COMPENSATING CABLES & EXTENSION LEADS FOR THERMOCOUPLE & RTDs

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Published: 09th July 2010
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INTRODUCTION : It is often desirable to connect a thermocouple probe, as a part of a very long circuit from the sensor itself to a remote reference unit and/or measuring instrument. Yet, we would avoid the expense of high specification thermocouple cable on a long run. Connecting cheaper cable


would be ideal but we need to do so without having to take particular care that temperature where the connection is made is known and taken in to account.


When connecting thermocouples to instruments, it is essential that a cable is used, which has the same emf output as the thermocouple; otherwise spurious emf is generated at these junctions. The best solution is to use the same material as the thermocouple (extension cable).


A cheaper alternative is to use compensating cables, the alloys of which are different from those of the thermocouple but have the same output over a limited temperature range.


Compensating and extension grades of wire are supplied in the bright-annealed condition.


For this to be possible, the thermo electrics properties of the additional conductor must not differ too much from those of the thermocouple itself. Extension and compensating cable provide convenient, economic solution-each with its pros and cons.


Extension cable uses wire of nominally the same conductor as the thermocouple itself, which thus inherently possess similar thermo power characteristics, and with no connection problems.


Miss-match error arising from high connecting box temperature is likely to be relatively small. These cable are less costly then thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distance typically as flexible wiring or multi-core cables. They are recommended for best accuracy.


Compensating cables, on the other hand are less precise, but cheaper. They harness quite different relatively low cost alloy conductor materials, whose net thermocouple in question, but which not them as faithfully as do extension cable. Thus, the combination develops similar output as those of the thermocouple, but the operating temperature range has to be restricted to keep


miss-match error acceptably small.





TECHNICAL CONCEPT:-





The thermocouple wires, made of different metals or alloys A and B, are joined together at one end H, called the hot (or measuring) junction, at a temperature T1.


The other ends, CA and CB (the cold or reference junctions), are maintained at a constant reference temperature T0, usually but not necessarily 32°F (0°C).


From the cold junctions, wires, usually of copper, lead to a voltmeter V at room temperature Tr.


Due to the thermoelectric potential gradients being different along the wires A and B, there exists a potential difference between CA and CB.


This can be measured by the voltmeter, provided that CA and CB are at the same temperature and that the lead wires between CA and V and CB and V are identical (or that V is at the temperature T0, which is unusual).


Such a thermocouple will produce a thermoelectric emf between CA and CB which depends only upon the temperature difference T1 - T0.





Thermocouple


(Type designation) Materials





B Platinum-30% rhodium/platinum-6% rhodium


E Nickel-chromium alloy/a copper-nickel alloy


J Iron/another slightly different copper-nickel alloy


K Nickel-chromium alloy/nickel-aluminum alloy


R Platinum-13% rhodium/platinum


S Platinum-10% rhodium/platinum


T Copper/a copper-nickel alloy








INSULATION:-





Insulation used in cable is:


• PVC


• TEFLON


• GLASS FIBER


• Silicone rubber: -


• Fire resistance mica cable: -





• PVC:


It is cheaper then other; it is used for the temperature range 30°C to 105°C. It is available in twisted, flat, &multi pair combination. There is arrangement of options-with ripcord, PVC sheathed, screened, with a copper wire drain, or steel wire armoured construction-the conductor itself being solid or stranded.











• TEFLON:


It is used for high temperature range 273°C to 250°C, or 300°C for short period. It is available in twisted, flat, & multi pair combination, but not steel wire armoured versions. It withstands attack from virtually all known chemicals, oils & fluids. All over the cable is made in extruded form and is


therefore gas, steam & water tight which makes them most suitable for application such as autoclaves or sterilizers.





• Glass fiber:


It is used for higher temperature range. It handles up to 50°C to 400°C, in some cases it is used up to 800°C. Single and multi-pair varieties are available in flat and twisted with many of the options. Ceramic fiber cable are used up to 1400°C which are suitable for use at normal air ambient temperature where there is a possibility of a hot spot, which might


damage lower rated cables as PVC.





• Silicon rubber:


Excellent properties for the propagation of flame by in corporation of flame retardant silicon rubber compounds. Suitable for situations where there is a risk of fire. Ideal for application where, for short period of time, the temperature can fluctuate, which would cause other cable to become


inflexible and brittle.





• Fire resistance mica cable:


Resistance to a temperature of 750°C for at least three hours in accordance with the flame test requirement. Essential for situations where it is of strategic importance to ensure that the cable continues to function during a major crisis-involving fire. The cable incorporates a high temperature rated MICA glass tape with a XLPE insulation on the cores and low smoke and fume material on the bedding and/ or outer sheath.Sheath material used is halogen free.





• Color-coding &specification:


Extension lead & compensating cable are distinguished by color code and letters, to ease identification of the whole circuit. Although the code used to be different from country to country, standard color has been adopted for all standard thermocouple.


B S 4937 part 30(1993) provides standards of coding. In this there is no difference between extension & compensating cable-color codes. With compensating cable, the different alloy used is distinguished by KCA & KCB.





• Armoured flame retardant PVC:Armoured flame retardant PVC is extremely useful where there is a need to run a number of thermocouple signals back to instrument. All cable incorporate insulated cores, bedding and overall sheath in flame retardant PVC, which has good properties for the reduced propagation of flame. The mechanical properties of these cables meet the requirement of BS EN


60811:1995.





• Non-Armoured flame retardant PVC:


Non Armoured flame retardant PVC is extremely useful where there is a need to run a number of thermocouple signals back to instrument. All cable incorporate insulated cores, bedding and overall sheath in flame retardant PVC, which has good properties for the reduced propagation of


flame. The mechanical properties of these cables meet the requirement of


BS EN 60811:1995.








RTD Cable:-





RTD Cable are used to carry the RTD signals to the control room or field mounted instruments.


they will be in three wire four wire for simplex and ; six and eight wire for duplex


RTD.


The conductor normally used for RTD cable is Nickle Plated Copper (NPC) or Silver Plated Copper (SPC) or


Bare Copper (BC).





ARTICLE SOURCE:-http://www.compensatingcables.net


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