7-1
MODEL 54 C
SECTION 7
THEORY OF OPERATION
SECTION 7.0
THEORY OF OPERATION
INDUCTIVE (TOROIDAL) CONDUCTIVITY
The basic conductivity measurement is made using
two toroids. The drive toroid induces a current in the
liquid loop. The second toroid (receiver) computes the
liquid current loop. By keeping the number of turns
(windings around each toroid) and the drive voltage
across the transmitter toroid constant the current in the
liquid loop is directly proportional to the conductivity of
the loop.
The actual value of the drive voltage generated by the
Model 54C (either 1 volt peak-to-peak or 0.1 volt peak-
to-peak) is determined by the conductivity being
measured. The current sensed by the receiver toroid
is converted at the analyzer to an AC voltage through
a high performance amplifier. This voltage is further
converted to a DC time-proportional signal which is
used by the microprocessor to measure and compute
absolute conductivity. The microprocessor also meas-
ures the raw DC voltage used to generate the drive
voltage and the zero input at the time-proportional
converter. These measurements are used to correct
for ambient temperature and line voltage variations.
Rosemount Analytical also offers a booklet titled
“Conductance Data for Commonly Used Chemicals.”
This booklet includes conductance information for over
100 industrial chemicals commonly used.
This section is a general description of how the analyz-
er operates. This section is for those users who desire
a greater understanding of the analyzer’s operation.
CONTACTING CONDUCTIVITY
The conductivity sensor produces a “conductance
signal” that is proportional to the conductivity of the
process solution. The 54C Analyzer subtracts a base-
line zero conductivity signal from the sensor signal
and multiplies the result by the cell constant and the
cell factor. This absolute conductivity is then corrected
to the reference temperature (usually 25°C). by accu-
rately measuring the process temperature by means of
a RTD located in the conductivity sensor. The micro-
processor also adjusts the amount of correction
required for temperature compensation by means of a
temperature slope adjustment. This slope may be
adjusted between 0 to 5%/°C either manually via the
keyboard or automatically during bench or process
calibration. This slope controls the amount of correc-
tion required in the temperature compensation circuit,
and is specific to the process, giving you the most
accurate conductivity reading possible.
Summary of Contents for 54C
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