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Metal Casting Technologies : September 2007
89 ■ Dimensional inaccuracy caused by swelling of the casting. ■ Erosion scabs. High pouring temperatures cause increased rate of sand expansion resulting in increased tendency for erosion. MEASURING METAL TEMPERATURE IN THE LADLE The most common means of determining metal temperature in the pouring ladle is by the use of an immersion pyrometer which provides a fast, accurate and reliable measurement. Fig. 5. Immersion pyrometers generally utilise an expendable thermocouple tip in a protective tube which is assembled onto the lance probe by push-fit. The thermocouple is connected to a digital recorder which measures the voltage in the circuit which is proportional to the temperature difference between the two junctions of the thermocouple. The voltage is displayed as a temperature on the instrument. PROCEDURE OF USE AND CARE OF THERMOCOUPLES For consistent and reliable measurement of temperature, it is important that the correct procedure for use of the thermocouples is followed. The following is a general recommendation. 1. To insert on lance, hold the thermocouple by the cardboard tube, not the non-splash cover, and push it up over the lance as far as it will go. This should be done firmly to ensure a position fit between the contact block and the thermocouple. Do not ram the thermocouple on as that can damage it. 2. The thermocouple is now ready for use; this will be indicated by the green (ready) light. 3. Place the end of the thermocouple 5cm into the metal bath, being careful not to immerse it past the non-splash sheath onto the cardboard tube. The yellow (measurement) light will glow. 4. The thermocouple should be held in the bath until the red (complete) light comes on. When this light comes on, it indicates that the temperature has been measured and a final reading will be indicated on the display or chart. 5. After one dip the thermocouple should be taken off the lance, put to one side and allowed to cool. 6. At least three thermocouples should be used in rotation. By the time the third is used, the first should be cool enough to use again. This system of rotating the thermocouples will result in maximum life and accuracy for each thermocouple. Fig. 6. 7. Do not leave the thermocouples on the lance between dips. ● Figure 4. Expansion defect Figure 5. Metal temperature measurement using immersion pyrometer Figure 6. Stand for rotating and cooling thermocouples WHO'S WHO OF METALS -- ANNUAL 2007/8 References: 1. Heraeus Electro-Nite literature 2. Atlas of Casting Defects, American Foundrymens Society, 1974