Re: [PSUBS-MAILIST] Oxygen Analyzer

["Sean T. Stevenson" <cast55@telus.net>]

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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