Blog / Thermoelectric temperature sensors (thermocouples)

Thermoelectric sensors react to temperature changes by changing the thermoelectric force of the thermocouple built into them.
Thermocouple – two conductors made of different materials, connected to each other at one end and forming part of a system that uses a thermoelectric effect to measure temperature (the Seebeck effect).
The thermoelectric effect consists in creating a thermoelectric force due to the temperature difference between two welds: the measuring (combined ends of the thermocouple), which it acts in the measured temperature and reference (free ends of thermocouples), which is at a known temperature (usually 0°C).

Thermocouple types

Thermo-electric characteristics of thermocouples (short-form data) /EN 60584-1/

Tolerances for thermocouples (EN 60584-2)

(*) For thermocouples type S and R permissible deviation is calculated according to the formula:: $[1+(t-1100)·0,003]°C$

Sheted thermocouples

Sheathed thermocouples are shielded by a nickel-chromium steel tube within which are housed the thermo-electric wires and isolation
material in the form of heavily compressed manganese oxide (MgO).
The material of the sheath provides the mechanical and chemical shield for the fused measurement end and high resistance to many
aggressive environments.
At one end the thermocouple is welded creating the fused end for measurement. The external shield is hermetically sealed by welding.
The other end of the thermocouple is connected to a compensating cable, plug- socket connector or connector block in the sensor head.
Due to the heavily compressed isolating layer and the metallurgical structure of the thermo-electrode and sleeve, the thermo-elements
are bendable and can be bent to a minimum radius of five times their external diameter.
The main advantage of sheathed thermocouples are: small external diameters, long service life and flexibility permitting them to be bent
and screwed in place, high resistance to shock and vibration and low thermal inertia.

Dynamic properties of sheathed thermocouples

Thermocouple measuring junctions

	1a	Single measuring junction (single thermocouple), with galvanic ground connection with sheath. Short time constant. Measuring junction isolated from chemical and mechanical environmental influences. Applied in non- conducting environments
	1b	Single measuring junction, galvanically isolated from sensor sleeve. Longer time constant. Applied in conducting environments or where electric isolation from the measurement system is required.
	2a	Twin measuring junction (twin thermocouple), with galvanic ground connection with sheath. Short time constant. Measuring junction isolated from chemical and mechanical environmental influences. Applied in non-conducting environments. May be applied in demanding situations allowing continuous measurement of process temperature (damage to one measuring circuit does not cause failure)
	2ab	Twin measuring junction., isolated from sheath and connected together. Longer time constant. Applied in conducting environments and when isolation from the measurement system is required.
	2b	Twin measuring junction, isolated from sheath and from each other. Longer time constant. Applied in electrically conducting environments and when isolation from the measurement system is required.
	1c	Single measuring junction, not protected or protruding outside the sheath, exposed. Shortest time constant. Measuring junction sensitive to mechanical damage and not resistant to atmospheric corrosion
	2c	Twin measuring junction , not protected or protruding outside the sheath, exposed. Shortest time constant. Measuring junction sensitive to mechanical damage and not resistant to atmospheric corrosion. May be applied in demanding situations allowing continuous measurement of process temperature (damage to one measuring circuit does not cause failure).

Compensating cables (EN 60584-3)

Compensating cables are made of the same materials as thermocouple or an alloy substitute that is not identical to the thermocouple,
but have the same properties as a thermocouple.
Alternative materials are used for K and N-type thermocouples and for thermocouples of precious metals type R and S (for B-type
thermocouples are applied copper wires in the temperature range up to + 100 ° C).
Compensating cables can be used in the temperature range up to 200 ° C.
Wires for compensating cables are standardized in EN 60584-3.
The thermoelectric voltages within acceptable operating temperatures correspond to the thermoelectric voltage for thermocouples
according to standard EN 60584-1. Deviation limits for compensation wires are given in standard EN 60584-3.
Compensating cables are manufactured in two classes.
Class 1 – compensation cables made from the same materials as the thermocouple.
Class 2 – compensation cables made of the same materials as the thermocouple or substitute materials.
Colour coding of compensating cables is in accordance with EN 60584-3.
We supply cables to class 1.

Thermoelectric temperature sensors made of thermocouple wires

These temperature sensors comprise a thermocouple conductor in a protection sheath. The maximum operating temperature of such a
sensor is dependent on the type of isolation of the conductor. Thermocouple sensors are made with the following isolating materials

The external cable isolation may be enclosed by a stainless steel braid or a stainless steel armored hose.
As sensor core we apply thermocouple solid wires (diameter 0.5mm) or thermocouple stranded wires 0.22mm² , 0.35mm² or 0.50mm² .

Thermoelectric temperature sensors with connection head

These temperature sensors comprises of a protection tube, measurement insert and a connection head with an internal terminal block.
The insert may be a replaceable element of the sensor or not. Sensors are available with heads in several arrangements, which differ
according to form, material of construction and dimensions.