7.3
Temperature compensation and
medium temperature compensation
7.3.1
Temperature influences during pH measurement
There are two essential influences of
temperature on pH measurement:
• The slope of a pH measuring chain is
temperature-dependent. This change of
slope is well-known (Nernst voltage) and
can therefore be compensated by the
instrument.
• The pH value of the medium to be
measured changes with temperature
(temperature-dependent dissociation).
This change is different for varying media
and often unknown. To compensate it, the
temperature dependence of the medium
must be determined in the laboratory,
if required, and stored in the instrument.
• This is necessary, for example, if the pH
value of a process with high temperature
shall be compared to a laboratory value,
which is usually determined at a lower
temperature. Entry into Mycom CPM 152
is effected in form of a table.
7.3.2
Temperature compensation of measuring chain
Temperature compensation for the slope of
the pH measuring chain can be carried out
either automatically via the connected Pt 100
temperature sensor (ATC) or manually by
entering a temperature value (MTC). MTC is
suitable for constant process temperatures.
7.3.3
Medium temperature compensation
Tables for three different measuring media
can be entered in the Mycom CPM 152 for
medium temperature compensation. The
appropriate table can be selected as the
active medium before the process begins.
The compensation consists of up to ten value
pairs (temperature and relevant pH level).
Procedure:
• Take a sample from the process. Its pH
value should approximate the set point of
the process as well as possible.
• Heat the sample in the laboratory up to the
process temperature and determine its pH
value.
• During cooling-down, record the pH and
temperature value pairs for temperatures
prevailing at later measurement
(e.g. process temperature and ambient
temperature in the laboratory).
• Enter the value pairs into the instrument.
As reference temperature, select the
temperature for which the set point of the
process is defined (e.g. ambient
temperature in the laboratory).
→
Commissioning
→
Temperature
☞
7.3.4 (for pH)
→
Temperature
☞
7.3.5 (for Redox)
pm152e07.chp
mycom CPM 152
Instrument configuration
Hauser
37
