Myron L PS6FCE - Manual (Page 48)

44 45
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 P
ool
P
ro
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. The ORP Sensor
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 31, pg. 43, 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 P
ool
P
ro
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
The basics are presented in pH/ORP, pg. 42, because sources of
error are much the same as for pH. The junction side is the same, and
though the platinum surface will not break like the glass pH surface,
its protective glass sleeve can be broken. A surface film will slow
the response time and diminish sensitivity. It can be cleaned off with
detergent or acid, as with the pH glass.