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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