There are many ways researchers describe the performance of semiconductor gas sensors. What was sensitivity earlier is now termed Response but the notation as "S". Though one understands that sensitivity is now accepted as response over concentration, there exists several ways of defining response(Rg/Ra, DeltaR/Ra, Ra/Rg). Even the transient curves are sometimes plotted as Response vs Time curves. There appears no standard way to define a response. In that case, how does one compare the performance of two sensors? This varies greatly with journal to journal and school of thoughts of the respective researchers. Comparison tables in many journals don't convey full meaning since they show partial data which suits a particular way of expression. How does one characterize the performance and compare them?
Yes, it is a big problem about definition of gas sensors performances. It also depend on the type of semiconductor (n-type or p-type). The most simple way is the representation of gas response (for n-type as Rair/Rgas and for p-type as Rgas/Rair, where Rair is the resistance in air and Rgas is the resistance under exposure to gas). But this parameter does not take into consideration the concentration of gas. Thus, many authors provide the table with comparation of gas sensors parameters with indication of gas concentration. The sensitivity is a parameter which takes into account the concentration of gas and is defined as [(Rair-Rgas)/Rair]*[100%/concetration] for n-type and as [(Rgas-Rair)/Rair]*[100%/concetration] for p-type, which gives the %/ppm.
The many authors give only gas response, usually as ratio of resistances as was mentioned becuase it is a more easier way.
If you will compare the gas sensors using gas response you need to take into account the concentration of gas.
Yes, it is a big problem about definition of gas sensors performances. It also depend on the type of semiconductor (n-type or p-type). The most simple way is the representation of gas response (for n-type as Rair/Rgas and for p-type as Rgas/Rair, where Rair is the resistance in air and Rgas is the resistance under exposure to gas). But this parameter does not take into consideration the concentration of gas. Thus, many authors provide the table with comparation of gas sensors parameters with indication of gas concentration. The sensitivity is a parameter which takes into account the concentration of gas and is defined as [(Rair-Rgas)/Rair]*[100%/concetration] for n-type and as [(Rgas-Rair)/Rair]*[100%/concetration] for p-type, which gives the %/ppm.
The many authors give only gas response, usually as ratio of resistances as was mentioned becuase it is a more easier way.
If you will compare the gas sensors using gas response you need to take into account the concentration of gas.
I agree in principle with Vasile, but another parameter that has to be taken into account when comparing two sensors is the operating temperature.
In general, there is an optimum temperature for the maximum response for each pair sensor material/sensing gas, for the same gas concentration.
As the overall performance is concerned, the so-called 3S-criterium is considered:
Sensitivity-Stability-Selectivity.
One may have a look into our work on sensors in
www.laser-applications.eu
What Vasile tried to explain is the fundamental parameter of gas sensors.
In fact, the ratio of the resistance of the sensor in air and with the sensing gas in air simply defines the response of the gas sensor ( when multiplied by 100 is known as percentage response.
But gas sensitivity is the response per unit concentration of the sensing gas. To find out the gas sensitivity the response parameter has to be divided by the concentration of the gas mixed with air during sensing measurements. This is as simple as that.
.....at a given operating temperature.
Besides, this definition of sensitivity holds only when there exists a linearity between the response and the concentration (constant slope). Otherwise this definition of sensitivity is wrong.
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