Q&A with Gas Sensors


1.  What is a ‘combustible gas mixture’?


Combustible gas mixture is the proper ration of fuel and air within the ignition limits.


2.  Explain the operating principle of a gas detection system.


Gas detection system measures the imbalance in the current loop caused by its active and in-active filaments in the presence of a combustible gas.


3.  Why is gas detection as important as fire detection?


Presence of combustible gas Is the first stage of a possible occurrence of a fire.


4.  What are the components used in a gas detection loop?


A gas sensor, 3 wire system and a gas monitor.


5.  What is a ‘flame arrester’ in a gas sensor? What does it do?


Flame arrester is used on the sensors head. The sensors are continuously powered to burn the combustible gas for measuring. The flame arrester prevents the occurrence of fire outside the sensor.


6.  What are the name of the filaments inside the gas sensor?


There are two filaments. They are called ‘active’ and ‘non-active’ filaments.


7.  How much approximately is the resistance of a gas sensors filament?


From manufacturer to manufacture the sensor resistance varies. It is approximately 3 ohms on each of the filament (refer to the station drawings for the exact parameters).


8.  How to check a gas monitor without test gas?


Some gas monitors are provided with a self-test facility to check its alarm and shutdown functions(levels).


9.  Why is a three-wire system used on the sensors?


The three-wire system is used to compensate the line resistance. The active and non-active filament loop form a whetstone bridge in the gas monitor. The amount of unbalance in the current loop during the occurrence of combustible gas is detected as the percentage of combustible gas.


10.  What is test gas used to calibrate a gas monitor?


Methane gas is used for calibrating a gas monitor.


11.  How much is the gas concentration in the test gas cylinder?


Generally, methane 2% by volume is used as a test gas to calibrate a gas monitor.


12.  What is the important factor set on a gas monitor loop?


Depending on the manufacturer, either it is the loop current or the voltage at the sensor head which has to be set precisely for better accuracy and sensitivity.


13.  why is it that Halon is not released on a high gas shutdown alarm in a gas turbine hood?


Halon (fire extinguishing agent) is not released on detection of combustible gas because the presence of gas is not a fire.