Thermocouples

General

Thermocouples are sensors for measuring a temperature. The main components of the thermocouples are a thermocouple, which generates the so-called thermal voltage, and a connector, for example a connector. Thermocouples are used in many areas of temperature measurement. They are characterized by a high flexibility of construction and a quick response time.

Overview thermocouple types

Following thermouple types are specified in ISO EN 60584-1:

Type	Positive wire [plus]	Negative wire [minus]	Color ISO [plus / minus]	Color ANSI [plus / minus]	Temperature range [°C]
J	CuNi	Fe	black / white	white/ red	-200 +750
K	NiCr	NiAl	green / white	yellow / red	-200 +1100
N	NiCrSi	NiSi	pink / white	orange / red	-200 +1200
S	PtRh10%	Pt	orange / white	black / red	0 +1600
R	PtRh13%	Pt	orange / white	black / red	0 +1600
B	PtRh30%	PtRh6%	grey / white	grey/ red	+600 +1700
C	WRe25%	WRe5%	red / white	(black / red)	0 +2200

Various combinations of metals / alloys have proven to be advantageous since they have an almost linear curve of voltage over temperature.

Good to know: The thermal voltage is not generated in the tip but between the two wires. Most of the voltage is generated in the area of the highest temperature difference, for example in the transition from the furnace hot-zone to the insulation.

Thermospannung

The thermocouple consists of two wires, which are made of different alloys. Each of these wires develops a certain tension as soon as one end is warmer than the other. If you now connect both wires, the difference in the respective voltage in the wire can be measured. This difference is called thermal voltage.

How is the thermal voltage generated?

There are free electrons in every electrical conductor. They can move in the conductor and thus enable, for example, an electrical current to flow.

Physically, to put it in simplified terms, these free electrons occupy a certain space that grows with temperature.

This space is roughly the same size in a uniformly heated copper conductor. This is shown schematically in Figure 1.

Now you heat one end of the conductor.

This increases the range of motion of the free electrons in this area. Due to the effect that the same charges repel each other, the more heated electrons “push” the rest towards the cold end.

As a result, there is a lack of electrons at the heated end and an excess at the cold end. This creates a voltage.

Good to know: The level of the tension is mainly influenced by the chemical composition of the wire and the temperature difference along them!

However, no signal can be generated with the single voltage of the wire. So you need a second wire. Both wires should be connected at the highest temperature point.

The voltage difference between the two wires serves as a signal and is defined as thermal voltage.

Figure1 : Example of a uniformly warm copper conductor at room temperature.

Figure 2: Example of the copper conductor above, the end of which is now heated.

Thermospannung entsteht zwischen zwei unterschiedlichen, vebundenen Metallen

thermo-control

thermo-control Körtvélyessy has been producing reliable and durable thermocouples since 1983. Specializing in high-temperature applications, the only platinum-rhodium thermocouples with a 4-year guarantee are manufactured here.

All versions have the following design features / options:Individual process connection possible according to specification

Vacuum-tight and pressure-resistant up to 20bar
100% helium tested, max. permissible leak rate 10-6mbar · l / s
Manufacturer’s certificate with manufacturing data and test result helium leak test
Calibration certificate according to AMS 2750 / CQI-9
Pre-assembled with cables and plug connections

In our web shop you will find a large selection of sheathed thermocouples and accessories.