71
MAN0530 Issue 09 October 03
Searchline Excel
2104M0506
4. CROSS-DUCT EXCEL
Ensure that before using the gassing cell for the first time, remove the ends of the plastic
nozzles (with a knife or wire cutters). Rubber end caps are used to seal the tubes afterwards.
Air Shield
Mounting
Cell
Receiver
Transmitter
Gas In/Out
To get the best accuracy when using the gassing cell:
(1)
Ensure that the gassing cell contains fresh air, and perform a zero calibration on the
Excel
(2)
Apply the test gas and allow sufficient time to fully fill the gassing cell and check that the
Searchline Excel output stabilises.
A flow rate of less than 0.5 litres per minute is advisable, to avoid pressurising the gassing
cell.
(3)
For a methane calibrated unit tested with 100%v/v methane, the expected reading can
be calculated from:
Gas reading = 100%LEL / (2 x duct width)
A few examples are shown in the following table, and the tolerance to be applied is
±
20% of reading or
±
10%LEL (whichever is greater). The variability in the readings are
due to a number of factors such as variation in air pressure, flow rate, and tolerances in
the gas composition.
Test Gas
Duct Width
Expected Reading
* Expected Reading (FM)
100% v/v Methane
0.5m
100%LEL
±
20%LEL
100%LEL/m
±
20%LEL/m
100% v/v Methane
0.8m
60%LEL
±
12%LEL
60%LEL/m
±
12%LEL/m
100% v/v Methane
1.0m
50%LEL
±
10%LEL
50%LEL/m
±
10%LEL/m
100% v/v Methane
1.5m
33%LEL
±
10%LEL
33%LEL/m
±
10%LEL/m
100% v/v Methane
2.0m
25%LEL
±
10%LEL
25%LEL/m
±
10%LEL/m
* For a definition of %LEL/m units for the FM version of the Excel Cross-Duct see
Appendix B
.
(4)
After checking the reading, ensure that the gas is flushed out of the gassing cell, with
clean air or nitrogen, until the reading returns to zero.
(5)
Finally replace the rubber caps over the gassing ports.
