High Temperature Cables – What is best for your application?

Thermocouple and RTD cable

Here at Process Parameters, we use a wide range of high-performance cables in the manufacture of our temperature sensors. We can also supply cable for making connections between your sensors and instrumentation. In this article, we look at the pros and cons of some of the more common types.

 

PVC

The first cable in our list is probably the most common and is used on almost every appliance we have around us in the home and workplace. Its main benefit is that it offers a very low-cost cable but for industrial use, care needs to be taken.

In general, the operating temperature range for PVC is just from -10 to +70°C, however, we use a modified version of PVC which increases the maximum to 105°C. This may suit many HVAC applications but in general, is insufficient for industrial processes.

Otherwise, PVC can offer a good range of properties such as good resistance to UV, acids, alkalis and oils. It is also non-flammable and offers good electrical resistance properties.

Negative aspects of PVC tend to relate to long term stability as it can degrade over time at elevated temperatures which can affect its properties. There are some compounds that can affect the material quite dramatically. It is also a dense material and combined with relatively thick insulation layers can lead to cables being relatively heavy.

  • Good resistance to UV
  • Good resistance to acids and alkalis
  • Good resistance to oils
  • Good electrical resistance
  • Degrades over time at elevated temperatures
  • Dense material
  • Thicker insulation than some materials – heavier cable

 

Silicone Rubber

An excellent alternative to PVC is silicone rubber as this offers a highly cost-effective, flexible cable.

Operating temperature is extended to an extremely useful -60 to +180°C continuously with high temperature possible for short times. We also now stock a special version with a continuous temperature rating of up to 240°C making it extremely competitive with PFA and PTFE cables in certain applications.

The prime advantage of silicone is its extreme flexibility and lack of springiness. The cable remains flexible to very low temperatures making it ideal for deep freeze storage applications.

At the other end of the scale, silicone rubber has excellent resistance to moisture and steam making it the cable of choice for use in many immersed applications and those in steam sterilising autoclave processes. High-pressure steam only has an effect on the material above 150°C.

In general silicone rubber is considered to be physiologically inert and is therefore suitable for use in food and medical applications. It also has excellent weather resistance and is, therefore, suitable for use in outdoor applications.

  • Excellent resistance to heat and cold
  • Excellent flexibility
  • Excellent resistance to UV
  • Excellent resistance to moisture
  • Generally good resistance to oils
  • Poor resistance to acids and alkalis
  • Not considered flame resistant unless manufactured with retardants

 

PTFE – PFA – TeflonTM

At Process Parameters, we refer to this group of products under the single name of PTFE. In reality, there are very minor differences between them. The main difference is how the material is manufactured.

PTFE is an extremely high-performance material and offers excellent features when used for the manufacture of industrial temperature sensors. For most applications, this is our “go to” cable insulation.

One stand-out feature from a temperature measurement point of view is the operating temperature range which is from -200 to +250°C. In addition, the cable retains excellent flexibility to a temperature of -70°C, below this the cable should be held static.

Another property is that PTFE is practically chemically inert making it suitable for use in almost any environment including string acids and alkalis. It is food safe and is also waterproof.

PTFE based materials are typically manufactured to be very thin. This helps cables to remain small and relatively lightweight unlike those of PVC or Silicone. Where weight is an issue such as motorsport or aerospace applications, PTFE can be a good choice.

Despite all these excellent properties, PTFE is almost impossible to seal against with sealants and adhesives. This makes using the cable in a probe assembly very tricky when trying to protect against the presence of moisture. At Process Parameters, we have used processes by which we are able to chemically etch the insulation prior to potting and this has given excellent results.

Aside from its wide operating temperature range, this type of insulation has plenty of other useful features.

  • Exception chemical resistance – practically chemically inert
  • Excellent resistance to UV and weather
  • Good flexibility, but can be a little springy unlike silicone rubber
  • Ideal for use in food and pharmaceutical/medical applications
  • Very low coefficient of friction.
  • Very difficult to bond or seal – but can be etched.

 

Glass Fibre

For very high temperatures there is no alternative but to move away from polymeric materials and use fibre based insulations. The most common of these is Glass Fibre and is available in two temperature ranges.

For use up to 400°C, we offer our standard glass fibre insulation and this covers a wide proportion of applications. For those where 400°C is insufficient, then an alternative is our high-temperature version is rated to 600°C. Apart from the temperature rating is there any other difference? In general, the higher temperature version is a bulkier larger cable as it has multiple layers of insulation. Otherwise, they are very similar.

Whilst these cables are able to withstand very high temperatures, and often there is no alternative, it is worth being aware of some other features of the cable which are not necessarily positive.

Being a woven fibre insulation, the finished cable is most definitely not resistant to water. Indeed the insulation will act like a sponge and absorb moisture readily. This can cause a low insulation resistance of short within the measurement circuit of the sensor and cause incorrect readings.

For obvious reasons, Glass Fibre insulation is not very robust and is susceptible to damage from abrasion and flexing. To avoid this, it is common to supply the cable with a stainless steel overbraid. This takes the form of thin stainless steel wires woven onto the outside of the cable in a similar format to the glass fibre underneath. This provides excellent protection against mechanical damage.

  • Excellent high-temperature resistance to 400°C or 600°C.
  • Relatively chemically inert.
  • Not suitable for use in presence of moisture
  • Not suitable for use below 0°C.
  • Relatively low resistance to damage from flexing and abrasion. Improved with stainless steel overbraid.

 

Other types of cable insulation

The four types of material described above are the most common found in temperature measurement but they are by no means the only types. For special applications, it is possible to supply sensors and cables with insulations of the most readily available types including Kapton (Polyimide), Thermoplastic Elastomer (TPE), Polyurethane (PU) and ceramic fibre.

 

This article is intended as a guide and does not replace our own free technical assistance which is always readily available.

If you would like additional assistance, we can help you. Please contact us and we would be glad to discuss your application and help with your product selection.

T: 01628 778688

E: sales@processparameters.co.uk