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Re: [APD] Dissolved O2 membranes

> Date: Sat, 2 Dec 2006 21:31:07 -0800 (PST)
> From: Thomas Barr
> Subject: Dissolved O2 membranes
> Both David and Vaughn has expressed some doubts as
> to the ability of a DO memebrane used for DO meters/
> readings for adaptation to CO2 pH probe measurements.
> Nice try,...


Well, since my original message appears to have been garbled somehow let me
try another way.

The original round of discussions that centered on the visual CO2 "meter"
stressed the importance of keeping the reference solution inside the tank to
equalize the temperatures. The point being made at the time was that even a
slight temperature difference between the two chambers would result in
distillation, its condensate affecting the reference solution in the long

> ...but the KCL solution is retained by the DO memebrane
> just like a bicarbonate solution would be and the transfer
> rates for CO2 are similar across the teflon or the higher
> permability(but less durable) membranes...

Of this I have no doubt, only the problem with osmosis is not the transfer
of salts across the membrane but _water_ instead. And while a few are
hashing out the *long- term* considerations - like "bacterial sludge" - that
might come into play during continual monitoring (read "automated systems"),
I'm wondering if perhaps the reference solution might take on or lose water
over some period of time and its value start to drift.

Ah, one of the mysteries of Nature just waiting for a fresh, inquiring mind.
Tonight's episode: "Will my 'brane stop passing gas before it gets too
slimey to the touch?" Check your local listing for air times...

> ...I've spoken with American Marine and with Milwaukee
> already about converting their DO probes over to a pH/
> baking soda ref solution.
> This way we have a nice screw on cap membrane and the
> pH probe in there instead of an O2 probe. We do not
> use a KCL electrolyte, rather a baking soda electrolyte...

Errr, aaahhhh...

I'm looking at the probe for my Pinpoint Conductivity Meter, and a pinpoint
of light appears above my head...

Someone mentioned the ready availability of membrane sheets, and being in a
"cheaper- by- the- dozen" frame of mind I'm thinking it may be the most
economical choice - easier to replace than to maintain, maybe?

A DIY approach (and the reason I mentioned the probe) might be to put a
threaded PVC sleeve over the end of the pH probe. The sleeve should be tight
enough to be easily sealed to the barrel of the probe. The threaded end of
the PVC can extend just past the end of the probe, and its end polished
smooth. Drill a nice- sized hole through a threaded endcap and grind the top
of the cap down to reduce the resultant "window frame" to a minimum
thickness. Again polish everything and every edge.

Up- end the "hooded" probe and fill the cavity with the reference solution.
Lay a small square of membrane across the end - leave no air underneath.
Seal it by screwing on the endcap, leaving the membrane to fill your nicely-
prepared frame.

If the reference solution is prepared to mimic the tested alkalinity of the
tank, then there's an increased likelihood that osmotic differentials would
be slim. But if the solution begins to either take on or lose water over
time, then perhaps there will be a noticeable change in the membrane's

In other words a small, fixed volume in a sealed environment should work to
advantage. The small volume will equalize more quickly (15 minutes, you
say?), and since the only chamber wall that will yield to pressure changes
is the membrane, I'm wondering if these membranes are flexible enough to
exhibit such changes, and to what degree.

Perhaps an alternative approach to the wearing of "socks and garters"?...


David A. Youngker
jaafaman at comcast_net

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