Monday, March 17, 2014

Exposed thermocouple junction | tip insulation

As we know, there are three types of thermo-couples according to junction type:

- Grounded junction
- Under-grounded junction
- Exposed junction


The following are some methods to electrical insulate exposed junction:

Using hot shrink tube:
A shrink tube of the correct size may be used to electrically insulate the exposed junction of thermocouple.

Using PVC insulation tape:
This is a very simple method but the PVC tape has low thermal conductivity.
Using beryllium oxide:
Source: http://www.instructables.com/answers/I-am-looking-for-a-material-that-is-thermally-cond/


The most thermally conductive electrical insulator is beryllium oxide.  It has the thermal conductivity of metallic aluminum and the electrical conductivity of aluminum oxide.  Insulating washers of this material are available to fit most bolt-down  transistor and diode cases, and this is the only practical way to use it, since BeO is a hard and very refractory ceramic.  Note that BeO dust is carcinogenic.  It's not hazardous in pieces too large to accidentally ingest, but don't even think of grinding a notch in a BeO insulator to fit an odd corner.


Thermal conductivity [W/m.K]

Dielectric strength [kV/mm]
330
Water resistance
Yes
Wear resistance
Yes
Maximum temperature (melting point)   [°C]
2507

Using motor | transformer winding enamel | insulation

Key words: wire enamel, insulation varnish


Thermal conductivity [W/m.K]

Dielectric strength [kV/mm]
3.3
Water resistance
No 
Wear resistance
No 
Maximum temperature [°C]


Using thermal conductive silicon pads

Using mica


Using diamond:

Diamond has the highest thermal conductivity on earth. Also, it has a very high hardness, so it is wear resisting. It can withstand high temperatures. For sure, diamond does not seem to be an economic solution in most cases.


Thermal conductivity [W/m.K]
 2000
Dielectric strength [kV/mm]
 1000
Water resistance
 Yes
Wear resistance
Yes 
Maximum temperature [°C]
 Min. 3500

Using thermal conductive epoxy:


This method seems to be the most feasible and simple preparation method for relatively low temperature range. Thermal conductive epoxy is used in electronics field to seal components that are sensitive to water for water proofing like and in solid state electronic components like relays, diode bridges, and thermo-switches.

Epoxy pack is composed of two tubes (materials): one contains the resin and the other contains the catalyst.

The method is already used by Omega engineering for thermocouple types J, K, T, E (check the link below):
http://www.omega.com/pptst/TC-PVC_EPXOY_TC_PROBE.html

The highest thermal conductivity Omega provides provide is 0.712 W/m.K [model OT201]

3M adhesives (one of the leading companies in adhesives) also produces thermal-conductive epoxy with two options:
* 1-1.4 W/m.K [code TC2801]
* 0.72 W/m.K [code TC7207]


Thermal conductivity [W/m.K]
 1.4
Dielectric strength [kV/mm]

Water resistance
Yes
Wear resistance
Yes 
Maximum temperature [°C]