I saw one of these pop up on ebay and then seeing it sell for over $300 about knocked me on the floor. I can understand a sentimentality for a kit a relative built, or perhaps someone used in their youth… but as far as functionality, egads.
The Heathkit unit works by detecting differences in the thermal conductivity of the exhaust gas relative to air. Its a much simpler and less expensive method than infrared spectroscopy and it should give a reasonable ball park for values less than stoke… provided you sample the exhaust gas stream AHEAD of the catalytic convertor. I use the term should, as the fuels back in the day when this unit was developed are quite a bit different than today. I dont know if this will or will not affect the accuracy.
That being said, the unit raised my curiousity a lot, so I had to dig into the details a bit. It uses a pair of matched thermistors in a bridge configuration, with one in a “air” reference environment, and one in the exhaust stream. The thermistor model is matched pair of Fenwall G126B, a 2K thermistor at 25C with a 200ohm value at 150C. Sadly this part is not a catalog item by any means, but Honeywell (who bought out Fenwall) still offers the 126 series… no idea on minimum order quantities.
You can see a schematic and manual for the Heathkit CI-1080 at the Wedge Labs.