Subject: Re: Dissolved oxygen test

From: [email-masked] (Ronald Payne)
Date: Thu, 1 Jun 1995 18:05:20 GMT

In article <[email-masked]> [email-masked] (Tim Welch) writes:
>
>We are looking for a dissolved oxygen test that is easier than the Winkler
>method. Anyone have any ideas?
>
> --
>PEERS Project Media Specialist          All I know is that I know nothing
>                                        and I am not even sure of that.
>EOSC LaGrande, OR                       Socrates  
>[email-masked]

Have you considered electronic DO probes.

-- 
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
R. Payne                            I exist in my own little world
Professional Student                 and in my Realm, grass is Blue.
email: [email-masked]

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From: [email-masked] (stevec)
Date: 1 Jun 1995 14:04:30 GMT

>method. Anyone have any ideas?

Depends. I have worked with activated sludge wastetreatment systems and found
that meters are the way to go, but you get what you pay for, usually. They can
be a trick lot! One of the problems with using a meter is that you have to know
something about the total dissolved solids in the the water you want to measure 
oxygen in if you want a really accurate measurement. Unlike the classical
Winkler test where you use a known volume of water and then determine the
milligrams of oxygen in it, you are measuring the partial pressure of oxygen
dissolved in the water as a function of its percent saturation. Temperature and
dissolved solids affect the saturation concentration. Many of the available
probes are temperature compensated, and some have a "compensation" adjustment
for salinity ( a form of TDS) to adjust an air calibration for the change in
saturation concentration from chlorinity. 

The BIGGIE is the calibration procedure chosen. We found in our research that
there were two acceptable approaches for us. The first was to calibrate the
probe to read 100% saturation or some arbitrary concentration, like 8 ppm, in a
sample of the water that had been filtered and aerated vigorously. Then the
sample was measured and the other critical parameters (temperature and TDS)
were measured. TDS was measured in filtered sample using a refractometer. The
actual O2 was then calculated on a PC using the equations given in "Standard
Methods for Examination of Water and Wastewater". This worked well for
evaluating samples that varied a lot from one another. The second method was
to calibrate in air using a table created on the PC that corrected for
temperature and TDS.

If your samples don't vary a lot, a **well maintained** probe is very
convenient to use and may give you the relief your looking for. If you have
samples that vary in composition a lot, or you must conform to some particular
protocol, it may not be the answer. As long as I was working in wastewater
treatment research, we never were able to totally resolve differences between
the Winkler test and the electrode. We chose to use the electrode.

A lot of companies make good probes, but one word of caution. My experience
with the Leeds & Northrup / L G Nester probes was that when they fail, they
don't fail catastrophically. It has been about 6 yrs since I used them, but it
was perplexing at first. The probe would begin to drift a lot, then one day we
would discover that the probe was not responding to step changes in
concentration, even though it appeared to calibrate. I liked the units, and 
if they have fixed the problem or have a procedure for identifying a failing
probe, I would recommend the units.

Your needs might be met with a standard Clark oxygen electrode, like the YSI
units use. These are used by a lot of folks, and were not as expensive as the 
L&N units were at the time I was using them. We were working with with a really
tough system - hot brine in an pure oxygen activated sludge system. It was a
trip.

I know this is long, but just to offer a simple answer might be misleading.

Steve