Of all the temperature sensors used in industry and research, the thermocouple is the sensor of choice on grounds of temperature range, speed of response, ruggedness and cost.
Though there are other important sensors the eight most commonly used thermocouples known are Types J, K, T, E, N, S, R and B. These letter designations, now used world wide, follow the recommendations of the IEC (International Electrotechnical Commission), the ISA (Instrumentation, Systems and Automation Society), the ASTM (American Society for Testing and Materials) and the ANSI(American National Standards Institute).
Type J is useable up to 720°C . It is not very susceptible to aging up to about 540°C. It is very cost effective and is the thermocouple of choice in the plastics processing industry where temperatures rarely exceed 400 °C. The iron conductor is subject to oxidation at higher temperatures and when unprotected.
Type K is useable up to 1150°C in an oxidising atmosphere. Metallurgical changes can cause a calibration drift of 1 to 2°C in a few hours, increasing to 5 °C over time. A special grade of Type K is available that can maintain special limit accuracy up to ten times longer than the regular grade.
Type E – Chromel/Constantan is useable up to 820 °C. It has the highest mV output
of all the thermocouples and has similar calibration drift to that of Type K so the same precautions are recommended.
Type N – Nicrosil/Nisil is useable up to1260°C. It was developed to overcome several problems inherent in Type K thermocouples. Aging in the 300 to 600 °C range is considerably less. Also Type N has also been found to be more stable than Type K in nuclear environments, where Type K has been the sensor of choice.
Type T – Copper/Constantan. Oxidisation of the copper limits the useable temperature to about 370 °C. It has been the thermocouple of choice for applications down to – 200 °C.
Types R – (Platinum 13% Rhodium. Platinum) and S (Platinum10% Rhodium/Platinum) are usable up to 1480°C. They are extremely stable but reducing atmospheres are particularly damaging. This type should be protected with a gas-tight ceramic tube and a secondary tube of porcelain, silicon carbide or metal outer tube, as conditions require. Type R delivers some 15% more mV than type S.
Type B Platinum-13% Rhodium/Platinum 6% Rhodium) is usable up to 1700 °C, ). Also easily contaminated, and damaged by reducing atmospheres. The same protective measures for R and S shown above apply to type B thermocouples.
Wire Size and Atmosphere. The upper temperature limits and life expectancy of all thermocouples are very dependent on atmosphere and wire size. Don’t count on going to the above limits in most applications. For longer life and higher temperature use, choose the larger size wires. For speed of response choose the smaller sizes; but note that a protection tube will dominate response time. With the platinum alloys the high cost and the long life of the materials usually dictate small wire sizes, typically 0.35mm.
Construction Basic:.Take two wires, join them at the hot end, measure the mV at the other end; who needs more? Often nothing wrong with that. Poke it in a duct or a non-aggressive liquid, or clamp it to the process under a screw and washer or a hose clip.
For protection from damage, abrasion and corrosion I’ll give a few choices here. But for the hundreds of real workable answers look in the many excellent supplier catalogs and web sites. Here are just a few design examples:
1.Twin or quad bore alumina with the wires inside. The junction may be exposed, given a non-aggressive environment. Otherwise it may be fitted in metal or ceramic outer tube to protect from atmosphere or damage.
2. Fiberglass or ceramic-fiber insulated wire inside a stainless steel tube; hot junction welded to the closed end. This is a very common on plastics machinery and good for fast heat transfer.
3. The metal clad MgO (magnesium oxide) compacted design is very rugged, easily bent and good for protection from aggressive media. There is a wide choice of sizes and metal sheath grades for different media.
4. The thermowell is a tough handy fitting for giving access to the contents of a vessel or pipe. You can slip your thermocouple inside it and still get to it for service or replacement without draining the contents. The hot junction stops short of the closed end of the well. To aid heat transfer you might use a blob of electrically insulating, thermally conducting material at the tip. Thermowell assemblies can be big, strong and slow so can mess up your control system response. Keep them small as possible.
Response Times of thermocouples. Time constants go from 0.1S (e.g. exposed tip in a fast stream) to some 15S (e.g. a thermowell in a tank). Time constant is the time taken to reach 63% of the final value. Watch that you don’t hurt the control response by putting a slow thermocouple on a lively process.
Adapted from an article originally written by Arthur Holland, Holland Technical Skills, for Eurotherm.