse
Adjustment
Auto-
Range
M
U
X
A to D
Converter
Digitial to
Analog
Converter
(DAC)
0-1 V dc
Concentration
and Range
4-20 mA dc
Concentration
and Range
Difference
Amplifier
Auto Zero
Amplifier
Central
Processing
Unit
(CPU)
Alarm 1
Alarm 2
System
Failure
Alarm
RS-232
Range
Contacts (4)
External
Valve
Control
Remote Span
Control
Remote Zero
Control
Cal
Contact
Displays
Processing
Power
Supply
A to D Conv
To CPU
IR
Source
Preamp
Preamp
Detector
Detector
Block Diagram of the Model 7300A Electronics