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> >Went to Ginnie this weekend and dove 36%O2. We got refills at a local >shop and the fellow calibrated the Miniox sensor by running "100%" O2 >across the sensor. We were shown in the ANDI class to calibrate to > ~21% in the surrounding air. I asked the filler if the sensor gave valid >readings all the way from 1% to 100% and he gave me that..."youre a >dumbass look". Obviously filler boy has never pondered the question... > >What is the best way to calibrate the sensor short of >calling Miniox and asking for specs for the sensor? Is this gizmo affected >by other factors like temp or humidity? > >jbentley@cr*.co* Jeff, Thought I may be able to shed a little light on the subject. Down here in OZ the debate re Mini Ox accuracy has been a point of contention for some time. As the sensor is a galvanic cell, and uses an electric curent for detection, the unit is definately affected by both temp & humidity. The temp & humidity affect the curent generated from the sensor, and depending on both factors, your readings can vary up to 2% from atual - either high or low. As far as accuracy, if you want what IANTD publish in their standards, that is +/- 1%, then the unit MUST be calibrated using a known content certified calibration gas which is no more than 5% away from the expected result for the gas you are analysing. This means that for true +/- 1% accuracy, the unit should be calibrated with a certified calibration gas with an Oxygen content of ideally 35%. This would give the required accuracy for EANx mixes in the range 30% - 40% - which here in OZ are the most common mixes used. We all regularly use either 100% Oxygen or air, and failing having a calibration gas, I would first calibrate using 100%, then check against air before analysing your mix. This way, at least you have two referance points prior to your analysis. This is all to do with the linear accuracy of the unit. The unit is accurate to 1% accross the scale, however once the unit has been correctly calibrated with either air or 100% Oxygen, the unit is simply not able to give more that +/- 3% when testing the usual mixes in the 30% - 40% ranges. The ONLY way to get true +/- 1% using air or 100% Oxygen and analysing the usual dive mixes is to use very expensive para-magnetic analysers, which are lab only instruments. Hope this is of interest. Safe diving Tony D
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