Spot measurement
The measurement of cathode lining voltage drop is a well-established method of monitoring cathode-lining condition in many smelters. Despite being a relatively infrequent measurement at many smelters the cathode lining voltage drop, if measured accurately, provides important information on how a cathode is performing with age. Accurate measurement of cathode lining voltage drop also enables the optimisation of cell target voltage. Furthermore, with accurate CVD measurements, regular monitoring can also highlight cathode-lining abnormalities such as sludge and ridge build up. A further application of CVD measurements is the ability to compare the performance of different cathode types, or different cathode suppliers.

The successful implementation of the above applications depends upon a reliable and accurate method of measuring cathode lining voltage drop. Unfortunately, no uniform system has been developed for CVD measurement, therefore smelters have had to develop their own measuring techniques. A common method of voltage drop measurement includes the use of an iron rod immersed to the bottom of the cell and connected to a voltmeter. The circuit is completed by a connection from the voltmeter to the busbar. The use of this type of measurement is generally regarded as cumbersome, labour intensive and often provides poor accuracy and reliability of measurement

With the above limitations in mind the C-V-Therm™ disposable sensor has been developed to provide a combined measurement of cryolite bath temperature and cathode lining voltage drop. The combined sensor for bath temperature and CVD measurement* consists of a Pt-PtRh disposable thermocouple as described earlier with a thin wire connected from one of the thermocouple element legs to the outside of the cardboard protection tube. The wire provides an electrical contact with the Al metal pad to enable measurement of the cathode lining voltage drop.

The measurement is carried out in 2 stages. In the first stage the cryolite bath temperature is measured as described earlier when using disposable Pt-PtRh thermocouples. When bath temperature has been obtained the sensor is then immersed to the bottom of the pot to measure the cathode lining voltage drop. The total measurement cycle takes less than 20 seconds. A dedicated hand held, portable data logging instrument is available for use with the sensor.

Brand names: C-V-Therm™