Crowcon Detection Instrument Custodian C Portable Gas Detector
Got a “Drager Polytron SE NO2” surplus but seems that old sensors don't work too well and ended up tossing it. Maybe electronics worked fine maybe they didn't but cost to replace sensor was outside of my budget at the time
Micromax / MPRO series (ex: MPRO-4ABCD seems to be popular model)
Nice digital unit