Myron L 6P and 4P User Manual

Page 49

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