Det-Tronics PW9200x Pathwatch IR Gas Detection System - Manual (Page 17)

B. The hydrocarbon vapor cloud is smaller than the
path length.
NOTE
If the cloud length cannot be approximated from
the installation parameters, it must be assumed
that the cloud length is equal to the path length.
The calibration graphs provided in the APPENDIX
correlate the RATIO output voltage to the PPM-M
product. To determine the average vapor concentra-
tion in the cloud or path, divide the PPM-M product by
the cloud or path length in meters.
For Example:
Condition A - Cloud larger than path, or cloud length
unknown. (Gas is propane, PathWatch
has Heavy HC filters. See Figure A6.)
Assume: - RATIO output of 2 vdc
- Path length of 50 meters
- (cloud is equal to path)
Read: Calibration Graph indicates
that 2 vdc output corresponds
to approximately 1000 PPM-M
Then: 1000 PPM-M ÷ 50 M
Result: 20 PPM average vapor con-
centration over the 50 meter
path.
Condition B - Cloud smaller than path
Assume: – RATIO output of 2 vdc
– Cloud length of 10 meters
Read: Calibration graph indicates
that 2 vdc output corresponds
to approximately 1000 PPM-M
Then: 1000 PPM-M ÷ 10M
Result: 100 PPM average vapor con-
centration over the 10 meter
cloud.
All factory calibrations are performed with the SENSI-
TIVITY switch set in the HI position (maximum sensi-
tivity).
HYDROCARBON VAPOR
ALARM THRESHOLD ADJUSTMENT
In critical applications, it is recommended that the
calibration be checked before setting the alarm
threshold. Based on the specific application of the
instrument, select the concentration in ppm at which
the Hydrocarbon Vapor Alarm will activate. It is
important to remember that the open path detector
measures the average gas concentration over a con-
tinuous series of points along a path. Because the
gas concentration over a 100 to 300 foot path could
vary from only a few ppm to explosive levels and
because the user must know when the gas concen-
tration at any location along the path is approaching
an explosive level, the threshold must be set at a
much lower level than a point detector. Typically, the
open path detector is set to alarm for an average con-
centration in the 0.05% to 0.10% LFL range for heavy
hydrocarbons.
To determine the equivalent RATIO output (volts) for
the selected concentration (ppm), refer to the calibra-
tion graph provided in the APPENDIX.
1. Multiply the path length (M) by the selected vapor
concentration (PPM) to arrive at PPM-M.
NOTE
If the cloud length can be approximated from the
installation conditions, a more accurate result is
obtained by multiplying the CLOUD length (M) by
the selected vapor concentration (PPM) to arrive
at PPM-M.
2. From the calibration graph, read the equivalent
RATIO output voltage for the PPM-M product
determined in Step 1.
To set the hydrocarbon vapor alarm threshold:
3. Attach a digital voltmeter between the HC ALARM
SETPOINT jack (grey) and SIG GND (green) on
the front panel of the receiver.
4. Adjust the HC ALARM ADJUST potentiometer on
the front panel so that the Hydrocarbon Alarm set
point voltage matches the RATIO output deter-
mined from the calibration graph in Step 2,
above.
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