Memosens CPS76D

2

Endress+Hauser

Function and system design

Measuring principle

pH measurement

The pH value is used as a unit of measurement for the acidity or alkalinity of a liquid medium. The

membrane glass of the electrode supplies an electrochemical potential which is dependent upon the

pH value of the medium. This potential is generated by the selective penetration of H+ ions through

the outer layer of the membrane. An electrochemical boundary layer with an electric potential forms

at this point. An integrated Ag/AgCl reference system serves as the required reference electrode.

The transmitter converts the measured voltage into the corresponding pH value using the Nernst

equation.

ORP measurement

The ORP potential is a unit of measurement for the state of equilibria between oxidizing and

reducing components of a medium. The ORP is measured using a platinum or gold electrode instead

of the pH-sensitive glass membrane. Analog to the pH measurement, an integrated Ag/AgCl

reference system is used as a reference electrode.

rH measurement

The rH value is defined as the negative logarithm of partial pressure of hydrogen in a solution. The

pH value and ORP value of a solution must be measured simultaneously to calculate the rH value.

The value is calculated using the following equation:

rH = 2 . (mV/S) + 2 pH

pH = measured pH value

mV = measured ORP value in mV + 207 mV (Ag/AgCl system)

S = slope of pH electrode

The rH value is an indicator of the oxidation or reducing ability of a process solution. The measuring

range is from 0 to 42. A process medium with rH values between 0 and 9 has a high reducing ability;

a low reducing ability with rH 9-17; an indeterminate ability with rH 17-25; a low oxidation ability

with rH 25-34, and a high oxidation ability with rH 34-42.