Abstract:
A inductive conductivity sensor for measuring conductivity of a fluid includes a housing supporting a controlled impedance loop and a transducer. The transducer includes a driver for inducing a first current into the fluid adjacent the housing, and for inducing a second current into the controlled impedance loop. The transducer further includes a detector for inductively measuring the first and second currents.

Description:
FIELD OF THE INVENTION 
     This invention relates to inductive conductivity sensors for measuring conductivity of a sample fluid, and particularly to inductive conductivity sensors having a diagnostic resistor in parallel with a current path in the fluid. 
     BACKGROUND OF THE INVENTION 
     Inductive conductivity sensors are used for measuring the conductivity of a fluid, such as a liquid or dispersion of solids suspended in the liquid. Inductive conductivity sensors are used to investigate the properties of electrolytes in the fluid, such as the degree of disassociation, the formation of chemical complexes, and hydrolysis. 
     A toroid inductive sensor is a common form of inductive conductivity sensor that employs two spaced-apart “toroid” transformer coils. A drive coil is electrically excited by an alternating current source to generate a changing magnetic field. The changing magnetic field induces a current loop in the sample fluid; the magnitude of the induced current is indicative of the conductivity of the fluid. A detection coil inductively detects the magnitude of the induced current and provides a measure of the conductivity of the fluid. The body of a toroid sensor is typically cylindrical, and the coils are positioned near opposite ends of the cylinder. The axial passage of the cylinder defines part of the induced current loop in the fluid. 
     One problem associated with inductive conductivity sensors is that an open circuit condition in either the drive or detection coil circuits is difficult to detect. More particularly, an open circuit condition in the drive coil circuit results in no current being induced in the fluid. However, ion content of the fluid may generate noise in the detection coil that may be mis-analyzed as a conductivity value. An open circuit condition in the detection coil circuit results in no current being supplied to the analyzer from the detection coil, which might be mis-analyzed as a highly resistive (zero conductivity) fluid. The present invention is directed to a technique that permits diagnostics to be performed on the sensor to detect an open circuit condition in either the drive or detection coil electronics. 
     SUMMARY OF THE INVENTION 
     An inductive conductivity sensor for measuring conductivity of a fluid includes a housing arranged to be positioned adjacent the fluid. The housing supports a controlled impedance loop and a transducer. The transducer includes a driver arranged to induce a first current into fluid adjacent the housing and to induce a second current into the controlled impedance loop. The transducer also includes a detector arranged to inductively measure the first and second currents. 
     The current measured by the detector comprises the current representative of solution conductance (the first current) offset by the induced in the controlled impedance loop (the second current). The offset or second current provides the advantage of sensing open circuit in the sensor circuits, as well as to offset the detector current to overcome noise. 
     In some embodiments, the controlled impedance loop includes a conductive wire inductively coupled to the driver and the detector and a resistor coupled to the conductive wire. 
     In one form of the invention, the driver includes a first magnetic core adjacent a first end of the housing and a first coil arranged around the first core. Similarly, the detector includes a second magnetic core adjacent a second end of the housing and a second coil arranged around the second core. The controlled impedance loop extends through the first and second cores. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an inductive conductivity sensor mounted within a conduit according to a first embodiment of the present invention. 
     FIG. 2 is a side view of the sensor shown in FIG. 1 coupled to a power source and measurement circuit. 
     FIG. 3 is a section view of the sensor shown in FIG. 1 taken at line  3 — 3  in FIG.  1 . 
     FIG. 4 is an equivalent circuit of a sensor coupled to a power source and measurement circuit employing the principles of the present invention. 
     FIG. 5 is a section view of an inductive conductivity sensor according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one form, known as an insertion toroid conductivity sensor, the sensor is immersed in the fluid whose conductivity is to be measured. One example of an insertion sensor is one that is inserted into a pipe or conduit through which the fluid is flowing. The insertion toroid conductivity sensor measures conductivity of the fluid flowing in the conduit. Another form of toroid conductivity sensor is known as a flow-through toroid conductivity sensor in which the conduit carrying the fluid is in an axial passage through the sensor. The principal difference between the two types of sensors is that the drive and detection coils and circuits of the insertion-type sensor must be protected from corrosive fluids being measured, whereas the coils and circuits of the flow-through type sensor do not. 
     FIGS. 1-3 illustrate an insertion toroid inductive conductivity sensor  10  according to one embodiment of the present invention. Sensor  10  is mounted inside conduit  12  carrying the fluid  42  whose conductivity is to be measured. Body  16  includes an arm or strut  14  that is mounted to a wall of the conduit to support body  16  within the conduit. Alternatively, sensor  10  may be mounted to the wall of a tank or other container, such as in a chemical process stream. Sensor  10  is an insertion-type sensor, so named because the body  16  of the sensor is inserted directly into the conduit or container for the fluid whose conductivity is being measured. Thus, sensor  10  is in direct contact with the fluid. 
     Body  16  encloses and supports first and second toroidal coils  18  and  20  comprising electrical wires wrapped around respective toroidal ferromagnetic cores  22  and  24 . Body  16 , which is shown in phantom in FIG. 1 for sake of clarity, is constructed of non-magnetic, non-conductive material, and forms a non-magnetic region  19  between the regions of cores  22  and  24 . Coil  18  is a driver coil and is electrically coupled by wires  26  to an alternating current source  28  (FIG.  2 ). Coil  20  is a pick-up coil and is electrically coupled by wires  30  to a measurement circuit  32  (FIG.  2 ). 
     As shown particularly in FIG. 3, wire  34  is supported within body  16  and forms a continuous conductive loop through coils  18  and  20 . The loop formed by wire  34  includes resistor  36 . In the embodiment shown in FIGS. 1-3, body  16  is arranged to be supported within conduit  12  carrying the fluid whose conductivity is to be measured. Consequently, coils  18  and  20  and wire  34  are protected from any corrosive nature of the fluid being measured by the material of body  16 . 
     In operation of the toroid inductive conductivity sensor of FIGS. 1-3, application of an alternating current to coil  18  generates an alternating magnetic field within magnetic core  22 . This magnetic field induces an alternating current in the fluid, represented by loop  38 . The electrical resistance of fluid  42  impedes current flow in loop  38 . At the same time, the magnetic field in core  22  induces an alternating current in the loop formed by wire  34 . The resistance value of fixed resistor  36  impedes current flow in wire  34 . The flows of current in wire  34  and in loop  38  induce an alternating magnetic field in magnetic core  24 , which in turn induces an alternating current in coil  20 . The current induced in coil  20  is measured by circuit  32 . 
     The equivalent circuit of the sensor is illustrated in FIG.  4 . It will be appreciated by those skilled in the art that the fluid resistance  42  and fixed resistor  36  are electrically in parallel. Consequently, the current, I 20 , induced in coil  20  is proportional to the sum of the inverse of the resistances          (       I   20     ∝       1     R   42       +     1     R   36           )     .                          
     The current induced in wire  34  is in parallel with the current induced in the fluid (loop  38 ). Consequently wire  34  and its resistor  36  provide a base output in pick-up coil  20  indicative of a closed circuit. While resistor  36  represents an impedance in parallel with the resistivity of the fluid being measured, the effects of the resistor can be electronically offset in measurement circuit  32 . By calibrating the measurement circuit to provide a zero readout due solely to the resistor  36 , the measurement circuit will provide an output representative solely of the resistivity (conductivity) of the fluid. If an open circuit condition occurs in either the drive circuit of source  28  and coil  18  or the detection circuit of coil  20  and measurement circuit  32 , the absence of the resistor  36  in the induction loop causes the measurement circuit to provide a negative output, indicative of the open circuit. 
     Another feature of the present invention resides in the fact that the controlled impedance loop provides a current to the detector to offset the sense current through the solution. If the solution has a high impedance (low conductance), the current through the solution, I 42 , will be low. In prior systems, noise induced in the detector current I 20  could adversely affect the ability to measure low solution currents. The offset of the low solution current to a higher detection current due to the loop of conductor  34  and resistor  36  diminishes the effect of noise. 
     FIG. 5 illustrates a second embodiment of a sensor  60  according to the present invention. In this case, sensor  60  is a flow-through toroid inductive conductivity sensor that includes a body  62  having an axial passage  64  for receiving conduit  66  carrying the fluid whose conductivity is to be measured. Conduit  66  includes a non-conductive section  72  and conductive washers  74  and  76  at each end of body  62  in contact with the fluid in conduit  66 . Wire  78  is coupled to washers  74  and  76  to complete a loop circuit around cores  22  and  24  for current flowing in the solution in conduit  66 . Coils  18  and  20  are coupled to ferromagnetic cores  22  and  24  which in turn are supported by body  62 . Wires  26  and  30  couple coils  18  and  20  to the source of alternating current and measurement circuits, as in the case of the sensor of FIGS. 1-3. Wire  68  forms a loop through cores  22  and  24  and includes a fixed resistor  70 . 
     Sensor  60  illustrated in FIG. 5 operates in the same manner as sensor  10  shown in FIGS. 1-3. In this case, however, the fluid is carried by conduit  66  and is not in contact with sensor  60 . Consequently, it is not necessary to protect the drive and detection circuits of coils  18  and  20 , wires  26  and  30 , and cores  22  and  24  from the fluid being measured. Likewise, it is not necessary to protect wire  68  or resistor  70  from the fluid. In this case, it may be advantageous to employ an external resistor  70  that might be changed for different applications of sensor  60 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.