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Figure 1. Stripped metal sheathed cable with two leads. Leads are normally insulated by heavily compressed magnesia (MgO).
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Figur2 2. Cross-section of a metal sheathed cable.
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Figure 3. The bending radius should exceed twice the diameter of the cable.
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Figure 4A. Insulated
Junction
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Figure 4B. Grounded Junction |
Figure 4C. Exposed Junction |
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| Figure 5. Metal sheathed cable with 4-6 conductors is appropriate for the built-in Pt100-sensor. |
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Description
Metal sheathed cable consists of two or more leads which are insulated by heavily compressed magnesia (MgO). Different diameters of sheath are available. See table below. The thickness of the sheath can vary among the manufacturers though 10-15 % of the outer diameter is typical. The lead diameter is typically 20 % of the sheath outer diameter. See figure 1 and 2. Metal sheathed cable is a good alternative when you require the following properties:
• High ambient temperature
• Lengthy protection against moist and steam
• Plastic bendability
• Abrasion resistance
Insulation
The insulation normally reaches more than 1000 MOhm at room temperature. Higher temperatures, greater lengths and decreased diameters reduces the insulation resistance. If moisture comes in direct contact with the compressed magnesia, the insulation is also reduced. A greater percentage than 50 RH in the air will dramatically reduce the insulation within minutes.
Despite these facts, metal sheathed cable is normally the type of cable that has the best qualities to cope with high surrounding temperatures, used in thermocouples, Pt100 and as electrical conductors. With moderate demands for insulation and lifetime, the metal sheathed cable can be used in temperatures raising over 1000 °C.
Plastic bending
Metal sheathed cable is plastic and allows bending. Though, the bending radius should exceed twice the diameter of the cable to avoid fractures and short circuits. For example, a cable with the diameter of 3 mm needs to be bent with a radius greater than 6 mm (practically using a tube with the diameter of 12 mm). See figure 3.
Applications
Metal sheathed thermocouples are made out of metal sheathed cables, where the conductors are chosen to fit the specific type of thermocouple. The point of measurement is constructed in different ways, depending on the characteristics needed. The most common construction at Pentronic is the ‘insulated/ungrounded hot junction’, see figure 4(A). It is the most mechanically secure construction which also, thanks to the insulation, allows differential measurements without additional insulation amplifiers (as long as the highest temperatures are avoided). The second construction is the ‘grounded junction’ (see figure 4B), which is appropriate when a short response time is desired. The third construction is the ‘exposed junction' (see figure 4C). The choice of the exposed junction is normally based on an extremely high demand for a short response time.
Metal sheathed cables do also exist with double thermocouples. This means that the cable consists of two pairs of conductors of the same type of thermocouple.
Built-in Pt100
As an alternative to metal sheathed thermocouples, a Pt100 detector can be built in. It is then important that the strands are electrically sufficient conductors, for example nickel. Typical dimensions of this type of construction are 3 mm, 4,5 mm and 6 mm. The number of conductors are normally four to six, symmetrically placed on the same radius (see figure 5). The temperature should not exceed 600 °C for this type of construction.
Common dimensions
Conductors and sheaths come in varying types of material. The maximum continuous length is normally not limiting, and it depends on the diameter and subset of length demanded according to specifications. For example, material with the diameter of 3 mm is possible to produce to a length of at least 100 meters. Theoretically, the length of 400 meters is the limit.
Pentronic normally uses the following dimensions of cables, according to the metric system. (Other dimensions could be used if required)
Metal sheathed cable,
diameter (mm)
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Type
K
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Type
N
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Type
J
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Type
T
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Pt100
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Color code ref. to
IEC 60584-3
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-
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0,5
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X
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0,8
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X
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1,0
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X
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X
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X
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X
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1,5
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X
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X
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X
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X
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2,0
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X
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X
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X
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X
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3,0
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X
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X
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X
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X
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X
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4,5
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X
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X
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X
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X
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6,0
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X
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X
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X
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X
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8,0
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X
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