SafEye
™
™
Series 200 IR Gas Detector - User’s and Maintenance Manual
SPETCREX INC.
Doc. No. TM792100, Rev. B, Oct 1997
- 3 -
1. 4. 3.
Microprocessor Based
The incoming signals are analyzed by the built in microprocessor. A sophisticated
mathematical algorithm that calculates between the various functions of the detected
signal thresholds, and rate of rise is employed. Statistics, ratio algorithms, correlation,
data communications, diagnostics and other functions are performed.
1. 4. 4.
GAS SENSITIVITY
The Safeye IR Model uses wavelengths around 3.4
µ
spectral band to measure air
flammability potential between 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
x
m and high sensitivity of
0-2 LEL
x
m according dip switch setting. However since the desired detection
information is air flammability and the actual measurement is radiation absorption
around the 3.4
µ
spectral band, the detector has a difference in 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.
Under/Over Estimation:
In cases where a detector is calibrated to the least sensitive gas composition possible
in a protected area, then it will over estimate the actual flammability of all other gas
compositions. For example, if calibrated for methane and the mixture is 98%
Methane, 2% Propane, the over estimation factor will be 2.5. The detector will read
3 LEL
x
m, when the actual gas concentration mixture between source and detector is
1.2 LEL
x
m. Calibrating the detector to the above mentioned gas mixture may result
in under estimation of actual air flammability for pure methane. An under estimation
of 0.4 will result in the detector reading 3 LEL
x
m only when the actual gas
concentration between source and detector is 7.5 LEL
x
m.
From the above, it is clear that over estimations can result in false alarms and under
estimations can result in the detector not responding to a real alarm situation, hence in
a protected area a substantial under estimation of gas calibration is inadmissible while
a substantial over estimation should be avoided if possible.
1. 4. 5.
Gas and Mixture Selection and Setting
At the 3.4
µ
spectral band of the Safeye, the least sensitive gas is 100% methane and
the highest sensitive gases are various mixtures of Methane with heavier paraffins
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 a built-in 4 gas calibration settings that can be
changed by dip switches.
