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11 Appendix
the transmitter.
With the symmetrical connection, even difficult electrical ambient conditions
can be compensated.
If, for example, an electric motor for a mixer conducts a fault current into the
substance being measured, this will result in a potential shift with regard to the
system ground.
With the usual asymmetrical connection, a fault current may flow to the
system ground via the stray capacitance (which occurs in all instruments), thus
causing a measurement error.
In case of the symmetrical connection, both inputs are fed to the instrument
electronics via operational amplifiers. These operational amplifiers cancel the
fault current (up to a certain degree), thereby preventing measurement errors.
Impedance monitoring
Impedance monitoring of glass pH combination electrodes make high
demands on the transmitter electronics. The measurement needed for this is
performed in parallel to the acquisition of the main measurement variable. To
minimize the load on the electrode, the reaction time may be up to a minute.
With the asymmetrical connection of the glass and reference electrode, the
cumulative impedance can be monitored.
Monitoring the reference electrode is not recommended, since the measured
value is difficult to interpret.
Impedance measurement depends on the cable material, cable length and the
components that are used. Special JUMO cables for pH measurement may be
up to 10 m long.
If ISFET sensors or impedance converters are used, then impedance
monitoring is not possible.
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(1) Glass electrode
(2) Reference electrode
(3) Operational amplifier
(4) Grounding pin
(1)
(2)
(3)
(4)
(3)
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