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Sensor Cross Sensitivity

Electrochemical sensor cross sensitivity

Example: Sensoric HCN hydrogen cyanide sensor at 20°C

H2S Sensor Cross Sensitivity

Gas Conc.Response
CO
SO2
NO
NO2
H2 		300 ppm
5 ppm
35 ppm
5 ppm
1000 ppm		≦ 1.5 ppm
about 1 ppm
< 0.7 ppm
about – 1 ppm
≦ 10 ppm
CO2
Methyl sulphide
Ethyl sulphide
Methyl mercaptan
Propylene
100 ppm
100 ppmm
100 ppm
1000 ppm		No response
15 ppm
10 ppm
about 45 ppm
about – 0.5 ppm
Isobutylene
Ethylene
Toluene
Turpentine
MTBE		 1000 ppm

10000 ppm
3000 ppmm
1000 ppm		about – 0.3 ppm
No response
No response
about 70 ppm
≦ 60 ppm

Effect of citrus solvents on H2S and COSH sensors

  • Orange peels contain d-limonene, the same "citrus oil" solvent included in many household cleaners
  • ALL H2S sensors respond to limonene as well as other VOCs like butadiene if the vapors are present in high enough concentrations
  • Instrument responds by going into alarm, making this a "Fail Safe" alarm condition
  • Requires high concentration for the H2S sensor to go into alarm
  • Although H2S sensors are not designed to function as general VOC detectors, the fact that they respond to high concentrations of VOC has saved the life of at more than one worker

Cross sensitivity to VOCs actually benefit to customers

  • Case study example:
    • A worker at chemical plant entered production area to remove a bucket of "product" that contained mix of 1,3 butadiene (48%), C6 hydrocarbons (22%), and C5 hydrocarbons (15%)
    • Butadiene is an extremely volatile and toxic VOC
    • The worker's personal H2S detector immediately went into alarm, but the worker did not believe that the alarm was valid
    • The worker left the area, replaced his H2S detector with a new one, donned a second detector for SO2, and went back into the area
    • Both new instruments went into alarm but , unfortunately, this time the worker collapsed
    • The worker was rescued, but had to spend significant time in the hospital
    • The alarms were caused by response of sensors to butadiene