Both work the same way.
There is a current control circuit that holds current constant once the
conductivity has increased to the point that the current wants to go higher
than the set point.
In essence, it will "run away" very slowly until the current control puts
the brakes on and holds the speed steady.
The way it keeps current the same regardless of the conductivity going
higher is by automatically dropping the voltage.
The auto off circuit is a voltage comparator.
It is fed a specific internal voltage to compare the electrode voltage to.
When the electrode voltage drops to the point that both voltages match
while keeping current constant as conductivity rises, it turns itself off.
Since electrode exposure, spacing, current, conductivity and voltage are
all interdependent, it works exactly like a conductivity meter. Like a
conductivity meter, there are ways a careless person can make it read wrong.
Conductivity meters that operate in the pure water ranges, used with
care, come pretty close to AA Spectrophotometer results.
The SG6s I've seen are set at around 15 microsiemens max with a dial to set
it lower and no way to go higher
The Silverpuppy is set at around 15 microsiemens with no 'go lower' dial
but with a bypass so you can go as high as you dare.
The "derived" relationship of microsiemens of conductivity to PPM at one
to one [ Or Trems one to 1.2 ] is only valid between around 10 and 20 PPM
and includes a fudge factor for unreadable "colloidal" content . Under 8 a
meter reading slews high, over 20 it slews low.
That means, if a meter is reading 25 uS, the actual silver content could
be as high as around 30 PPM. If it's reading 35, it could be as high as 50
PPM. There's no way to tell.
A "PPM" meter adds another fudge factor to a conductivity reading
specific for various metal salts to make yet another approximate
"association"...NOT CS.
Each salt has it's own fudge factor.
It does not read "PPM" in any substance.
Given a calibration at 300 PPM in saline and a range of 0 to 2,000 PPM, a
+/- 2% range read error can amount to over 20 PPM which is somewhere
around 40 microsiemens when used in the 5 to 10 PPM ranges.
They are in fact better than that but far from perfect.
Two identical and identically calibrated PPM meters can often both read
zero in distilled water, 2 PPM in one sample, then one at 6 PPM and the
other at 8 PPM in the same higher PPM sample.
That's a difference in reading of around 4 PPM/uS "equivalency in
silver" in the same sample.
"Conductivity" meters are far more repeatable with differences rarely
going over .5 uS between meters.
6 PPM by a PPM meter in "silver water" is about 12 PPM and an 8 PPM
reading on that PPM meter is around 16 PPM by the spectrophotometer.
I've had state run water labs return results as high as 50 PPM by their
methods that tested at 12 PPM by the AA Spectrophotometer set up
specifically for silver at the "colloidal sciences" lab.
Finding actual silver content is not easy even for the pros in
multimillion dollar labs that do nothing BUT test water.
Thinking that a $14 PPM meter using 3 fudge factors all lined up and
calibrated at 30 times the use range will do better is pretty arrogant.
It's amazing they do as well as they do.
ode
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