n2mp said:
Yes, everything talked about above will happen in a conductivity
meter, and in any system involving electrochemical reactions, as
far as current is flowing in the solution.
I was under the impression that a conductivity meter measuring
ultrapure water doesn't have any electrochemical reactions.
: is the AC signal in a conductivity meter adjusted so that no
electrochemical reaction occur at the electrode ?
The one I designed (which was low-cost and low-accuracy)
simply took a DC reading, then took another DC reading with
polarity reversed. That worked fine.
From my webpage at [
http://www.guymacon.com/SMC/INDEX.HTM ]:
"...One interesting technical challenge was measuring the
resistivity of the water. 100% pure water has a bulk
resistivity of around 18 Megohms per cubic centimeter. This
resistivity is also strongly affected by the temperature of
the water. A Microcontroller has the ability to measure
resistance by measuring the rise-time of a digital pulse
through an RC circuit, but the range it measures is usually
only a few Kilohms, not the Megohms I needed to measure. I
managed to get the range up to 250 Kilohms by careful
capacitor selection, but the impedance of the
Microcontroller pins wouldn't allow me to go any higher than
that. I solved this problem by designing a parallel plate
sensor with an area of 200 square centimeters and a distance
between plates of one centimeter. This converted a bulk
resistivity of 18 Megohms per cubic centimeter to a
resistance of 90 Kilohms - well within the capability of the
Microcontroller. I used the same Microcontroller to measure
a thermistor to detect the water temperature and wrote a
calibration table in software to compensate for temperature
and sensor nonlinearity. The Microcontroller had limited
memory, so I wrote the table with 11 temperature values from
0 degrees C to 100 degrees C in 10 degree steps and 21
resistivity values from 0 ohms to 20 Megohms in 1 Megohm
steps. From these values, I extrapolated 1 degree and 0.1
Megohm steps, which were displayed on the LCD. The same
sensor also served as a water level detector; when the tank
level got too low, there was air between the plates instead
of water and the open sensor reading would trigger the
addition of more ultra-pure water. I wrote code so that if
the water stayed on for too long without filling the tank it
would give up and display an error. I used the same scheme
so that if the water stayed on for too long without
increasing the resistivity to an acceptable level, it would
give up and display an error.
"Another interesting challenge occurred when the prototype
started acting strangely. Over the course of several hours,
the bulk resistivity of the ultra-pure water as measured by
the Microcontroller would rise above the theoretical maximum
of 18 Megohms per cubic centimeter, while the commercially
available resistivity meter would show no change. Eventually
the Microcontroller software would erroneously conclude that
the water level was below the plates and would add
ultra-pure water. As soon as a small amount of additional
ultra-pure water was added, the reading would immediately go
back to normal. I found that stirring the water made the
problem go away - it only happened when the water was very
still. It turns out that the ions in the ultra-pure water
were moving away from one plate and collecting on the other
plate. I programmed the Microcontroller to swap which plate
was grounded and which plate was exited at each reading. At
this point, the sensing system tracked the commercial meter
perfectly."
