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Re: [PSUBS-MAILIST] Oxygen Analyzer
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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