DESCRIPTION
The SST Model GIR901 Infrared Combustible Gas NOVA-Sensor is a completely self-
contained device that measures and displays the concentration of gas accumulated in a
protected area, performs local control functions, and transmits this information to a
central control point.
Principle of Operation
The GIR901 uses the absorption of hydrocarbons in the 3.3 μm to 3.5 μm range for the
detection of any hydrocarbon gas. The absorption of the hydrocarbon compounds is a
result of the oscillation of the H-C structure when exposed to energy. The GIR901 uses
an incandescent lamp pulsed at 4 Hz in conjunction with two piezoelectric light sensors
all contained in a stainless steel plug-in housing. In a patented configuration, the sensors
are close to each other and are exposed to the direct light from the lamp, with no mirrors
or other optical structures involved. In addition, almost no light is reflected by the walls of
the housing thus eliminating all possible influences from wall contamination. The active
sensor has an optical filter tuned to 3.4 μm; the reference sensor sees the wide-band
infrared signal from the lamp.
The presence of hydrocarbons will decrease the signal to the active sensor significantly,
while the signal to the reference sensor is almost unchanged. The electronics module
contained in the GIR901 constantly measures the signals from both sensors, averages
them to filter out noise and then calculates the ratio of the averaged active and reference
signals. With a constant length of the optical axis, the relation between the active/refer-
ence ratio and the actual gas concentration is logarithmic. The length of the optical path
yields an output for hydrocarbons that allows measurements with a resolution of 1%-2%
of reading. The ratiometric principle makes it possible to eliminate almost all effects of
temperature, aging, or fogging that would otherwise spoil the measurement. The optical
operation principle has the following advantages:
• Readings independent of Oxygen, wind, etc.
• Readings NOT influenced by cross-sensitivity and poisoning
• No burn-out, saturation, shift or drift when the detector is exposed to high gas
concentrations
Each SST NOVA-Sensor includes a high reliability microcontroller based transmitter/con-
troller in the associated explosion proof junction box. A digital read-out is provided to
continuously display operating status and the actual concentration of gas present in
percentage of the Lower Explosive Limit (%LEL). The transmitter converts this reading
to a standard 4-20 mA signal. This signal may be connected to a suitable SST NOVA-
5000 Gas Detection Module, or to any other device with a standard 4-20 mA input.
Connections between the transmitter and control device are normally made with 3
conductor cable [+24 VDC, 24 V return, 4-20 mA signal]. Dry contact relay outputs are
provided for low alarm, high alarm, and fault conditions. The low and high relays operate
at user adjustable alarm setpoints; the fault relay operates upon loss of power or internal
failure of the unit. Relays are suitable for controlling local HVAC or equipment shutdown.
MODEL GIR901 INFRARED COMBUSTIBLE GAS NOVA-SENSOR
901-2 June 2011 901-2
