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Because voltage (EMF) which is created by the difference of temperatures between junctions will change when cold junction’s temperature is changed but hot junction’s temperature is constant, different temperature values are measured. Therefore, mV tables of the thermocouples are designed assuming that the cold junction is at 0°C for standardization. For example, voltage values of thermocouple at 200°C are found when hot junction is at 200°C and cold junction is at 0°C

Thermocouples are installed into protecting tubes venturing the environment conditions. Protecting tubes are made by different materials because of these conditions. Because element wires are places into different poles, they are isolated by ceramics.

Voltage values of thermocouples are very small. Therefore, that environmental noise of 50-60Hz power supplies may affect the output voltage of thermocouple. According to this, it is recommended to apply low pass filter to thermocouples’ output. In addition, an amplifier may be used to amplify the signal.

Output signals of thermocouples are not linear; therefore, if the signal is used in PC or digital measurement equipments, linearization should be applied to the signals.

Bacause of their reliability, easy usability and the ability of measuring wide temperature range, thermocouples are used commonly in industry, while selecting thermocouples, here are the points that user should note about:

• Temperature range
• Rezistance of thermocouple or protecting tube against chemical mattersdayanıklılığı
• Resistance against corrosion and vibration
• Installation requirements(may need being compatible with an existing device, existing mechanical structure may determine immersion length or the diameter of protecting tube.)

Here are the structural parts of a thermocouple:

1- Element Wire
2- Isolator
3- Primary
4- Protecting Tube
5- Mounting Part
6- Connection Head
7- Flange, Mounting Bush

The most commonly used element wires in industry that are compatible with the standarts of DIN 43710 and IEC-584 are below:

If temperature-mV curve of thermocouples is examided, it can be seen that these curves are not linear. Each element wire has a region of temperature that is more linear than the other regions. For example, if we compare Fe-Const thermocouple that is measuring 800 °C with a NiCr-Ni thermocouple that is measuring 1200°C , we can see that Fe-Const has a more linear curve that NiCr-Ni in the temperature range of 300°C -500°C . So In this temperature range, Fe-Const is more reasonable than the NiCr-Ni thermocouple. If PtRh-Pt thermocouple element wire is examided, it can be seen that it is not linear below 800°C, 800°C -1600°C is the reasonable temperature range for PtRh-Pt thermocouple.

Thermocouple element wires have two juctions, one is (+) and the other is (-). They should be noticed while connecting the thermocouple. Some color codes are used in international standards.

After welding the end point of the element wires, if it is placed in a protecting tube or not, (+) and (-) junctions are isolated. Generally KER-610 ceramic isolators are used for isolating. Furthermore, KER-799 Pre Aliminium is used in special processes (over 1500°C ). Generally, single part isolators are used for PtRh element wires, double part isolators are used in with Fe-Const, NiCr-Ni or the others.

Limits of the elements wires are changed by the diameter of element wire. As it widens, temperature limit increases, also it operates in a longer periode. Sensibility and protecting tube diameters are another importans points.

Thermocouple protecting tubes are selected according to the process conditions. To make more strong whether element wire or protecting tube, process conditions should be looked over correctly.

Generally, Metal sheaths are used till 1200 °C, over 1200°C , it is reasonable to use ceramic staths. Special alloyed sheaths can be used until 1250°C also.

Here are the most common types of thermocouples at thermocouple processes:

1.4301 DIN Stainless (304 Quality)
1.4541 DIN Stainless (321 Quality)
1.4571 DIN Stainless (316 Quality)
1.4749 DIN Stainless (446 Quality)
1.4841 DIN Stainless (314 Quality)
2.4816 DIN Stainless (INCONEL-600)
C-2 pure iron
Alloy-25

Selection of the protecting tubes can be found in thermocouple coding forms and also in download section as a pdf document.

If process temperature is over 1200°C, ceramics tubes are commonly used as protecting tubes.

The most commonly used ceramic tube types are below:

KER-530

KER-610

KER-799

Element wires and compensation cables are connected in right polarities to the ceramic terminal in connection head that the thermocouple tubes are fastened A-type large and B-type small heads in standards are used. Connection heads are available having dimensions according to DIN 43729. Metal heads are suitable for a maximum ambient temperature of 200°C. Protection class of heads is IP67. If any head is ordered without standards, EX-PROOF head or special heads are also produced.