I used to own a Gunson 4125 Gastester, which I bought to check/set CO on the 930 when it still ran CIS (it would idle identically for a wide range of settings, but anything less than 3.25% would result in lean surging climbing inclines on small/fixed throttle openings).
When calibrated and used per instructions, it took a long time, 15 minutes IIRC, idling to get a reading. Which ended up giving results very close to my wrench's 5 gas analyzer.
They're cheap units, so a bit touchy, not to mention slow. And a one-trick pony that's unnecessary with a wide-band lambda.
Unless you need it for emissions and don't have a wrench with a real gas analyzer, take a look at Souk's threads:
https://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html
https://forums.pelicanparts.com/porsche-911-technical-forum/185926-its-time-year-again-cis.html