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Carbon dioxide sensor

Former Member

Basic connectivity

I was able to calibrate the carbon dioxide sensor earlier today. In  the process, I also learned that the sensor is much simpler than I  originally thought. For our purposes, the sensor will require a total of  three connections:

  1. a common ground
  2. a source of between 6 and 12 VDC, at about 150 milliamps
  3. an analog sink for between 0 and 4 VDC at very few milliamps of current

The 6 to 12 volts enables the onboard sensor heater and the output  signal. The output signal is an analog voltage which is inversely  related to the amount of carbon dioxide present at the sensor. In other  words, as the amount of carbon dioxide increases, the output voltage  decreases. The sensor appears fairly sensitive and responsive and seems  to support a sample rate of up to twice per second. Based on these  details, we should be able to both identify the changes in the peak  carbon dioxide output of the subject and measure the time between the  nth and (n + 1)th peaks, to infer an exhalation rate.

Functional operation

At 7 VDC input, the sensor draws a steady 150 milliamps. It also  generates a fair amount of heat around the sensor housing. I'll have to  take that into account when I mount it in the plastic snorkel tube.

I've also learned that the sensor output value responds very  quickly to an increase in ambient CO2, but rather slowly when the CO2  source is removed. I'll need to see how useful it will be once the  subject being measured starts breathing quickly. Instead of being able  to measure the full range of CO2 output levels, we may instead only be  able to determine the peaks

  • Assuming you are using NDIR methodology to measure CO2 it is very difficult to measure exhaled breath concentration with that technology. A number of commercial operations have tried.

     

    CO2 diffuses, so response will be as you expected, in fact you are probably not seeing the true peak as the sensor response will lag slightly, which you are seeing as the value decays.

     

    Using a light source generates heat, which affects density, which effects the reading as you are in effect 'counting molecules' when measuring cocentration. If they are all more widely spaced because the gas is less dense, they'll be less in the way of the measured source, so it'll mis-read the concentration. The other area in exhaled breath is the high humidity, not in itself an issue - until it condenses. This then can effect the internal volume of the gas measuring chamber, any reflectivity that it may be dependent upon, and long term may corrode components.

     

    Be interested to hear more about what you are doing though.