User Manual

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 Po o l Pr o 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 Po o l Pr o

uses the offset value determined during calibration to 7 in pH calibration

(pH 7 = 0 mV). Sensitivity of the ORP surface is xed, 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 lm will slow

the response time and diminish sensitivity. It can be cleaned off with

detergent or acid, as with the pH glass.