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Operational Theory 2
chamber while the rear chamber absorbs radiation at primarily weaker
absorption bands. The absorption causes the gas to heat up and the
differential nature of the absorption process causes the front chamber to
heat up more than the rear chamber. Since the chambers are charged with
gas, the pressure in the primary chamber becomes higher than in the
secondary chamber. This pressure differential causes a net flow of gas
from the primary chamber to the secondary chamber through a tiny orifice
connecting the 2 chambers. The gas cools in quick order and the flow
reverses until the pressures are once again equal.
A mass flow sensor is placed in the orifice between the 2 chambers
and senses the mass transport between them. It is designed in such a
manner as to be able to sense minute flows in either direction. The sensor
produces a signal resulting from an electronic imbalance each time mass
flow is detected (in either direction) through the orifice. The signal is
passed along to a preamplifier and then to a voltage to frequency converter
for enhanced signal processing. The microcontroller retains this
information as a zero gas reading for calibration and offset in real
measurements.
When the process is repeated and a span gas is introduced into the
sample cell, a slightly different condition exists. Now IR absorption takes
place within the sample cell. Less energy is received at the detector. But
since the primary chamber is smaller than the secondary chamber and
differential absorption takes place at predominately strongly absorbing
wavelengths within the primary chamber, the difference in energy of the
gas in the primary chamber is less than when there is no IR absorption in
the sample cell. The energy of the gas in the secondary chamber is also
less but the change is not as dramatic. Hence the patented balanced design
detector produces a different signal when an IR absorbing gas is
introduced in the sample cell. The resulting signal is inversely related to
the concentration of the gas of interest in the sample cell.
Between the IR window and the detector is the filter cell. Depending
on the nature of the sample gas, some applications could experience
interference in the absorption band spectra. For instance, both CO and CO
absorb at wavelengths in the IR very close to each other. The presence of
CO
could produce a measurement error in a system designed to detect
2
CO. The filter cell is a sealed volume of gas specifically designed to
"comb out" the offending absorption line or lines before the radiation
reaches the detector. The filter cell in some cases acts as a thermal barrier
to keep the detector from experiencing sudden temperature fluctuations.
10
Teledyne Analytical Instruments., - Rev. 3
IR7000
2
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