Thermocouple Probes

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,
by solder.


It is possible to make either an electric welding (but as for mechanical contacts,
there is a risk of poor contact), or a solder with a soft metal (this is harmful for
accuracy), or a welding with a blowlamp (with a risk of damage for metals and,
mainly, for alloys), or a welding with a TIG or, even better, with a laser YAG
which gives the perfect junction.


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
connection.

In both cases, after the normalised plug or just
at the direct connection level, it is necessary,
for the user, to foresee, in his equipment, a
temperature compensation named "cold welding
compensation", otherwise the different metals in
contact create thermo-electric effects which could
harm accuracy.

Thermocouples can be realised with uninsulated leads
(O.D. from 12,7 microns or 0.0005"), insulated leads
with PVC, Teflon, Glass Silk Jacket, or other.


They also can be covered with metal like Stainless steel or Inconel,
in several diameters and with different shapes. They can be assembled
in a probe with or without a junction head, with or without transmitters.

Thermocouples

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:

CODEPositive elementNegative elementmedium e.m.f
in µV/C.
Temperatures
in C.
T
Copper
Constantan
51-200/+350
J
Iron
Constantan
55-200/+600
E
Nickel Chrome
Constantan
78-200/+600
K
Nickel Chrome
Allied Nickel
41-200/+1.000
N
Nicrosil
Nisil
380/+1.150
S
Platinum 10% Rhodium
Platinum
110/+1.550
R
Platinum 13% Rhodium
Platinum
120/+1.550
B
Platinum 30% Rhodium
Platinum 6% Rhodium
80/+1.650
W
Tungsten 5% Rhenium
Tungsten 26% Rhenium
130/+2.150

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

and, finally, an European Spec, in view to replace the others :IEC 584-3 in which the
insulating jacket and the insulation of the positive element are of the same colour.

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.

For any further information, don't hesitate to contact us.
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