Thermocouples are made with 2 leads of
different metals or metal alloys which
deliver a directly proportional voltage
vs. environmental temperature.
These two metals are assembled in contact, known as "hot welding" either
per a mechanical contact (due to metal oxidation, this isn't perfect), or with
an intermediate metal as in the case of molten in the aluminium (or other
metals) industries but with a poor accuracy, or, which is the main cases,
These thermocouples are extended with compensation
cables (made with the same materials as
thermocouples) and are connected to the data
acquisition equipment with normalised plugs
(also made of similar materials), or with a direct
In both cases, after the normalised plug or just
Thermocouples can be realised with uninsulated leads
if two leads of different metals are welded together, end to end, when the solder is exposed
to a temperature, a voltage will appear at the other end of these leads.
Such voltage is proportional to temperature.
A thermocouple is realised.
The solder is named "hot melting".
A lot of metals are able to be used for thermocouples.
They are choosen for their resistance towards temperature and environmental,
also for their e.m.f (electro motrice force).
the main used couples are as follows:
|CODE||Positive element||Negative element||medium e.m.f|
Platinum 10% Rhodium
Platinum 13% Rhodium
Platinum 30% Rhodium
Platinum 6% Rhodium
Tungsten 5% Rhenium
Tungsten 26% Rhenium
A coulour code permits to make a distinction between the different thermocouples,
following several specifications:
French Spec NFC 42-324
American Spec ANSI/MC 96-1
British Spec BS 1843
German Spec DIN 43714
Japanese Spec JISC 1610
e.g. thermocouple "K"
following British Spec, insulating jacket is red, positive element is brown and negative is blue;
following American Spec, insulating jacket and positive element are yellow, negative is red;
following European Spec, insulating jacket and positive element are green, negative is white
(as for all the other thermocouples).
Several possibilities exist for the realisation of the hot melts.
Accuracy and reliability of thermocouples will depend on the solder quality.
It's necessary to avoid using a third metal which will introduce a third parameter
not necessarily cancelled by the couples it makes with the two other metals.
It's difficult to realise a solder with a blowlamp without using a third metal,
electrical melting isn't really very solid.
Plasma TIG and YAG Laser welding are surely the best processes.
Thermocouples leads exist in a lot of sizes from 12,7µm(0.0005"), which
AS-LASER assembles with a YAG Laser.
Leads can be uninsulated or insulated (with PVC, Teflon, Glass silk tissue),
or encapsulated in a tube (Stainless steel or Inconel).
Various shapes can be supplied and always custom designed.