Nice analysis Bob (Tai-Pan) BUT I have to disagree (unless you
have a different TDS-1 then I) - The manual from Hanna mentions
they do not have a standard solution for the Ultra pure (UPW) unit,
so they use a calibrate resistor, so I checked the TDS and PWT and
they both read, per Hanna correctly, with the calibrate resistors
suggested 100K = 10uS (4.3 mg/L of NaCl)! At 1uS they say 1Meg
ohm (0.4mg/L of NaCl).
That implies a conductivity measurement, there being no outside
source of voltage in MY resistor! My digital meter only reads to
0.1mv, and I see no voltage on the resistor (or open circuit), so they
must be using 100uvolts or less! Can you measure below that range?
You had me going for a while with some great ideas, but damn,
blown out of the water (Cs) again!
The gain here is that you can calibrate your PWT with a 1% resistor
(5 cents at Radio Shack), using uS/cm = 1Million/ohms resistance
(I Meg=1uS, 100K=10uS, etc.) - which we all knew (conductivity is
reciprocal of resistivity) but never thought of using as a meter
calibration (I actually bought a case of 84uS NaCl test sol last year)!
[email protected]
------------------------------------------------------------------------
Bob Lee said:
<snip> No electric voltage (potential) or ac signal is at the TDS-1 probes.
I have measured them and found nothing. The ionic potential of the water
and whats in it provides the voltage which causes a current to flow in
the input bridge. The bridge is very high imdedance and only a very
small micro current (microvoltage) in needed to cause current flow in
the bridge circuit. Well, where do we get this from in the CS?. Any time
we disassociate ions from atoms (electrons) we have a potential
(voltage) no matter how small. In a battery we disassociate them at a
fast enough rate to get a large current flow. When we don`t the battery
will be dead. The ions (electrons) will flow around the circuit trying
to equalize the potential caused by disassociation. Even in the CS we
have disassociation taking place (if we didn`t there wouldn`t be any
ions) and the small micro potential in the CS will cause a current
(microcurrent) to flow in the TDS-1 bridge.<snip>
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