Tonight I was able to mount the Parallax CO2 sensor and my homemade "flow sensor" on the repurposed respirator mask. The flow sensor is small biscuit fan from an old Sun SCSI tape drive.
To make this all work, I first removed the exhalation cage and its check valve from the mask. Then I removed both input filter cartridges, their sealing gaskets and one of the inhalation checkvalves. I covered the remaining check valve with packing tape. So the subject was breathing too many volatiles from the adhesive on the tape, I covered the adhesive which would have been exposed through the input hole with a reversed piece of tape. I'm assuming the shiny side of the tape is less volatile than the adhesive.
I then cut two pieces of 1/4 inch plywood, one which fits in the filter cartridge mount and the other which is a bit larger than the biscuit fan. I bolted the fan to that square of plywood, made a gasket from poster putty to go between the square of plywood and the exhalation check valve mount and held the fan in place against the mask with several small cable ties. The CO2 sensor is, fortunately, exactly 5/8 inch in diameter and I have a 5/8 wood bit. Offset from the center of the circle of plywood cut to fit into the filter cartridge mount I drilled a 5/8 hole for the CO2 sensor. I then wiggled the sensor into that hole and made another gasket from poster putty. The sensor and circle of plywood are also held in place with a cable tie.
Lastly, I fashioned a sort of strain relief for the cable, which will be connected to the master microcontroller, from yet another cable tie. That cable is a short section of four conductor telephone wire.
Tomorrow I'll write some more software for the microcontroller and try to get some readings from both sensors. My concerns at this point are that the humidity in the subject's exhalations will bother the CO2 sensor. I'm also not confident that normal breathing will be able to consistently spin this particular fan. I'm on the lookout for a much smaller fan.
