Bacharach RDM800 - Manual (Page 14)

HGM300 Refrigerant Gas Leak Monitor
NDIR Bench Measurement Description
Pyroelectric Detector
Sample Flow
NDIR Source Dual Optical Bandwidth Filters
Gas passing through the bench absorbs some I-R energy. Each of the 24 gasses in the onboard library
has a very specific absorption signature defining the amount of energy any of these gasses will absorb in
relation to the concentration present at the time of measurement. In order to increase accuracy and
eliminate false alarms a narrow (8.1 to 9.4 uM) optical bandwidth filter is used to limit the optical
spectrum to only those gasses listed in our library. All gasses outside of this bandwidth are ignored. The
output voltage from the pyroelectric detector is processed to eliminate any extraneous noise, amplified,
and digitized. This information is then sent to the onboard microcontroller, which analyzes and displays
the data. The unit is purged automatically which brings in a clean air sample. This air sample contains no
refrigerant gas, therefore, does not absorb any I-R energy. The energy measured in this purge sample is
used as a reference or baseline, when calculating PPM values for actual input channels, which allows us
to compensate for any changes that may occur over time in the measurement electronic circuitry, I-R
source, or I-R detector. This also eliminates for field calibration of the unit. All measurements are
temperature and pressure compensated to standard conditions to eliminate reading variability due to the
climate or altitude of the unit.
What specifically makes the HGM300 Single-Beam-Single-Wavelength NDIR measuring bench so
different?
1.) We gold plate the inside of the sampling tube. Gold is virtually untarnishable so this improves the
reflecting properties of the optical path, which improves overall measurement bench sensitivity and
accuracy.
2.) Because the optical properties have been improved we can reduce lamp intensity thereby improving
the I-R source life, which improves long-term reliability. We estimate source life to be beyond 5
years.
3.) I-R source modulation is required of all such devices. We do this electronically thereby eliminating
any mechanical parts and improving long-term reliability.
4.) Sample temperature changes can directly affect short-term measurement stability. Our auto
rezeroing function engages every 10 minutes or whenever the sample temperature changes by more
than 0.2 deg. C, or the enclosure interior temperature changes by more than 2.0 deg. C. Thereby,
improving measurement stability and accuracy, and completely eliminating the effects of sample-
component temperature drift anywhere in the monitor.
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