Spot measurement
A recent introduction to the sensor field is a disposable sensor for combined measurement of bath temperature and liquidus to determine superheat, thus eliminating the potential errors inherent in calculating superheat from separate bath sampling for temperature and composition (theoretical liquidus). The direct shop floor measurement method is based upon thermal analysis. The sensor is immersed into the cryolite bath and held until a stable temperature plateau is obtained. After approximately 25 seconds the sensor is removed from the bath, together with a sample of cryolite which is held in the metal cup. The bath sample cools whilst being agitated. When the liquidus temperature (primary freezing point) is reached heat of fusion is released and the cooling rate decreases. From the time - temperature trace, the inflexion point that corresponds to the liquidus temperature can be determined. The total measurement cycle takes about 40 seconds to complete after which time the bath temperature and liquidus are used to calculate the superheat.

In order to provide a user-friendly measurement system that can be used by trained personnel, it was necessary to develop dedicated instrumentation and probe holding equipment. The equipment consists of battery operated data logging and temperature display instrumentation, designed to withstand the strong magnetic field around the pots. The instrument is capable of storing more than 5,000 measurements, together with pot number, time and date of measurement. Connected to the measuring instrument is a vibrating lance to which the Cry-O-Therm sensor is connected. The lance is vibrated during the measurement cycle to induce a stirring action of the cryolite sample within the metal cup. This action promotes sharply defined cooling characteristics and helps suppress undercooling of the cryolite sample, which would otherwise make thermal analysis interpretation meaningless. A 24V DC battery powers the vibration lance and instrumentation, making the complete system portable for use on the potline.

Superheat measurements enable a smelter to control better the heat and mass balance of reduction cells. In this way Cry-O-Therm optimizes process control of this highly complex, interrelated operation.

Brand names: Cry-O-Therm®