Honeywell ACM 150 Manual To Installation, Operation, And Maintenance page 79

ACM 150
Noted by: Date/Time:
Note
Store spectra of all Alarm 1/Alarm 2 Incident(s).
For an expert evaluation, contact your local representative.
You can e-mail files directly to Honeywell Analytics.
Signal-to-noise ratio
Important warning and disclaimer:
Enabling signal-to-noise ratio (SNR) on samples could lead to a situation in which
legitimate gas detection is inhibited. Any gas in the sample cell absorbs in a given
infrared (IR) region. A Fourier Transform Infrared (FTIR) uses this infrared absorption
to detect gases. However, if the infrared absorption is in an SNR region, it decreases
the infrared signal and therefore decreases the SNR. If infrared absorption is
decreased below the SNR cutoff, the ACM software can classify the spectrum as
"low SNR" and reject it before the spectrum is analyzed for gas concentrations. This
is a potential false negative reading of gas1, caused by an innocuous interfering
gas2.
For example, if the following noise areas are chosen for SNR calculation with an SNR
setpoint of 100:
• Sample/Background RMS* Noise Area 1 = 968-1008, 100
• Sample/Background RMS Noise Area 2 = 2480-2530, 100
• Sample/Background RMS Noise Area 3 = 4380-4430, 100
*RMS = root mean square
The ACM 150 monitor software will check the RMS SNR over these regions and, if
configured to do so, will ignore readings of any gas if the SNR reading in any of the
three regions is <100. However, a release of a 250 PPM concentration of C
559-40-0, ACGIH TWA 2.5 PPM) will absorb in RMS Noise Area 1, decreasing the
SNR from over 500 to less than 10. Since 10 < SNR setpoint of 100, the ACM 150
monitor will ignore the 250 PPM reading of C
the other fourteen gases scanned at that point.
Enabling SNR in the background does not inhibit gas detection. When the
background signal-to-noise ratio is calculated on "background gas," there are no
interfering gases since backgrounds are done with dry nitrogen.
F and gas concentration readings of
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