11 Appendix
94
11.3 Glossary
Calibration
timer
The calibration timer indicates (if required) when the next routine calibration is
due. The calibration timer is activated by entering a number of days, after
which recalibration has to be carried out (plant or operator requirement).
MIN/MAX value memory
This memory acquires the minimum or maximum input variables that have
occurred. This information serves, for example, to decide whether the sensor
that is connected is suited to the values that are actually present. .
The MIN/MAX value memory can be reset: Operator level / Display / MIN/MAX
value memory / Yes,
see “Operator level parameters”, page 84ff.
Asymmetrical connection of pH electrodes
pH electrodes are usually connected to the transmitter asymmetrically. The
connection corresponds precisely to the arrangement of a pH electrode with
regard to the impedance.
In the case of the asymmetrical connection, the glass electrode has a high-
resistance connection to the transmitter electronics and the reference
electrode a low-resistance one. Most transmitters are designed for this type of
connection.
For both asymmetrical and symmetrical connections, the input impedance of
the transmitter must be about 1000 times higher than the impedance of the
glass electrode that is connected. The impedance of a glass electrode can be
up to 1000 MOhm.
Symmetrical connection of pH electrodes
The symmetrical high-resistance input is an alternative method of connecting
pH electrodes to a transmitter. In this case, both glass and reference electrode
have a high-resistance connection to the transmitter. This connection type
makes it imperative to make an additional connection of the liquid potential to
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(1) Glass electrode
(2) Reference electrode
(3) Operational amplifier
(1)
(2)
(3)
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