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>From: Jeff Bentley <jbentley@cr*.co*> > >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? Just to add to what Tony David wrote, the other main factor to affect readings is the pressure. Galvanic cell O2 sensors actually measure partial pressure of O2 rather than fraction of O2. Obviously at 1 ata, the partial pressure is the same as the fraction of O2. The implication is that in taking measurements, you need to be careful not to raise the pressure on the cell from too high a flow rate through any sort of restriction of the gas being measured. I have been discussing the application of these sensors with the folks at Teledyne and they also cautioned that if you are using a Tee adapter to run the gas past the sensor, then you should attach a length of tubing to the outlet end of the Tee adapter. Apparently without this, you can get a back eddy effect which mixes some room air with the gas being measured. On the point of teperature sensitivity, they recommend that you calibrate under the same conditions in which you measure due to the cooling effect of gas coming out of a pressurised vessel. I.e: rather than calibrating to room air, you should calibrate to air out of a SCUBA cylinder using the same setup you use to measure an unknown mixture. A point of interest is that although many O2 sensors are temperature compensated (I don't know whether this applies to a Mini-Ox), this compensation is usually in the form of a thermistor at the opposite end of the sensor enclosure from the gas interface. Hence, it is only compensating for variations in room temperature and NOT to variations in gas temperature. This info applies to Teledyne sensors, I'm not sure about other makes. Finally, I would like to remind everyone concerned about highly accurate FO2 measurements of Richard Pyle's post a week or so back on whether you know your body's sensitivity to PO2 within 1% !! I.e: The Mini-Ox's accuracy is probably plenty for this application, whether you calibrate at 100% 21%. Cheers, David. ______________________________________________________________________________ David Giddy, | Voice: +61 3 9253 6388 Telstra Research Laboratories, | Fax: +61 3 9253 6144 P.O. Box 249, Rosebank MDC, 3169, AUSTRALIA | Internet: d.giddy@tr*.oz*.au* ------------------------------------------------------------------------------
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