For most hydrocarbons in low concentrations (ppm) we used "Drager Tubes", very specific. Higher levels we usually grouped them togather and used an "Explosive Meter" made at the time by Mine Safety Appliance. For low levels of O2, PPM to lower percentages, we used an instruments made by "Teledyne". For verifying breathable air we used various makes of O2 analyzers. These were mainly for vessel entry.
If you are checking low levels of O2 around welding I understand that they are acceptable ones now. Several years ago none that we tried worked.
Depending on the levels you can get most at a laboratory supply company. Matheson Gas is/was a good supplier of analyzers. The bigger safety supply house and welding suppliers carry the explosive meter and some O2 analyzers.
