The Science of Gas Detection

Gas Detection Scientist at Work

In response to a question regarding cross sensitivity of H2S sensors to carbon monoxide when dealing with the four gas mixture of 10 ppm H2S/ 300 ppm CO/ 2.5% methane/ 17% oxygen/ balance nitrogen we mentioned in the last post, here is what one technical expert had to say:

"It’s possible for an instrument to adjust for the amount of CO it is seeing. But in reality, there are variations from time to time and from sensor to sensor, which means 100% accuracy is difficult to achieve and maintain. A simple way to improve accuracy is to use a lower concentration of CO and a higher concentration of H2S in the 4 gas mixture. For example, we specify 50 or 100 ppm CO and 25 ppm H2S for our mixture."

You're going to ask me what this means, aren't you? And it is kind of important to remember that H2S is, after all, a poison.

Well, it's simple, gas detector manufacturers know that for most of their four gas instruments, the presence of CO in the calibration gas or the environment falsely drives up the H2S reading. My son asked if this could be dealt with by re-writing the calibration software which would eliminate the bias via a suppression calculation. The gas detection scientist responded that, although it was a good idea, it would basically be work, so instead his company preferred to use a calibration mix that kept the H2S and CO concentrations closer so whoever was using the instrument wouldn't notice the bias during the calibration.

Let's just say that this is helpful information, except that when the responder goes into a situation where the carbon monoxide is in fact higher that the range the instrument is calibrated to, then the H2S reading will still be high for the environment because the H2S sensor is still sensitive to carbon monoxide (CO).

I asked another gas detection expert about this, and he said, "Look, if it reads falsely high, the responder will leave sooner."

After considering this response, I said, "Hey, we're the ones that might end up breathing this stuff. Why not just compensate for the bias and give us an accurate reading?"

"No one," he said, "ever asked me to look into it."

There are potential dangers associated with cross sensitivities for sensors. Do you know what gases your sensors are cross sensitive to? I'll post a few later this week for you to get a look at.

Meantime I won't talk to any gas detection scientists, because it's a lot like talking to pigeons who read German.