Myron L 6P and 4P - Manual (Page 49)

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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 II senses the sensor well
temperature and compensates the reading.
B. ORP/Oxidation-Reduction Potential/REDOX (6P)
1. ORP as an Indicator (6P)
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 (6P)
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. The ORP Sensor (6P)
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 (6P)
Figure 34, pg. 44, shows the platinum button in a glass sleeve. The
same reference is used for both the pH and the ORP sensors. Both
pH and ORP will indicate 0 for a neutral solution. Calibration at zero
compensates for error in the reference junction.
A zero calibration solution for ORP is not practical, so the Ultrameter II
uses the offset value determined during calibration to 7 in pH calibration
(pH 7 = 0 mV). Sensitivity of the ORP surface is fixed, so there is no gain
adjustment either.
5. Sources of Error (6P)
The basics are presented in pH/ORP, pg. 43, because sources of error