8. Connection Cables

Resistance thermometers generally use standard copper cable (if possible, with a cross-section of 1.5mm2) for the electrical connection to downstream measuring equipment. The connection between a (sheathed) thermocouple assembly and the measuring equipment/the reference junction may only be created with a compensation or extension cable that matches the thermocouple (see Section 9). Care needs to be taken to ensure that this cable is connected in the correct polarity. The above-mentioned connection cables are available as stranded conductors and as solid conductors with various insulations (e.g. PVC, silicon, PTFE, FEP and fibre glass) and, where appropriate, with additional sheathing (wire meshes) and shielding. When selecting and laying the connection cables, care should always be taken to ensure that :

• the insulation materials used are resistant to the thermal, mechanical and chemical stresses that occur at the usage site
• all cables at the junctions are uncoated and free from corrosion, moisture, dirt and loose parts, in order to keep the transition resistance small; 
• all cables are laid more than 0.6m away from parallel energy cables or at right angles to them, so that the measurement signal to be transmitted is not falsified by electromagnetic
• interference. This type of interference can also be avoided/reduced by using cables with static shielding (foil or wire mesh) and paired stranded wires;
• the junctions of all wires are subject to the same temperatures, so as not to generate any parasitic thermoelectric voltage through the creation of  intermediate elements. All connection cables should be regularly checked to ensure their mechanical and electrical integrity.  
  

8.1 Sheathed Cables    

In the case of extreme operating conditions (such as operating temperatures of 500°C or above, radioactive waves , high mechanical stress etc.), in which standard connection cables can no longer be used, mineral-insulated sheathed cables can be deployed. These consist of a heat-resistant and corrosion-resistant metal tube (sheath tube), in which up to 8 inner conductors are surrounded on all sides by highly compact, mineral insulating powder. Copper wires (for resistance thermometers) or thermocouple wires (for thermocouple assemblies) are used as inner conductors. Sheathed cables are manufactured according to special procedures, which involves drawing the entire material compound (protection tube, insulation and inner conductors), through the appropriate deformation steps, from an outer diameter of approx. 15 to 25mm to the desired end diameter (0.25 to 15mm) and winding it up into a ring shape. These cables are also used in the manufacture of sheathed thermocouple assemblies and sheathed resistance thermometers. The sheathed cables can be bent without impairing their technical properties, whereby the bend radius should be greater than 5 times the outer diameter of the sheathed cable. It is also easy to spirally coil the cables, enabling them to be used as "elastic" cables. As the mineral insulation suffers significant loss of its electrical resistance through absorption of moisture from the air, the cable ends need to be sealed with a sealing compound in a moisture-proof manner during storage and must not be unprotected for an extended period during processing.  

  
Go to Section 9   (Compensating and Extension Cables)

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