[APD] Adaptation of the Drop checker using memebranes, a very significant increase in response times

[Thomas Barr <tcbiii at yahoo_com>]

In effort to increase response time for the pH probe method
using a KH reference solution, I realized that I needed to forgo
the air gap common in drop checkers. Ininitally I adapted the
air gap similar to that of a drop checker CO2 test with the KH
ref solution that Vaughn came up with..

The velocity at which the flux of gas transfers across the water
to air and then from air to water is compounded and rather
slow(2x phase changes, gas[aq]=> gas[g]=> gas [aq]). 
This is 10,000 slower than gas=>gas for *each* change.

Perhaps 2 hours to equilibrate or so based on several folks
measurements(My simple method: use 2 of them, one in the tank as
reference for the tank's CO2 ppm, then add another and time the
color change till both the units are equally resolved). Test Ref
tank (t) to equilibrate equally to tank ref = response time. You
place the ref test inside the tank 1 day prior and assume the
tank's CO2 ppm is stable(caveat: ***it's not stable at longer
response times***, but this is close enough. But.....this is
suitable and avoids such assumptions ***if*** you use the pH
probe method and where the ****response times are reduced***
with the membrane visual checker).
So with the membrane method, there's a better way to measure the
response time's accuracy as well.

Note, this same test method works well for the below version I
recently tried out and also for testing pH probe set ups(but you
need two pH probes/meters).

I have been suggesting a membrane approach to transfer, this
involved no phase intereface changes which slow things down a
good deal and prevents large gas bubbles from causing some
potential issues. It also significantly reduces the distance at
which the gas must pass. 
 
[B]Seconds versus hours.[/B]
Now apply this same idea conceptually to the drop checker method
suggested by Vaughn.

A thin sealed box with the membrane snapped in with a notch and
a lip with an O ring. You need enough KH ref solution inside to
get a decent color resolve and placing a white backing on the
container would certainly help.

Now you have only to the membrane itself to cross, and no phase
shift changes.

This idea is a lot like the SeaChem (Greg Morin, are you
listening??) Ammonia Alert visual test "hang in the tank" idea
and could certainly be commercialized and adapted.

I think such a unit would only cost perhaps 5-10$ from a vendor.
Response time estimations have been about 5 mintues or so.

A bit better than the 2 hours?:-)

If you want to do DIY,a  simple small thin container you can
wrap the DO membrane around and keep the KH ref solution
seperated from the tank water and place in a semi high current
region. Change and clean as needed.

Cost is still about that of drop checker, but unlike that
method, this has a very rapid response time. 
 
Regards, 
Tom Barr

www.BarrReport.com



 
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