A Discussion on Pellistor Gas Sensor Responses
Pellistor gas sensors have been used for several decades to monitor the presence of combustible gases. They provide a measure of the combustibility of the environment in which they are placed and are capable of detecting almost all combustible gases within their explosive ranges. Pellistors are used worldwide in all major applications requiring detection of explosive gas levels. This article discusses the science behind the pellistor
response, why different gases respond by differing amounts and some of the factors affecting the gas response.
The name “pellistor” stems from the original description of this sensor type as a “pelletised resistor”. It consists of two parts, a detector element and a compensator element, both of which are formed as beads on wire coils. The coils serve to heat the beads when electrical current is passed through them and also to detect changes in temp- erature of the beads caused by ambient conditions. A pellistor is, in effect, a type of calorimeter and to provide stability with known temperature character- istics the coils are typically made from platinum wire. The detector
bead contains a catalyst to promote combustion of gases and the compensator is poisoned to prevent such combustion. They are typically used in a Wheatstone bridge arrangement where the out of balance voltage from the bridge is the measurement used.
Compensator V Detector
Typically, a voltage supply powers the detector in series with the compensator and also powers balancing resistors that form the other half of the Wheatstone bridge. The out of balance voltage of the bridge circuit is measured and converted to a reading of %lel. The voltage supply heats the coils so that the beads are both raised to a temperature in the region of 500°C. In the presence of combus- tible gas, the catalyst within the detector element causes the gas to combust. This releases heat of combustion to the detector element and raises the temperature of the detector coil. This leads to an increase in the detector coil resistance and the voltage across the detector element rises. The rise in detector voltage results in an out-of-balance voltage signal from the Wheatstone bridge.
The basic relationship leading to the response from a pellistor can be described as
Power In + Heat From Combustion – Power Loss = Response Energy
The Power In is effectively the electrical power heating the coil. The Power Loss comprises heat losses through convection, radiation and conduction of heat down the lead wires of the coils to their mountings. The Response Energy is converted into a temperature rise of the beads and therefore a resistance rise of the coils. The Heat From Combustion is a function of the heat of combustion of the gas and the flux of gas.
The Power In can be described as V x I where V is the voltage across the coil and I is the current flowing through the coil. The Power Loss can be described as f ( TBead bead and TAmb
– TAmb ) where TBead is the temperature of the
is the ambient temperature. The conversion of Response Energy to temperature can be described as M x Cp x ΔTBead
= Response
Energy where M is the mass of the element, Cp is the specific heat of the element and ΔTBead
is the temperature rise of the element.
For an ideal compensator there is no Heat From Combustion as it is a poisoned bead and isolated from the Heat From Combustion generated by the detector. So the compensator temperature can be determined from
ΔTComp = ( ( V x I ) – f (TComp – TAmb ) ) / ( M x Cp )Comp
As electrical power is applied to the compensator, the temperature of the compensator rises until it reaches a point where the Power Loss balances the Power In. For a fixed amount of Power In the temperature of the compensator reaches a level from which it is then affected largely by the thermal conductivity of the environment and the ambient temperature. In the absence of combustible gas the same applies to the detector element and, provided the detector and compensator elements are matched, the voltage signal from the Wheatstone bridge remains constant with changes in ambient conditions.
For the detector element there is the added factor of the Heat From Combustion and this leads to a relationship where the detector temperature can be determined from
ΔTDet = ( ( V x I ) – f (TDet – TAmb ) + Heat From Combustion ) / ( M x Cp )Det
For a fixed amount of Power In the temperature of the detector reaches a level from which it is also affected by the thermal cond- uctivity of the environment and the ambient temperature. However, the temperature of the detector also increases in the presence of combustible gas and this results in the overall pellistor response.
The catalyst is required to reduce the activation energy, a term introduced by Svante Arrhenius in 1889, which is the minimum energy required in order for a reaction to proceed. The activation energy, Ea, is a potential energy barrier between reactants and products that must be overcome for the reaction to proceed. It is linked to the energy of a transition state that exists in the reaction mechanism. A catalyst lowers the energy of the transition state and therefore lowers the activation energy, allowing the reaction to proceed at lower energy. As the source of energy to drive the reaction is the heat generated by the coil, the use of a catalyst allows the reaction to proceed at lower electrical power.
As the voltage across a pellistor is increased the energy of the beads increases and as sufficient energy starts to become available to combust the gas the pellistor signal rises. With increasing voltage this signal reaches a plateau stage where the heat from the combustion is balanced by the heat losses and then with further increase in voltage the heat losses start to increase and the response drops off. A plot of this response for a fixed gas concentration versus voltage is commonly called a peaking curve. Normally the pellistor is designed so that the methane response is on the plateau region at the normal operating voltage. Methane has a higher activation energy for combustion than most other commonly encountered combustible gases and the plateau region for other gases is typically at a lower operating voltage than that for the plateau region for methane.
Firth, Jones and Jones1 described the response of a pellistor to different combustible gases in %lel terms by considering the heat of
IET
Annual Buyers Guide 2009
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