Editing Thermocouple (section)

==Thermocouple insulation==

[[File:Thermocouple K (2).jpg|thumb|Typical low cost type K thermocouple (with standard type K [[electrical connector|connector]]). While the wires can survive and function at high temperatures, the plastic insulation will start to break down at 300&nbsp;°C.]]

=== Wires insulation ===
The wires that make up the thermocouple must be [[Insulator (electricity)|insulated]] from each other everywhere, except at the sensing junction. Any additional electrical contact between the wires, or contact of a wire to other conductive objects, can modify the voltage and give a false reading of temperature.

Plastics are suitable insulators for low temperatures parts of a thermocouple, whereas ceramic insulation can be used up to around 1000&nbsp;°C. Other concerns (abrasion and chemical resistance) also affect the suitability of materials.

When wire insulation disintegrates, it can result in an unintended electrical contact at a different location from the desired sensing point. If such a damaged thermocouple is used in the closed loop control of a [[thermostat]] or other [[temperature control]]ler, this can lead to a runaway overheating event and possibly severe damage, as the false temperature reading will typically be lower than the sensing junction temperature. Failed insulation will also typically [[outgas]], which can lead to process contamination. For parts of thermocouples used at very high temperatures or in contamination-sensitive applications, the only suitable insulation may be [[vacuum]] or [[inert gas]]; the mechanical rigidity of the thermocouple wires is used to keep them separated.

===Table of insulation materials===
{{more citations needed section|date=June 2014}}
{| class="wikitable sortable"
|-
! Type of Insulation
!data-sort-type="number"| Max. continuous temperature
!data-sort-type="number"| Max. single reading
! Abrasion resistance
! Moisture resistance
! Chemical resistance
|-
| Mica–glass tape
| 649&nbsp;°C/1200&nbsp;°F
| 705&nbsp;°C/1300&nbsp;°F
| Good
| Fair
| Good
|-
| TFE tape, TFE–glass tape
| 649&nbsp;°C/1200&nbsp;°F
| 705&nbsp;°C/1300&nbsp;°F
| Good
| Fair
| Good
|-
| Vitreous-silica braid
| 871&nbsp;°C/1600&nbsp;°F
| 1093&nbsp;°C/2000&nbsp;°F
| Fair
| Poor
| Poor
|-
| Double glass braid
| 482&nbsp;°C/900&nbsp;°F
| 538&nbsp;°C/1000&nbsp;°F
| Good
| Good
| Good
|-
| Enamel–glass braid
| 482&nbsp;°C /900&nbsp;°F
| 538&nbsp;°C/1000&nbsp;°F
| Fair
| Good
| Good
|-
| Double glass wrap 
| 482&nbsp;°C/900&nbsp;°F
| 427&nbsp;°C/800&nbsp;°F
| Fair
| Good
| Good
|-
| Non-impregnated glass braid
| 482&nbsp;°C/900&nbsp;°F
| 427&nbsp;°C/800&nbsp;°F
| Poor
| Poor
| Fair
|-
| Skive TFE tape, TFE–glass braid
| 482&nbsp;°C/900&nbsp;°F
| 538&nbsp;°C/1000&nbsp;°F
| Good
| Excellent
| Excellent
|-
| Double cotton braid
| 88&nbsp;°C/190&nbsp;°F
| 120&nbsp;°C/248&nbsp;°F
| Good
| Good
| Poor
|-
| "S" glass with binder
| 704&nbsp;°C/1300&nbsp;°F
| 871&nbsp;°C/1600&nbsp;°F
| Fair
| Fair
| Good
|-
| Nextel ceramic fiber
| 1204&nbsp;°C/2200&nbsp;°F
| 1427&nbsp;°C/2600&nbsp;°F
| Fair
| Fair
| Fair
|-
| Polyvinyl/nylon
| 105&nbsp;°C/221&nbsp;°F
| 120&nbsp;°C/248&nbsp;°F
| Excellent
| Excellent
| Good
|- 
| Polyvinyl
| 105&nbsp;°C/221&nbsp;°F
| 105&nbsp;°C/221&nbsp;°F
| Good
| Excellent
| Good
|-
| Nylon
| 150&nbsp;°C/302&nbsp;°F
| 130&nbsp;°C/266&nbsp;°F
| Excellent
| Good
| Good
|-
| PVC
| 105&nbsp;°C/221&nbsp;°F
| 105&nbsp;°C/221&nbsp;°F
| Good
| Excellent
| Good
|-
| FEP
| 204&nbsp;°C/400&nbsp;°F
| 260&nbsp;°C/500&nbsp;°F
| Excellent
| Excellent
| Excellent
|-
| Wrapped and fused TFE
| 260&nbsp;°C/500&nbsp;°F
| 316&nbsp;°C/600&nbsp;°F
| Good
| Excellent
| Excellent
|-
| Kapton
| 316&nbsp;°C/600&nbsp;°F
| 427&nbsp;°C/800&nbsp;°F
| Excellent
| Excellent
| Excellent
|-
| Tefzel
| 150&nbsp;°C/302&nbsp;°F
| 200&nbsp;°C/392&nbsp;°F
| Excellent
| Excellent
| Excellent
|- 
| PFA
| 260&nbsp;°C/500&nbsp;°F
| 290&nbsp;°C/550&nbsp;°F
| Excellent
| Excellent
| Excellent
|-
| T300*
| 300&nbsp;°C
| –
| Good
| Excellent
| Excellent
|}
Temperature ratings for insulations may vary based on what the overall thermocouple construction cable consists of.

Note: T300 is a new high-temperature material that was recently approved by UL for 300&nbsp;°C operating temperatures.