Thermocouples, Part 1: What Type Do I Need?
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, I will limit this article to the eight most commonly used thermocouples known as Types J, K, T, E, N, S, R and B. These letter designations, now used worldwide, follow the recommendations of the International Electrotechnical Commission (IEC), Instrumentation, Systems and Automation Society (ISA), American Society for Testing and Materials (ASTM) and American National Standards Institute (ANSI).
Type J. Usable up to 1,328oF (720oC), Type J is not very susceptible to aging up to 1,000oF (~540oC). It is cost effective and the thermocouple of choice in the plastics processing industry, where temperatures rarely exceed 750oF (~400oC). The iron conductor is subject to oxidation at higher temperatures and when unprotected.
Type K. Usable up to 2,100oF (~1,150oC) in an oxidizing atmosphere, Type K can be affected by metallurgical changes that can cause a calibration drift of ~2 to 4oF (1 to 2oC) in a few hours, increasing to 9oF (5oC) over time. A special grade of Type K is available that can maintain special limit accuracy up to 10 times longer than the regular grade.
Type E -- Chromel/Constantan. Usable up to 1,508oF (820oC), Type E has the highest millivolt output of all the thermocouples. It has similar calibration drift to that of Type K, so the same precautions are recommended.
Type N -- Nicrosil/Nisil. Usable up to 2,300oF (1,260oC), Type N was developed to overcome several problems inherent in Type K thermocouples. Aging in the ~570 to 1,110oF (300 to 600oC) 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. Oxidization of the copper limits the usable temperature of Type T to about 700oF (370oC). It has been the thermocouple of choice for applications down to -328oF (-200oC).
Types R -- (Platinum, 13 Percent Rhodium/Platinum) and S (Platinum 10 Percent Rhodium/Platinum). Types R and S are usable up to ~2,700oF (1,480oC). 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 percent more millivolt than Type S.
Type B -- Platinum, 30 Percent Rhodium/Platinum, 6 Percent Rhodium. Usable up to 3,092oF (1,700oC), Type B also is easily contaminated and damaged by reducing atmospheres. The same protective measures suggested for Types R and S apply to Type B thermocouples.
Wire Size and AtmosphereThe upper temperature limits and life expectancy of all thermocouples are 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 long life of the materials usually dictate small wire sizes, typically 0.35 mm.
Construction. In its most basic form, take two wires, join them at the hot end, and measure the millivolt output at the other end -- who needs more? Often there is nothing wrong with that. Poke it in a duct or a nonaggressive 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 on web sites. Here are just a few design examples:
- Twin or quad bore alumina with the wires inside. The junction may be exposed, given a nonaggressive environment. Otherwise, it may be fitted in metal or ceramic outer tube to protect from atmosphere or damage.
- Fiberglass or ceramic-fiber-insulated wire inside a stainless steel tube; hot junction welded to the closed end. This is a common on plastics and good for fast heat transfer.
- The metal clad magnesium oxide (MgO) compacted design is rugged, easily bent and good for protection from aggressive media. There is a wide choice of sizes and metal sheath grades for different media.
- 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 they can mess up your control system response. Keep them small as possible.
Response Times of Thermocouples. Time constants go from 0.1 sec (e.g., exposed tip in a fast stream) to some 15 sec (e.g., a thermowell in a tank). Time constant is the time taken to reach 63 percent of the final value. Watch that you don't hurt the control response by putting a slow thermocouple on a lively process.