Remote Conductivity Sensor Using Square Wave Excitation

The invention relates to remotely sensing conductivity of fluid flowing in a conduit, and in particular to sensing conductivity of dialysate in a dialysate preparation and supply machine.

Conductivity of dialysate in a dialysate preparation and supply machine is typically measured by sensors that are immersed in the dialysate in a conduit and are subject to long term drift, owing to formation of film and precipitates on the electrodes, and to other shortcomings.

Electrodeless conductivity sensors, e.g., those available from Great Lakes Instruments, Inc., Milwaukee, Wisconsin, have been used in water quality and process control. In one of these sensors, the conductivity of fluid flowing in a fluid flow conduit is remotely measured by providing a fluid loop connected to the conduit and two transformers coupled with the loop, inducing an electrical current in the fluid loop with one transformer, measuring the current induced in the other transformer by the current in the fluid loop, and determining the conductivity in the liquid using resistance, current and voltage relationships.

In general, the invention features simply and inexpensively driving the excitation transformer of a two-transformer/fluid-loop remote conductivity sensor with a square wave excitation signal provided by a digital timer and a flip-flop

In preferred embodiments the excitation transformer is bifilar wound and connected to both the true and complement outputs of the flip-flop; there are a current to voltage converter, AC amplifier, and synchronous detector connected to the sensing transformer; and there are additional turns about the transformers for use in calibrating the sensor.

In another aspect the invention features using an electrodeless conductivity sensor to sense conductivity of dialysate.

Other advantages and features of the invention will be apparent from the following description of the preferred embodiment thereof and from the claims.

The drawings will be described first.

FIG. 1 is a perspective view of a conductivity cell according to the invention.

FIG. 2 is a plan view, partially in section, of the FIG. 1 cell.

FIG. 3 is an elevation of the FIG. 1 cell.

FIG. 4 is a vertical sectional view, taken at 4--4 of FIG. 2, of the FIG. 1 cell.

FIG. 5 is a diagrammatic exploded sectional view of the FIG. 1 cell.

FIG. 6 is an electrical schematic of the electrical components of the excitation and sensing circuitry connected to the FIG. 1 cell.

Referring to FIGS. 1-5, there is shown conductivity cell 10, including plastic fluid flow conduit 12, excitation transformer 14 and sensing transformer 16. Excitation transformer 14 and sensing transformer 16 each include a toroidal ferrite core 18 and wires 20 wrapped around the core. Cell 10 is mounted in the dialysate flow path of a dialysate preparation and supply machine of the general type described in U.S. Pat. No. 4,371,385, which is hereby incorporated by reference, at a position along the dialysate supply conduit downstream of the mixing of concentrate with water and is connected to a processor to control the addition of concentrate to water.

Channel 12 has inlet 22, outlet 24, circular conduit portions 26, 28 (through transformers 14, 16) and connecting portions 30, 32 between them. In line with connecting portion 30 and extending beyond circular conduit portion 26 to inlet 22 is extension 31. In line with connecting portion 32 and extending beyond circular conduit portion 28 to outlet 24 is extension 35. Connecting portions 30, 32 have flat outer surfaces and circular inner surfaces 33 defining fluid flow channels therein (FIG. 4). As seen best in FIG. 4, the outer diameter of the circular conduit portion 26 is close in size to the inner diameter of toroidal core 18 and the wire turns on it, resulting in the largest practical cross-sectional area for the fluid flow path permitted by the dimensions of the cores. Also, the height of circular conduit portion is only slightly more than the thickness of toroidal core 18 and the wire turns on it, and there is a small distance between transformers 14, 16 (thus the length of the connecting portions between circular conduit portions is only slightly more than the diameter of the toroidal cores and the wire turns on them). These two factors provide a low value for the ratio of the length of fluid flow loop 38 (dashed line in FIG. 3) provided by connecting portions 30, 32 and circular portions 26, 28 to the cross-sectional area of the flow path, which in turn provides good sensitivity. While the transformers could be physically brought slightly closer together, to the point that the turns on one core overlap those on the other core and even contact the other core, this is not done, because it would tend to increase the likelihood of a leakage coupling between the transformers.

As is seen in FIG. 5, channel 12 is made of two identical pieces 34, 36, which are solvent bonded together after inserting transformers 14, 16 between them.

Referring to FIG. 6, there is shown the electronic circuitry providing excitation signals to excitation transformer 14 and receiving the signal relating to fluid conductivity from sensing transformer 16. Transformer 14 is connected to receive a 10 KHz square wave excitation signal on the left side of the schematic from oscillator 39 and driver 40. On the right side, sensing transformer 16 is connected to current-to-voltage converter 42, AC amplifier 44, synchronous detector 46, and filter/buffer 48, including, respectively, amplifiers 58, 60, 62, 64 (LF347).

Oscillator 39 includes timer 41 (7555) and flip-flop 42 (74HC74). Both the true and complement outputs of flip-flop 43 are connected to driver interface 45 (75451), the true and complement outputs of which are connected to transformer 14. The true output (Q) of flip-flop 43 is also connected by line 50 to synchronous detector 46.

Excitation transformer 14 is a bifilar wound transformer having 43 turns. Sensing transformer 16 has 89 turns. Both excitation transformer 14 and sensing transformer 16 each also have wrapped around them single wire turns 52, 54, connected to calibration pins 56, for connection to a resistor for use in calibrating the apparatus. The values or numbers of the remaining components on FIG. 6 are as follows:

In operation, dialysate flows into inlet 22 through loop 38 and out outlet 24, filling up the entire fluid flow path between inlet 22 and outlet 24 and providing a fluid loop coupled with transformers 14, 16. Because cell 10 is mounted so that the flow paths through connecting portions 30, 32 make a 45.degree. angle with respect to the horizontal, there are no corners at which air bubbles (which would distort measurements) could be trapped; thus, any air bubbles are displaced.

Oscillator 39 provides a 10 KHz square wave applied by driver 40 to excitation transformer 14. A square wave is advantageous because it can be simply generated from inexpensive components providing a constant amplitude, which need not be controlled, as with sinusoidal waves. Driver 40 increases the voltage of the square wave received from oscillator 39 from the 5 volt logic level to 12 volts.

Excitation transformer 14 induces an electrical current in fluid loop 38, which current is then sensed by sensing transformer 16. The current induced in transformer 16 is proportional to the conductivity of liquid in loop 38.

Transformer 16 is capacitively coupled by capacitor C9 to amplifier 58, so that the DC offset is blocked and only the alternating signal is amplified. The output of amplifier 58 is a voltage that is proportional to the conductivity of the liquid in loop 38. Capacitor C12 is used to block the DC offset so that amplifier 60 only amplifies the AC voltage.

Synchronous detector 46 converts the AC voltage from amplifier 44 to a DC voltage output, eliminating extraneous frequencies. When transistor Ql, driven by flip-flop 43, is turned on, it acts as a short to ground, and amplifier 62 operates as an inverting amplifier with a gain of -1; at this time the output of AC amplifier 44 is negative, resulting in a positive output from synchronous detector 46. When transistor Q1 is turned off, it acts as an open circuit, and amplifier 62 has a gain of +1; at this time, the output of AC amplifier 44 is positive, resulting again in a positive output from synchronous detector 46.

The output of synchronous detector 46 charges capacitor C16 through resistor R14. If there are frequencies other than 10 KHz, over a long period the negative and positive components average out; only the signal at the 10 KHz frequency consistently charges capacitor C16, and is passed through amplifier 64 of filter/buffer 48. The output of filter/buffer 48 is a DC voltage proportional to conductivity; it is converted through an A/D converter to a digital signal by a digital processor (both not shown) used to control a dialysate preparation and supply machine of the general type described in U.S. Pat. No. 4,371,385,

The circuitry of FIG. 6 can be calibrated by placing a resistor of known value between pins 56, draining the liquid from loop 38 (so that transformers 14, 16 are only coupled through single turns 52, 54) and comparing the output of filter/buffer 48 with the known resistance of the resistor.

Other embodiments of the invention are within the scope of the following claims.