Consider that parts of the system are the ratio of electrode surface area
to sample volume (and to mean distance between electrodes as well as
electrode material) and that the solutions are buffered for a reason, so
that interactions between sols and electrode materials are
controlled. Granted the ultimate result will be a voltage expressed on a
scale (LCD); but if a simple resistor would do it all, wouldn't they do it
that way?
For purposes of comparison, check out
http://www.extech.com/newsite/instrument/categories/water/subcategory/waterMeters.html
They have a wide array of instruments beyond the water stuff; their catalog
makes me drool!!
Take care, Malcolm
07:58 PM 10/10/03 -0400, you wrote:
url: http://escribe.com/health/thesilverlist/m63355.html
CS>Measuring very high ppms
From: Trem
Date: Fri, 10 Oct 2003 15:27:45
> Mike,
> The meter can easily be calibrated if you use the calibration
> solution from Hanna. I got one box of 16 bottles of solution that
> was defective and Ken got one of the bottles. It was the only
> batch I have gotten that was defective in many years. I think you
> can generally trust the solution to do what it was intended to
> do....allow calibration of the PWT.
> It will be very difficult to use a standard resistor since the
> sensing electrodes are not easily accessible and the meter would
> have to be disassembled to get at the electronics. Not something
> the average person would want to do. And it's really not a good
> idea when all one has to do is use the factory solution.
> Trem
Hi Trem,
Thanks for the update. A calibration solution may be accurate, but
there's no way to know that for sure. A mistake can occur during the
dilution, it can be contaminated somehow, or perhaps some of the
water may evaporate over time and change the reading.
Steve Young posted a table of salt concentrations and resistor
values some time ago. Ivan did also.
The salt dilutions look scary. Any mistake would destroy the
accuracy. I'm not good enough to tackle that.
The resistor method is probably the most accurate, but it would
require knowing the cell calibration factor. I don't have a Hanna
yet, but I understand it uses a special multiple probe arrangement
(the details escape me at the moment.)
I understand the probes have a small area, so the actual resistor
value would be larger than the standard table shows. Also the
calibration constant might be different for each probe due to slight
mechanical tolerances.
I found a manual for the hi98308 at
https://www705.vwh1.net/hanna4/downloads/instr/hi98308.pdf
It shows how to replace the electrode assembly, HI73308
I don't know what is inside the electrode assembly, but from the
pictures there seems to be a temperature sensor and two terminals
marked (1) in the diagram.
These terminals look quite large and sturdy. I'd have to get my
Hanna to take a look, but there might be a simple way to gain access
to them.
If so, I can diddle with external resistors to find the calibration
constant for the unit for various readings. Once I know the values,
I can refer to them later if I think there might be a problem.
Since little old ladies are allowed to change the electrode
assembly, I don't think I would have any problems taking it apart as
instructed.
The instructions are given in the manual so people can change the
electrode assembly when it becomes degraded.
As a matter of curiosity, how do you tell the difference between a
degraded cell and a bad calibration solution?
Best Regards,
Mike Monett
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