Why do the readings for H2S and NO2 go up when I breathe on the sensors in my G460? I notice they settle back down pretty quickly, but why do they go up in the first place?
Thanks very much for sending us your question.
Electrochemical sensors respond to sudden changes in humidity. They rapidly stabilize at the new humidity, but it can take a few seconds. This is true even for H2S sensors. The air in your exhaled breath is very humid (close to 100% RH) and very warm (98.6°F / 37°C). If you select the “PEAK” hold mode, then breathe on the H2S sensor in a G460, the H2S reading can climb as high as one or two ppm before it settles back down. However, this happens very quickly! If you don’t use the “PEAK” hold button to see the highest reading that is momentarily reached, the sensor stabilizes so quickly you probably won’t notice there was a change in readings.
Reducing gas sensors, (like H2S and CO sensors), use oxygen dissolved in the electrolyte to detect the gas being measured. The oxygen in the electrolyte comes from the atmosphere. Water is used and regenerated during the detection reaction, but the overall reaction does not consume water. The only thing that is used up is oxygen, and the gas that is detected. The overall detection reaction used in an H2S sensor converts hydrogen sulfide into sulfuric acid:
H2S + 2O2 → H2 SO4
H2S sensors exhibit only small transients when the concentration of water in the atmosphere changes, and recover very rapidly.
Oxidizing gas sensors, (like NO2, NO, Cl2, ClO2 and O3), use water from the electrolyte to detect gas. The water in the electrolyte that is used up in the detection reaction is replaced with water from the atmosphere. The overall detection reaction used in an NO2 sensor converts nitrogen dioxide into nitric acid:
NO2 + H2O → H2NO3
Because water is more directly involved in the detection reaction, breathing on oxidizing gas sensors (like NO2) can cause a bigger spike in readings.
Both reducing gas and oxidizing gas sensors stabilize in the new humidity as soon as it stops changing. However, it can take oxidizing gas sensors a slightly longer time to stabilize fully in the new humidity.
GfG Instrumentation mining industry customers use a lot of NO2 sensors. The sensors do very well in underground and high humidity situations. However, please make sure that you don’t hold a sweaty palm over the sensor compartment, and try not deliberately breathe on the sensors when the instrument is in normal operation. Also, if you move from an air conditioned low humidity area to a hot, high humidity area, give the sensors a minute to stabilize in the new humidity before you perform a fresh air zero.
Thanks again for the question.