balance resistance connected with L2 are elimi- 

 nated. It can be shown in this case that the 

 circuit is balanced when the balance capacity is 

 equal to the inductance of L-, divided by the 

 product of the resistance R]_ and the induced loss 

 resistance. Again the balance is found to be 

 independent of frequency. As in the other balance 

 system, Lj_ is assumed equal to Lg, and R-^ is 

 assumed equal to R 2 . No loss is assumed for In 

 and L2 and, as this is not the case in actual 

 practice, the circuit balance is slightly fre- 

 quency dependent. An advantage of this circuit 

 is that it eliminates the problem of a sliding 

 contact in the balance resistance which had been 

 found to introduce an uncertainty into the value 

 of the balance resistor. 



position, which is now a function of conductivity, 

 to the survey ship. 



As was mentioned previously, stray reactances 

 in components of the bridge circuit introduce a 

 reactive component into the null voltage which 

 reduces the accuracy with which the null can be 

 determined. The phase sensitive detector however 

 responds only to signals which are in phase or 

 l80° out of phase with the reference signal. 

 Since the reactive voltage component mentioned 

 above is made to be in quadrature with the refer- 

 ence signal it does not affect the output of the 

 detector. In the case of a laboratory or manually 

 balanced instrument, this type of null detector 

 has been found to give considerably more accurate 

 results than a simple amplitude null detector. 



THE IN SITU INSTRUMENT 



For convenient use as an in situ instrument, 

 the bridge circuit is supplied with a self- 

 balancing system to reduce or eliminate the inter- 

 connecting cable requirements between the instru- 

 ment and the survey ship. The balance system 

 used with this bridge utilizes the fact that the 

 signal out of the bridge to the null detector 

 reverses phase as the bridge is adjusted through 

 the null point. Therefore, the null point is 

 characterized by both an amplitude minimum and 

 a phase reversal. A block diagram of the balance 

 system used is shown in Fig. 2. 



The bridge output is amplified and fed to a 

 phase sensitive detector. A reference signal for 

 the phase sensitive detector is obtained from the 

 oscillator which is used to drive the bridge. 

 The output of the detector is a DC voltage whose 

 amplitude is proportional to the amplitude of 

 the input signal and whose polarity is a function 

 of the relative phases of the input and reference 

 signals . The phase of the reference signal is 

 adjusted so that the output of the detector is 

 positive on one side of the null and negative on 

 the other side. This signal is amplified and 

 used to drive a reversible DC motor which in 

 turn adjusts the balance resistance in the 

 bridge in such a direction as to effect a null. 

 A data telemetering system transmits the shaft 



SYSTEM ACCURACY 



Calibration of a conductivity meter in absolute 

 values is a difficult process when a high degree 

 of accuracy is desired or required. Solutions of 

 known salinity must be used and the temperature 

 must be known to a degree of accuracy similar to 

 the required conductivity accuracy. It is con- 

 siderably easier to make an estimate of the 

 expected accuracy by measuring the repeatability 

 of the device. To measure the repeatability, it 

 is only necessary to know the absolute salinity 

 and temperature roughly and to be able to measure 

 relative temperature to a high degree of accuracy. 



A sample of artificial sea water was made up 

 with an estimated salinity of 35 parts per thou- 

 sand. As a considerable period of time is 

 involved in making repeatability measurements, 

 evaporation from the sample was inhibited by 

 floating a film of oil on the surface of the 

 sample. The absolute temperature of the sample 

 was measured with a mercury thermometer and a 

 thermistor thermometer was used to measure rela- 

 tive temperature to a repeatability of 0.01°C. 

 The sample was stirred continuously to maintain 

 a uniform temperature. 



One preliminary laboratory model of the con- 

 ductivity meter has been tested for repeatability 

 under these conditions over a test period of 

 several days . The repeatability was found to be 

 -0.1 millimhos for a solution whose conductivity 

 was approximately 55 millimhos . A large part of 

 this error could be attributed to contact noise 

 in the adjustable balance resistor as was men- 

 tioned earlier. It is estimated that elimination 

 of the contact noise problem would improve the 

 repeatability of this particular device to 

 JT0.05 millimhos. As this device did not use the 

 best known physical configuration, it is esti- 

 mated that an improved model now under construc- 

 tion can improve this repeatability figure by a 

 factor of 10. 



27 



