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MAN0530 Issue 09 October 03 Searchline Excel
2104M0506
An open path alarm set at 1 LEL.m would be triggered by any of the situations shown below:
100% 50% 10%LEL
LEL LEL
1m 2m 10m
LEL.m monitoring is particularly beneficial while the Excel gas detector is protecting the perimeter
of a plant or process, often reducing the quantity of point detectors required. The open path
LEL.m system has the ability to detect leaks that point detectors may miss due to prevailing or
changing wind directions, and can provide early warning of a dilute gas cloud when point detectors
may not register any gas presence.
Path Averaged %LEL (%LEL/m)
FM versions of Excel Cross-Duct signal a reading which is the path averaged %LEL (Units of
measure are %LEL/m). This reading is the average %LEL concentration in the duct, based
upon the assumption that the gas concentration in the duct is uniform. Effectively, the reading is
calculated as below:
Path Av %LEL = Number of LEL.m in duct x 100 (%)
Duct Width (m)
Example
In a 1 metre duct, a 10%LEL/m reading will be signalled for 1 metre of 10%LEL gas, or
0.5 metres of 20%LEL gas, or 0.1 metres of 100%LEL gas. (This behaviour is similar to that of
conventional open path gas detectors which output readings in LEL.m, see previous definition)
Recommended alarm thresholds are 20 %LEL/m and 40%LEL/m.
When readings are displayed on the FM version of the SHC1 Handheld Interrogator, the %LEL/m
unit is displayed as %L/m.
B.3 ABBREVIATIONS
CENELEC European Committee Standardisation
CSA Canadian Standards Association
DSP Digital Signal Processor
EMC Electro-Magnetic Compatibility
APPENDIX B - GLOSSARY
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