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SELECTIVITY/CROSS SENSITIVITY
The electrochemical sensors are made to be as selective as
possible for a single gas, but in some cases interfere with
different gases, i.e a gas reacts to a sensor which is originally
intended for a different gas.
This is important to remember when designing
a gas warning system that will measure several gases.
Normally, this is not a problem since it is
rare that gases which interfere with each other occur together.
In many cases where interference occurs, this
is less important since an alarm will in any case tell you that
something has happened:
Exemple:
One of the gases that appears to be the most disturbing on
different sensors is nitrogen dioxide (NO2),
which interferes strongly in a negative way on a sulfur dioxide (SO2)
sensor.
If there is a risk of leakage of NO2
and SO2, both NO2
and SO2 sensors will be used.
If there is leakage of both gases
simultaneously, the NO2 gas
will, because of its negative effect on the SO2
sensor, reduce SO2
measurement, which may mean that the equipment does not give an
alarm for SO2, despite the
fact that the concentration is above the alarm limit.
However, this is an extremely unusual case,
and you will in any case have an NO2
alarm indicating that personal danger exists and this may be enough
and is also accepted by many users.
This means that you can often make use of a
sensor´s cross-sensitivity to other gases, and that you can
detect several gases with one sensor.
LIST OF CROSS SENSITIVITY
The list below gives an you idea of the cross sensitivity of
standard sensors.
When the interference has poor repeatability,
ie. interference may vary from time to time when exposed to a
particular gas, the following figures may be used as approximate
values. |
