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TMI Blog1992 (9) TMI 240X X X X Extracts X X X X X X X X Extracts X X X X ..... n this Chapter) for machines, appliances, instruments or apparatus of Chapter 90". The Assistant Collector classified the goods under Heading 8544.00 as "insulated cables", holding that the Heading would cover even cables which cannot conduct electricity and holding further that, being supplied in coil length to customers, it cannot be recognised as part or the accessory of the pyrometer. The lower appellate authority upheld the classification as determined by the Assistant Collector, holding that the goods in question are admittedly insulated cables used as conductors in temperature control equipment. Hence this appeal. 3. Shri Gopal Prasad submits that the compensating cables are a pair of wires having such e.m.f. temperature characteristics related to the thermo-couple with which the wires are intended to be used that, when properly connected to thermocouple, the effective reference junction is in effect transferred to the other end of the wires. The item in question is not an electric wire for carrying current but cables specifically used for transferring the thermo e.m.f. of the thermocouples to the thermocouple pyrometers for the measurement of temperature. Therefore, s ..... X X X X Extracts X X X X X X X X Extracts X X X X ..... en the measuring and reference junctions are at different temperatures. The measuring junction is that junction of a thermocouple which is subjected to the temperature to be measured and the reference junction is that junction of a thermocouple which is at a known temperature or which is automatically compensated for its temperature". Extension wire has been defined as "a pair of wires having such temperature e.m.f. characteristics relative to the thermocouple with which the wires are intended to be used that, when properly connected to the thermo-couple, the reference junction is transferred to the other end of the wires". The American National Standard further states that for minimum error the extension wire should be run from the thermocouple connection head to the instrument terminal or reference junction in one continuous length; junction boxes may introduce errors and can be avoided by design of the conduit system with the proper pull point. When splices are unavoidable, they should be made by compressing the two wires to be joined with a mechanical device to obtain intimate contact. No other electrical wires should ever be run in the same conduit with extension wires. The In ..... X X X X Extracts X X X X X X X X Extracts X X X X ..... s made of the same wires as the thermocouple it may cost almost about Rs. 50,000/-, hence substitute wires are used. The substitute wires are so calibrated as to match with the particular thermocouple. The idea being that the reading at the indicator should have an accuracy of almost +2oC. The chart below enumerates the different types of thermocouples and the compensating cables with appropriate wire. Nominal Temperature Range Type of Thermocouple Compensating Cable 134oC to 370oC Copper/Constantan Copper + -Constantan (eureka) 0oC to 760oC Iron/Constantan +Iron -Constantan (eureka) 0oC to 1200oC Chromel/Alumel (a) +Chromel(eureka) -Alumel (eureka) (b) +Copper -Constantan (eureka) (c) +Iron - Constantan (eureka) Platinum/Rhodium Platinum Rhodium +Copper Platinum (Pt Pt Rh) -Cupronickel (eureka) Generally the cable is guaranteed +2oC accuracy up to 100oC but when accuracy up to high temperatures is needed, eureka and cupronickel wires have to be imported. The price of Indian constantan is about 3 times that of copper and the imported wires after payment of duty comes to about 8 times of copper. Interpretation of National Test House, Alipur Report ..... X X X X Extracts X X X X X X X X Extracts X X X X ..... ue to the resultant loss in current due to the impurities or thinner cross section of wires. However, if the resistance is abnormally high it may cause a fuse in the mains. The second important test conducted is the thermal e.m.f. test. The thermal e.m.f. is generated when two wires of dissimilar metals are heated together. For example if an iron wire and platinum wire are formed into a pair and heated, a thermal e.m.f. is generated which can be read on a potentiometer. This is the basic principle of a thermocouple. However, if both the wires (positive and negative) are of the same metal say copper e.m.f. is generated when the pair is heated. Generally in electrical cables both positive and negative wires are either of copper or aluminium and when a pair of copper wires or a pair of aluminium wires are heated no thermo e.m.f. is generated. The generation of thermo e.m.f. will render the cable unsuitable for electrical use, whereas in compensating cable which is an accessory of thermocouples and is also known as thermocouple extension wire the primary and basic test is thermal e.m.f. test which in short describes the e.m.f. when the pair of positive and negative conductors are heat ..... X X X X Extracts X X X X X X X X Extracts X X X X
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