Mike
Without knowing exactly what you intend to use the equipment for its 
difficult to give a reply. 
I remember using the Seiger OXD1 when I was a ventilation officer for British 
Coal and I found it a very unreliable piece of kit. It was fine in a dry 
atmosphere but the slightest hint of dampness sent it out of calibration.
During the controlled flooding of my colliery I found myself travelling into 
low oxygen, high CH4, CO2 areas and the instrument I trusted and relied on 
most was my good old-fashioned miners relighter lamp. My attitude was that if 
it keeps burning the oxygen is sufficient and the methane I can keep a check 
on with my methanometer. Good job we didn't have a CO problem!
I remember a very embarrassing incident when the colliery manager and the HMI 
mines were measuring CH4 in a district using MSA D6D methanometers. The 
managers read 0.9 % methane and the HMI's read 1.5 %, both instruments had 
been issued by me that day after calibration. The variation in readings 
caused  me some embarrassment but the situation was clarified by both men 
using their relighters to check for a gas cap and neither finding one. 
Therefore the CH4 level had to have been less than 1.25 %. It proves though 
that the least sophisticated equipment is usually the most reliable.


Mark


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