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

http://www.honeywellanalytics.com/Technical%20Library/Americas/MicroMAX%20Pro/Datasheet/HWA6064%20H_MicroMAX%20Pro-DS_V4.pdf

Micromax / MPRO series (ex: MPRO-4ABCD seems to be popular model)

Nice digital unit