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Re: [PSUBS-MAILIST] Oxygen Analyzer
Just a clarification - you want the air passing the sensor to be sufficiently
dry to prevent condensation on the sensor membrane, but you don't necessarily
want to dry the air excessively, because then you won't actually be measuring
the cabin atmosphere. The water vapour in the air makes a contribution to the
total pressure which you would be ignoring if you dehumidified it before
measuring. Just keep the sensor isolated from a cold hull so that the
temperature at the air/membrane interface is above the dewpoint.
-Sean
On Wednesday 23 February 2011 17:26:44 you wrote:
> Thanks Sean,
> I'll do some flow level experiments near the air intake of the CO2
> scrubber, that's the only place where dry air will be flowing
> consistently. The other alternative is to make up a separate small fan for
> it.
> Regards Alan
>
> ----- Original Message -----
> From: "Sean T. Stevenson" <cast55@telus.net>
> To: <personal_submersibles@psubs.org>
> Sent: Thursday, February 24, 2011 1:48 PM
> Subject: Re: [PSUBS-MAILIST] Oxygen Analyzer
>
> > You want to avoid mounting an oxygen sensor in a manner that will lend
> > itself to moisture condensation on the sensor. In rebreathers, the
> > sensors are typically placed in the breathing loop after the scrubber
> > for this reason, as any water condensed on the sensor membrane will
> > affect the measurement. As for flow, the actual flow rate is not as
> > critical as its consistency. With zero flow, oxygen diffusion through
> > the sensor membrane will reduce the oxygen concentration in the region
> > immediately adjacent to the sensor, which will then read erroneously
> > (i.e. oxygen diffusion rate through the air becomes a limiting factor).
> > You want to have some flow so that the sensor reading accurately
> > reflects the gas composition you are trying to measure. Additionally, if
> > the gas is dry, flowing it across the sensor membrane can help
> > discourage condensation - another plus. If your flow is too fast,
> > however, the pressure at the sensor membrane does not accurately reflect
> > the pressure of the gas you are trying to measure, so the apparent PPO2
> > will be different, again leading to (slightly) erroneous results.
> > Ideally, you want just enough flow to continuously refresh the gas at
> > the sensor and keep it dry. Using a flow restrictor or some other means
> > of controlling the flow rate will ensure that your readings are
> > consistent from one measurement to another, which allows you to
> > accurately calibrate the sensor against atmospheric air, pure oxygen or
> > some other reference at the same pressure as your operating
> > measurements. This also allows you to to determine when a sensor is
> > starting to go bad.
> >
> > -Sean
> >
> > On Wednesday 23 February 2011 11:39:38 you wrote:
> >> Hi all,
> >> was putting my O2 analyzer together & read a link that I think Jens
> >> provided
> >> http://www.scubadiver.cc/forums/showthread.php?1272-Design-Construction-
> >> Op eration-and-Maintenance-of-an-Oxygen-Analyzer wich states the
> >> following...........
> >>
> >> Essentially all galvanic O2 sensors (such as those listed in the table
> >>
> >> above) function best with a gas flow around 2-4 liters per minute.
> >>
> >> Am wondering what other people do with regard to this flow or whether
> >> I've bought the wrong thing (El Cheapo O2 analyzer kit) Was thinking of
> >> mounting the O2 analyzer in the intake to the CO2 scrubber to get a bit
> >> of flow going through it. Regards Alan
> >
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