Instrument Signal Cables

A thermocouple is made up of two metallic conductors of different materials. When the junctions of the materials are at two different temperatures, an electro-motive force (EMF) is generated which can be measured. The voltage output is proportional to the temperature difference and hence if the cold junction temperature is known, the instrument can be calibrated to give a direct reading of the hot junction, or vice versa.

Thermocouple Extension Cables
Thermocouple Extension Cables are essential wherever it is necessary to carry out precise temperature measurements. They serve as an extension lead from the thermocouple, which is installed at the hot junction to the measuring instrument. The cable conductor is of the same material as that of the thermocouple.

Thermocouple Compensating Cables
As the conductor used within some thermocouples are very expensive, the expense of interconnecting between the thermocouple and the measuring instrument can be prohibitive, particularly when outside of the junction areas where high accuracy over the wide temperature range of the thermocouple is not required. In this area compensating cables can be used, whose conductors have similar EMF values as that of the thermocouple, but over a limited temperature range. Compensating cables provide an economical way of extending temperature measurement over long distances.

Features
In addition to using twisted pairs to reduce cross talk, screening of the cable is often required to protect signals from external electromagnetic, electrostatic and radio frequency interference. In case of critical applications screening can also be provided over individual pairs alongwith the overall screen.

Even if one installs the most accurate and reliable temperature sensors and uses the finest instruments to monitor and control, the system will be as strong or as weak as the connection between the sensor and instrument. Thermocouple Extension and Compensating Cables are available to meet the most demanding standards like ANSI, BS, IS, DIN, NFC, IEC.

Conductor
The conductor wires are carefully selected, matched and calibrated for specified limits of error in accordance with the specified standards. The standard conductor size for single pair armoured cable is 16 AWG and for multipair cables it is 20 AWG. Special sizes and stranded conductors are also available against specific requirements as mentioned in Table-2.

Insulation, Inner & Outer Sheath
The insulation, the inner and the outer sheath are provided so as to suit the temperature and the environmental conditions in which the cable is likely to be used.

The most commonly recommended material is PVC. For special applications HR PVC, FRLS PVC, Neoprene/Silicone Rubber, Fibre Glass, Asbestos, PTFE-Teflon, Ceramic yarn can also be provided to meet hostile and high temperature requirements.

Colour Code & Pair Identification
The positive core and the negative core of each pair and the outer jacket are colour coded in accordance with the specified standard. For various colour codes refer Table 3.

In case of multipair cables, one core in each pair is number printed at close intervals. Alternately a numbered polyester tape is applied over each pair for easy identification.

Individual Pair Screen
In case of critical applications individual pairs within the cable are screened with an aluminium-polyester tape with the metallic side down in continuous electrical contact with a 0.5 sqmm stranded tinned copper drain wire. An isolation tape is provided below and over the screen tape. Tinned copper braid shielding can also be provided.

Cable Assembly
Twisted pairs are laid up and assembled in concentric layers. A binder tape is normally provided over the cable assembly.

Overall Screen
An overall aluminium-polyester tape screen is applied with the metallic side down in continuous electrical contact with a 0.5 sqmm stranded tinned copper drain wire.

Armour
Galvanised steel round wire or strip armour in accordance with BS 5308 / IS 1554 respectively. The armour wire and strip confirms to IS 3975. If required, G.I square interlock armour or G.I / SS wire braid armour can also be provided.

Communication Pair
A pair of communication wires can be provided, if required. The standard conductor size is 0.5sqmm flexible annealed bare copper, insulated with the same material as the main cores.