Constant current and constant Voltage Gas detector 2

[DistContProc]

Hii All
we are using Catalytic gas detector for fire detection system in our turbine enclosure, and currently vendor supply 2 two type gas detector :
1. Constant Volatge and
2. Constant current

but i do not know the diference, but according to manual, both using the same technic "Whitstone Bridge", i want a further information on this, if anyone have experience and have applied them in the system.

thanks in advance

 

[roydm]

Some combustible gas detectprs work by sensing the temperature of a thermistor or RTD treated with a catalyst. As the gas reacts with the catalyst it's temperature (resistance) changes.
It you hold the voltage constant, the current will change.
If you hold the current constant the voltage will change.
In a Wheatstone bridge circuit one leg of the bridge would have a temperature sensor with catalyst, the other leg a temperature sensor without catalyst, change in ambient temperature would have no effect on bridge balance but introduction of gas would
Hope this helps
Roy

 

[JLSeagull]

Constant voltage or constant current pertains to the temperature compensation techniques for the transducer excitation circuitry. As a user instead of circuit designer, obtain matched components for compatibility as constant voltage and constant current sensors are not compatible. Also check the product specifications to select models applicable to your needs. Others can address the advantages of one transducer circuit scheme over the other. The manufacturers should have a selection table that lists their compatible components.

The older combustible gas detection technology is often called catalytic bead. The "bead" in catalytic bead sensors amounts to a solid-state ceramic doped with a catalyst. The resistance of this type sensor changes in the presence of a combustible gas due to changes in the thermal conductivity. Disadvantages of this type sensor include a requirement for an oxygen rich environment and depletion of the material when exposed to combustible gas, thus requires routine replacement. An advantage is that it senses hydrogen. The literature for one of the commonly used manufacturers states that constant voltage or constant current sensor types are available. Some models of the 4-20 mA transmitter for these detectors work with the constant voltage sensors; other transmitter models require constant current sensors.

The newer infra-red detectors work in oxygen depleted environment, are not depleted when exposed to combustible gas but do not sense hydrogen. Most prefer the IR sensors over catalytic bead unless hydrogen is a potential component of the combustible gas.