Myron L 9PTK - Manual (Page 68)

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5. Sources of Error
The basics are presented in pH and ORP/FREE CHLORINE, pg. 62.
a. Reference Junction
The most common sensor problem will be a clogged junction because
a sensor was allowed to dry out. The symptom is a drift in the “zero”
setting at 7 pH. This is why the Ultrameter III 9P does not allow more
than 1 pH unit of offset during calibration. At that point the junction is
unreliable.
b. Sensitivity Problems
Sensitivity is the receptiveness of the glass surface. A film on the surface
can diminish sensitivity and cause a long response time.
c. Temperature Compensation
pH sensor glass changes its sensitivity slightly with temperature, so the
further from pH 7 one is, the more effect will be seen. A pH of 11 at 40°C
would be off by 0.2 units. The Ultrameter III 9P senses the sensor well
temperature and compensates the reading.
B. ORP/Oxidation-Reduction Potential/REDOX
1. ORP as an Indicator
ORP is the measurement of the ratio of oxidizing activity to reducing
activity in a solution. It is the potential of a solution to give up electrons
(oxidize other things) or gain electrons (reduce).
Like acidity and alkalinity, the increase of one is at the expense of the
other, so a single voltage is called the Oxidation-Reduction Potential,
with a positive voltage showing, a solution wants to steal electrons
(oxidizing agent). For instance, chlorinated water will show a positive
ORP value.
2. ORP Units
ORP is measured in millivolts, with no correction for solution temperature.
Like pH, it is not a measurement of concentration directly, but of activity
level. In a solution of only one active component, ORP indicates
concentration. Also, as with pH, a very dilute solution will take time to
accumulate a readable charge.
3. ORP Sensors
An ORP sensor uses a small platinum surface to accumulate charge
without reacting chemically. That charge is measured relative to the
solution, so the solution “ground” voltage comes from a reference
junction - same as the pH sensor uses.
4. The Myron L ORP Sensor
Figure 34, pg. 63, shows the platinum button in a glass sleeve. The