Liquid conductivity and temperature are measured in respective sensitivity fields that are collocated—i. e., in volumes that nearly match by mathematical, geometrical, or functional criteria. Collocation is as distinct from mere adjacency or proximity; and is with respect to measurement volumes, not measuring hardware. Preferably pressure too is measured with sensitivity very generally collocated to the conductivity and temperature sensitivity. Preferably, respective temporal/spatial bandwidths of the two (or three) sensors are matched. Preferably the pressure sensor is a MEMS transducer, the conductivity sensor is a four-terminal device, the thermometer is a thermistor encapsulated in a silkscreened glass wall, and circuits (1) compensate for time lag between conductivity and temperature measurement, (2) remove artifacts due to detritus in or near either sensor, and (3) derive secondary parameters of the liquid.

Sensor and sensor system for liquid conductivity, temperature and depth

Improved circuits for measuring the conductivity of a solution confined between two electrodes in a cell compensate for series capacitance and parallel capacitance between the electrodes. A bipolar square-wave signal is applied to the cell. In one embodiment, the current through the cell is measured by an op-amp in current-to-voltage converter configuration. A feedback resistance employed with the op-amp in a feedback loop is controlled to a low value to ensure that the parallel capacitance is fully charged during an initial portion of each half-cycle of the drive signal. The feedback resistance is then selected so that the gain of the feedback loop is responsive to the range of the resistivity of the solution, and the measurement is made. In a further embodiment, charge proportional to the current through the cell is integrated across the op-amp, and the rate of charge is measured by measuring the time required to integrate the current to a predetermined level, and used to determine the resistivity of the solution. In this embodiment, the parallel capacitance is fully charged before each integration period begins. The voltage developed across the series capacitance is constant and can be compensated mathematically.

Methods and circuits for measuring the conductivity of solutions

A liquid conductivity measuring circuit with power-save features is described for use with a four electrode conductivity cell having two potential electrodes and two current electrodes positioned in the cell in spaced relation in a particular sequence with a first operational amplifier having its output connected and its inputs connected via an isolating capacitor and a buffer amplifier and a voltage setting resistance to a source of square-wave voltage, a second operational amplifier having its inputs connected to ground and via an isolating capacitor and its output connected to a reference resistance such as to control the current through the reference resistor to maintain one electrode at a constant potential, readout means being adapted to measure, on initiation of a single or small number of square-wave voltage cycles in a period short enough that no significant change of polarization of the potential electrodes can occur, the voltage between the positive and the negative half cycles or the change from zero to either half cycle of the voltage across the resistor, the measurement being a function of the current through the reference resistor and conductivity cell and therefore a measure of the conductivity of the liquid in or passing through the cell.

Liquid conductivity measuring circuit

An improved circuit for measuring resistance, as for the resistance of a temperature sensor, wherein the resistance varies over a wide dynamic range. The circuit provides two switch means to charge two capacitors through the unknown resistance. A time interval means measures charging time for one or both capacitors. Since charging time is directly proportional to resistance, a value of resistance is computed for one or both charging times. Control means, responsive to the time interval means, is provided to sequentially operate one or both switch means, the control means including means for selecting that charging time which is as long as possible yet within the range of the time interval means, thereby obtaining a higher resolution measurement in the range of sensor resistance where resolution would otherwise be inadequate.

Impedance measuring circuit

A liquid conductivity measurement system is disclosed that accurately measures a liquid's conductivity over a range spanning at least several orders of magnitude using just a single sensor, but minimizing the adverse effects of capacitive and other non-linear factors on the conductivity measurement. The system includes a sensor having two spaced-apart electrodes immersed in the liquid and a drive signal source that applies to the sensor an ac electrical signal having a selected frequency and predetermined magnitude. A voltage detector monitors a sensor voltage signal across the sensor's two electrodes and produces a measurement of the liquid's conductivity. The drive signal source is configured to iteratively adjust the frequency of the ac electrical signal based on the sensor voltage signal, to optimize the liquid's conductivity measurement.

Liquid conductivity measurement system using a variable-frequency AC voltage

An oscillator circuit wherein two conductivity cells are arranged in a circuit to provide a conductivity ratio which is converted into a linear variation of the period or frequency of the oscillator.

Oscillator circuit for providing a conductivity ratio of sea water

An oscillator circuit which converts the resistance variation with temperature of a non-linear thermistor into a variation in the period of an oscillator which is linear with temperature.

Linear period thermistor temperature oscillator

A standard conductivity cell having concentric thin-walled alumina cylinders with a thin film or layer of standard seawater contained between them. Electrical contact is made to the standard seawater by end electrodes and a pressure compensation arrangement allows for the cell''s use at great depths in the water. With a known relationship between conductivity and temperature, the cell also operates as a very rapid temperature sensor.

Lawrence C Murdock

Papers

Oscillator circuit for providing a conductivity ratio of sea water

Linear period thermistor temperature oscillator

Standard conductivity cell for measurement of sea water salinity and temperature