AN IN SITU CONDUCTIVITY METER 



D. D. SKINNER 



Westinghouse Electric Corporation 



Pittsburgh, Pennsylvania 



INTRODUCTION 



An indirect technique commonly used for 

 salinity determination is the measurement of the 

 electrical conductivity of the water. Conduc- 

 tivity cells using platinum or similar metallic 

 electrodes have been used for conductivity mea- 

 surements. However, electrode polarization and 

 contamination has limited the long term accuracy 

 of calibration of such devices. 



Some of the more recently developed conduc- 

 tivity meters have used induced currents in the 

 water to measure the conductivity. This tech- 

 nique eliminates the problems of electrode polar- 

 ization and contamination and appears to lend 

 itself to use as an in situ instrument. 



THE WESTINGHOUSE CONDUCTIVITY METER 



An induction conductivity meter suitable for 

 use as an in situ instrument is being developed 

 by the Westinghouse Electric Corporation. This 

 meter utilizes two toroidal inductors in a 

 balanced bridge circuit. A simplified circuit 

 diagram is shown in Fig. 1. 



The inductors, L-, and Lp, are wound on mag- 

 netic toroids made of a ferrite material and are 

 constructed as nearly identical as possible. 

 The resistors, Rj and Rp, are identical and their 

 resistance is approximately equal to the induc- 

 tive reactance of Lj_ and Lg at the frequency used. 

 As a result the bridge circuit should be balanced 

 there being no signal output to the null detector. 



To make measurements of conductivity, the 

 inductor, L 1; is submerged in the fluid whose 

 conductivity is to be measured. The fluid 

 around and through toroidal inductor L-, com- 

 prises a one turn secondary winding on L-, with 

 a resistive load inversely proportional to the 

 fluid conductivity. When placed in a fluid, the 

 fluid induces a resistive component into L^, 

 unbalancing the bridge circuit. A secondary 

 winding is placed on Lg and is loaded with a 

 variable resistor. This resistor and secondary 

 winding induce a resistive component into Lg and 

 the variable resistance can be adjusted until 

 the bridge is again balanced. The value of this 

 resistance is thus a measure of the conductivity 

 of the fluid surrounding L^. 



Fig. 1. A simplified circuit diagram of the 

 Westinghouse induction conductivity 

 meter . 



The inductor Lp is exposed to the surrounding 

 fluid in a manner similar to Lj_ and in close 

 proximity to L-]_, but the medium is prevented from 

 loading Lg by the presence of a thin, compliant, 

 electrically insulating membrane closing the hole 

 through the center of the toroid. Thus inductor 

 Lg is exposed to the same thermal and pressure 

 environment as L-^. Inductors L-j_ and L2 are 

 physically made as identical as possible. Thus 

 the thermal and pressure effects on the two 

 inductors should be the same and as a result the 

 bridge balance should be inherently independent 

 of pressure and temperature effects. 



Because L^ and Lp are identical as are R-j_ and 

 R2, the bridge balance is also inherently inde- 

 pendent of the frequency, amplitude and waveform 

 of the signal source. This is of considerable 

 value when the meter is used as an in situ instru- 

 ment where it might be difficult to maintain 

 accurate frequency and amplitude control on the 

 signal source. 



ACCURACY CONSIDERATIONS 



The accuracy of this circuit is fundamentally 

 limited to measurement of the smallest change in 

 conductivity that will produce a bridge unbalance 

 signal larger than the input noise of the null 

 detector. Therefore, electrical and physical 



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