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   Updated: 20 Sep 2018
 

 

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GAS DETECTION - CROSS SENSITIVITY

 

 

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.

 

SENSOR O2 CO CO/F H2S SO2 NO NO2
Std. measurement
range
25% 250 ppm 250 ppm 25 ppm 10 ppm 100 ppm 10 ppm
Max ppm measurem.
range
- 2000 2000 1000 500 500 100
Exposure indication of 100 ppm of the following gases (ppm)
CO   / / 7 0 0 0
H2S   340 10 / 200 35 -20
SO2 * 85 10 20 / 5 0
NO   30 10 0 0 / 0
NO2 * -60 10 -20 / 5 0
H2   40 40 5 0 0 0
Cl2   -5 0 -20 -15 0 90
CH=CH   100 100 0 180 0 0
CO2 * 0 0 0 0 0 0
HCN   20 0 0 30 0  
HCl * 5 0 0 30   0
 

* Acid gases such as SO2, NO2 and HCl including CO2 will increase the reading on the oxygen sensor.

Exemple:
1 % CO2 will give a 0.3% higher measurement reading, ie. at an oxygen concentration of 21%, the sensor will measure 21.6%.

Because of the molecular weight of carbon dioxide this increase will be offset so that the measured value will be fairly accurate. 25% CO2 or more will reduce the life time of the 02 sensor.

Blank cells indicate that data is missing.

 
SENSOR CL2 HCl HCN NH3
Std. measurement
range
10 ppm 10 ppm 25 ppm 50 ppm
Max ppm measurem.
range
20 100 100 200
Exposure indication of 100 ppm of the following gases (ppm)
CO 0 5 5 0
H2S -20 660 350 130
SO2 0 100 500 70
NO 0 285 0 20
NO2 120 -20 -120 0
H2 0 0 0 0
Cl2 / -30 -55 -50
CH=CH 0 2 50 0
CO2 0 0 0 0
HCN 0 25 / 30
HCl 0 / 35 -5
 

This table is not complete. If you wish information not included in the table above, please contact AP.

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