Precision Measurement Engineering, Inc.
Precision Measurement Engineering, Inc.
The fast conductivity channel consists of the 5346 Fast Conductivity sensor and associated circuitry. These circuits are located on the 5132 Conductivity- Temperature circuit board.
The 5346 Fast Conductivity sensor consists of a very small conductivity sensor potted into a stainless steel tube. The mechanical drawing shows that this sensor is designed to be mounted on the SCAMP and is bent such that it can be closely aligned with other sensors.
The Fast Conductivity sensor is manufactured by PME.
The image below shows a highly magnified view of the sensitive area of the sensor. The
sensor consists of 4 spheres of platinized platinum supported by a group of glass tubes.
This image shows the typical dimensions of this sensor. This sensor functions by being a
dimensionally stable contact between the salt water around the sensor and the electronic
circuit connected to the sensor. The electronic circuit makes a 4 terminal A.C.
measurement of the conductance of the salt water. The Fast
Conductivity's diagram shows the sensor's wiring and wire color definition.
LABELS
The picture below shows a less magnified view of the sensor.
The measurement made by the combination of the Fast
Conductivity sensor and its electronic circuit can be shown to result from a volume
weighted average of the conductivity of the salt water near the sensor. Conductivity
sensors thus have a spatial response. This spatial response is effectively a low pass
filter in space having a roll-off at approximately 4 cycles/cm. At the SCAMP travel rate
of 10 cm/sec this roll off appears at 40 cycles/second. For additional information, see
the REFERENCES section.
Conductivity sensors can also have a time-related response
due to salt water entrapment in boundary layers or in cavities located in the sensitive
volume. Due to the open construction of the Fast Conductivity sensor, time-related
responses are not a problem.
The block diagram of the electronic circuit shows
that this circuit consists of a control loop that maintains the A.C. voltage measured
between the conductivity sensor's voltage electrodes at a constant value by adjusting the
A.C. current supplied to the sensor's current electrodes. The block diagram shows the
sensor's voltage electrodes connected to a differential amplifier. This amplifier produces
an A.C. voltage that is rectified and compared to a constant D.C. reference. The
difference between these two is filtered to insure loop dynamic stability and is used to
control the amplitude of a 10 kHz sine wave. This sine wave is fed to the A.C. driver
which circulates A.C. current through the sensor's current electrodes, thus causing the
A.C. voltage that occurs across the sensor's voltage electrodes.
The current that flows in the driver-sensor circuit
is proportional to the solution conductivity. This current causes a voltage drop across
the scale selection resistor that is amplified, rectified, and filtered to give the
circuit's output. This circuit can be used over a wide range of solution conductivity by
proper selection of the scaling resistor. Fast C - example of a typical calibration of this channel. Note that the
response is linear, that 0 conductivity always occurs at approximately 0 channel ratio,
and that the slope of the calibration line depends upon the choice of scale selection
resistor. This calibration can be expected to vary substantially from sensor to sensor.
Head, M. J., The Use of Miniature
Four-Electrode Conductivity Probes for High Resolution Measurement of Turbulent Density or
Temperature Variations in Salt-Stratified Water Flows, Ph.D. Thesis, University of
California, San Diego, 1983.