Re: CS>Hanna Meter Model Number
From: Ode Coyote
Date: Sat, 16 Oct 2004 04:08:09
http://escribe.com/health/thesilverlist/m74024.html
> HI 98308 PWT [Pure Water Tester]
> When calibrating, store the meter and solution together for a
> while so both solution and the meter are the same temperature.
> Don't hold the solution container tightly in your hand because
> heat soak from your hand will toss everything out the window.
> The temp compensation doesn't work the way you'd think it
> would...and Hanna ain't talking.
> The PWT is the best of the sorta bad.
> I have 2 PWTs calibrated the same and they don't agree with each
> other anywhere on the scale except in the calibration solution.
> None are very 'good'.
> Maybe the $$$$$ bench top lab instruments are better..maybe not.
> But using conductivity to get PPM isn't exactly a valid way to do
> these things anyhow.
> It's just better than not being able to afford to do it right.
> It's better to do the job relatively badly than not at all, I
> suppose.
> Ode
Ken, thanks for the good info, especially on controlling
temperature!
As far as getting two units to agree, the spec is +/-2% of full
scale. This means +/-2ppm, which means one unit could read 4 ppm
higher or lower than another and they would still be in spec. A
slight difference in temperature would increase the discrepancy.
The problem is the units are design for higher conductivity than we
typically get in cs, so our readings are at the bottom 20% of the
range. This is the least accurate portion. Here's the specs from
Hanna's site:
Range : 0.1 to 99.9 uS/cm
Resolution : 0.1 uS/cm
Accuracy (@20C/68F) : +/-2% Full Scale
http://www.hannainst.com/products/testers/pwt.htm
Although individual units may differ by more than we would like, one
would hope they would be consistent and repeatable. As far as I can
determine from searching the archives, most people seem to feel the
units are repeatable. I know you have posted seeing different
results, but it is not clear what caused them.
So assuming the units give consistent readings (with special care
regarding temperature), they can be useful to determine if the cs
process is repeatable, and what changes can be made to improve it.
For example, the quality of the dw is very important, and poor dw
will give poor cs.
Also, the Faraday calculations can help by showing the maximum ppm
available with a given current and brew time, and the Hanna can be
used to verify the results. I posted a method earlier using Roger
Schlafly's Mercury program that makes these calculations very easy.
As far as designing a conductance meter to give higher accuracy, I
would tend to go with much larger plate area to capture a larger
portion of the solution. This would reduce the tendency to respond
to local variations in concentration of silver ions.
I posted the results of two experiments a while back that made the
silver and hydroxyl ions visible so you could see how they dispersed
through the solution. It was very clear that the ions move mainly by
convection currents, so there were miniature clouds and filaments
where the concentration was much higher, as well as voids.
So if the small Hanna probe encountered a local variation in
concentration, the readings could be quite different. Your comment
to store the meter and the solution together for a while would help
by allowing time for the ion concentrations to stabilize.
Yes, making good quality cs is not easy, and making high ppm cs is
much harder. But I am convinced it is worth the effort, by the
dramatic difference it had on the Shingles virus, cold sores, and
bacteria. And you know when you have the good stuff - your teeth
feel raspy after you hold it in your mouth for a while.
So I go for the good stuff:)
Best Wishes,
Mike Monett
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