For toxic sensors (CO, NO, NO2, SO2 etc.) two years lifetime is common. Experience shows that one can expect these electrochemical sensors to last about three years if the instruments are maintained properly, and the concentrations the sensors are exposed to are within the range regulations in most developed countries require.
There are, however, a number of factors that will affect he lifetime of electrochemical sensors. They will keep longer if stored at a lower temperature. It is advisable to keep them at a temperature of 20°C or less. Spare sensors are often stored in cellars (basements) or even in refrigerators. They must not be frozen, or the electrolyte will split the housing, but it is quite possible to keep them at about 4°C. Oxygen sensors will react wit hthe atmospheric air. Since this has a high concentration of oxygen, they will, perversely, wear out quicker in storage than in use. If an oxygen sensor is to be out of use for a longer period of time, then it can be removed from the instrument and stored in nitrogen. Theoretically, the whole instrument may be stored in nitrogen, but this is seldom practicable.
The toxic sensors will last longer if the system is thoroughly flushed with fresh air after measurements have been carried out. The time they last will be very dependent on the ranges they have to measure. If they are constantly at thelimit of range, then the lifetime will be reduced dramatically. In such cases it may be possible to use a sensor for a higher range, although this will necessarily reduce the sensitivity and hence the resolution of the system. An electrochemical sensor requires a certain level of humidity, otherwise the electrolyte will dry out and the membrane will be damaged. In flue gas applications this is not a problem, since there is sufficient water vapour present, but may be a factor in other applications. Here, it is probably possible to switch the air flow to ambient air at regular intervals and hence supply the essential water vapour pressure for a certain period of time. Failure to do so will lead to premature failure of the electrochemical sensor and unreliable results in the time beforehand. The sensor does not require much moisture, but completely dry gas is definitely not good.
Another factor that affects the lifetime of these sensors is cross sensitivity. If a component is present in high concentrations which causes a signal to be produced by the sensor, then this will consume the electrolyte of the sensor and reduce the lifetime accordingly. This may not be noticed if the instrument has a cross sensitivity compensation. It is nevertheless a much underestimated factor, as is the effect of extreme temperatures. The active part of a sensor is an electrode and electrolyte. If the electrolyte dries out, then the sensor will no longer function. For this reason, systems using electrochemical sensors should not be operated wit hcompletely dry gases. The level generally quoted is 5 % rH. This will keep the sensor in best condition and operating accurately.