Spectrex Series 300 Open-Path Gas Detection System - Manual (Page 12)

Spectrex Inc. - SafEye TM 300 Gas Detector Manual – TM 794100, Rev. D, May 2004
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2.3.5 Gas Sensitivity
The SafEye IR Model uses wavelengths around 3.4µ spectral band to measure
air flammability potential between the source and detector. At this wavelength,
all hydrocarbon materials have a strong absorption peak. This peak enables the
detector to achieve both regular sensitivity of 0-5 LEL.m or high sensitivity of 0-2
LEL.m according to functions set up.
In the short-range duct Model 301, the standard full scale is 2.5 LEL.m or 1
LEL.m by DIPSwitch.
However, since the desired detection information is air flammability and the
actual measurement is radiation absorption around the 3.4µ spectral band, the
detector has a different sensitivity to different gases or gas combinations.
This difference in sensitivity is irrelevant if in the protected area the gas
composition is known. However, if in a protected area the gas composition can
vary substantially, then the difference in sensitivity should be considered in
determining the Detector calibration.
2.3.6 Gas and Mixture Selection and Setting
At the 3.4µ spectral band of the SafEye, the least sensiti ve gas is pure (100%
vol) methane and the most sensitive gases are various mixtures of Methane with
heavier alkanes where the Methane percentage is less than 90%. For pure
Ethane the sensitivity is close to the high sensitivity gases and for pure Propane
it lies somewhere between the two extremes. Every SafEye has 4 built-in gas
calibration settings that can be changed by function set up The setting for each
Safeye is detailed on the calibration datasheet provided with each unit.
The calibration settings are designed for use in various applications:
Gas 1 - Pure methane and is for use in methane storage and piping
applications
Gas 2 - 92% methane, 4% propane and 4% ethane (default) Universal oil
and gas production mixture to be used in all cases where methane
concentration in the mixture does not exceed 98%. It can also be
used for pure Ethane applications.
Gas 3 - Pure propane and is good for propane/butane detection.
Gas 4 - 99% methane and 1% propane and is for use in detecting
methane mixture with heavier gases where the methane
component can vary between 100% to 95%. It is also good for
protecting areas where a leak can be either pure methane or pure
propane.
The four internal gas calibrations cover most of the flammable gas detection
applications. Actual selections should be made by the user in consultation with
experts to provide for safety requirements. However, for special cases where