Thermocouples: A Thousand Shapes and Sizes
October 9, 2006
Every different application of thermocouples demands that you choose a shape, size and material to match the process. That's all this column deals with. Matters of temperature, environment, thermocouple alloys and construction materials are largely outside this month's topic and are for another day.
For want of a better approach, I will look at your choices, in ascending order of size and mass, with corresponding increases in robustness and response time. First example:
A Junction of Two Bare Thermocouple Wires. The wires could be hair-thin, 0.001" (0.0025 cm) diameter and the junction would be the smallest possible welded bead. The mass of a 4" (10 cm) length would be about 0.4 mg. The response time constant could be around 3 ms -- less in a fast moving liquid or gas stream. You could use it in a very fast control loop.
What's time constant? In response to a sudden temperature change, the voltage output (v) of a thermocouple climbs the exponential curve represented by:
This is comparable to the charge on a capacitor being charged through a resistor. The Greek letter tau is commonly used to denote the time constant. Its units are seconds. It represents the number of seconds for the millivolt output (v) to reach 63 percent of the final steady value V. It takes six time constants (figure 1) to get within 0.25 percent of the final steady reading.
At the other end of the scale, a kiln thermocouple could weigh upwards of 2 kg (4. 4 lb) and have a time constant of some 1 to 5 min.
Here are two examples of physical support arrangements for a pair of super-fine wires:
- A small diameter metal tube packed with magnesium oxide (MgO) and the junction open to the medium being sampled (figure 2). Tough and flexible except for the tip.
- Twin or quad bore alumina rod with the wires inside and the junction likewise exposed. Stiff and fragile.
You would use either one in a non-aggressive space -- a room, an air duct or a liquid -- giving you a fast-responding thermocouple that would be even faster in a fast-flowing medium. The MgO-packed tube can be closed off and the junction welded to the inside of the tip.
If damage is a threat, open junctions can be protected by a cage or a perforated metal tube with minimal effect on the response time. For further protection from damage, oxidation and corrosion, here are more examples:
- Fiberglass or ceramic-fiber-insulated wire inside a stainless steel tube. The hot junction can be welded to the closed end to speed up the response. The junction can be isolated from the protection tube if the measuring circuitry demands it.
This construction, typically 0.25" dia. and up, is used in large numbers on plastic processing machinery, fitted in deep holes drilled in barrels, molds and dies. It uses a spring-loaded bayonet thimble that ensures good thermal contact of the junction at the end of the blind hole just short of the polymer.
- The metal clad MgO-compacted design is very rugged, easily bent and good for protection from aggressive media. It is commonly called “mineral insulated” or MgO but it also has attracted the usual confusing bunch of trade names. There is a wide choice of sizes and metal sheath grades for different temperatures and media. Sheath diameters are available down to 0.01" (0.25 mm), giving a very fast response. One design is the needle probe. It is used to penetrate the medium for fast indication of internal temperature in the food processing industry.
- A bare wire junction on an adhesive plastic substrate is yet another simple arrangement for fast surface-temperature measurements in a fixed location. On a metal substrate it is called a shim thermocouple. Two more methods of attachment are by means of a pipe clamp or by bolting a special washer-like design to the process.
- A special fast hand-held surface thermocouple consists of a very thin conforming ribbon of two thermocouple alloys that meet at a smooth weld-line that becomes the area of contact. This greatly mitigates the problem whereby other hand-held designs make point-contact and cannot pick up all of the surface temperature.
This attention to response time is critical in fast temperature-control loops. Its neglect can make some processes uncontrollable.
Slower and TougherFrom the metal-sheathed design used on plastics machinery, we move into the field where robustness and long life at hostile environments and high temperatures are more important than speed of response (figure 3).
Heavy-duty industrial ovens, kilns and furnaces often use a design with up to 8 AWG (0.1285" [3.26 mm]) base metal wires in twin-bore alumina and inserted in a closed ceramic sheath. The thick wires are necessary for a long service life, and the closed sheath protects them from the furnace atmosphere. The very expensive noble metal wires, luckily, manage to survive without being anywhere near so thick. A protective closed-end tube of refractory metal is used on some furnaces.
A heavy duty cast iron or aluminum head holds the ceramic connection block. The whole assembly, depending on probe length, could weigh some 5 lb (2.3 kg).
The thermowell is a tough, closed-end tube that is screwed or welded into a vessel or pipe. The thermocouple, slid inside, senses the contents; it can then be serviced or replaced without draining the process. Thermowell assemblies add to the normal lag of the sensor. For all my cautions about control response, the sluggish nature of most industrial heat processes acts in your favor.
For measurements on a clean or reasonably conducting metal surface, you can use a foundry probe. This is a spring-loaded pair of sharp spikes of different thermocouple materials, e.g., one of chromel, the other of alumel. These are connected by the appropriate thermocouple extension cable out to your instrument. The metal that they prod completes the hot junction but that intermediate metal makes no thermoelectric interference or errors with the temperature measurement. This probe is usually used for spot measurements of billets and castings rather than for control.