How to Protect a Thermocouple
The construction of a thermocouple is incredibly simple; it’s really just two different metal wires that connect to each other at the two ends. One of those connections, the reference junction, can usually be tucked away in a safe place, far from hostile environments. However, the measurement junction will always be in the thick of things, potentially enduring measurements within extreme temperatures, pressures, or corrosive process mediums. Leaving these wires exposed to the environment will shorten their lifespan, especially in the hot temperatures above 800ºC where no other temperature sensors go. That said, there are a handful of ways of how to protect a thermocouple and add protection to the measurement junction wires.
If none of that jargon made sense to you, be sure to read through “What is a Thermocouple?” to get a primer on this amazing instrument. Otherwise, keep reading to explore the variety of options that are available for thermocouple protection and add longevity to these hard-worked temperature sensors.
1. Thermocouple protection: Sheath it in Steel
Stainless steel is a standard in temperature sensor shielding. The material is cheap, plentiful, and comes in all sorts of sizes to fit many designs of thermocouples. The protective tubing is typically found in ¼” diameters and can run as long as a few feet in length, or even longer. The size of ¼” provides a good compromise between cost, rigidity, strength, and practicality. Smaller diameters, such as ⅛” or even tinier: like needle-sized thermocouples used for medical procedures, do exist. But the skinnier the probe the less physical abuse it can withstand and the shorter the probe. Alternatively, larger diameters exist but at a higher material cost and less responsivity.
You’ll find these wide and strong thermocouples in specialized industrial uses, like the measurement of oil well environments where the monetary cost and decrease in accuracy easily outweigh the risk of breaking or damaging a sensor thousands of feet deep underground.
Figure 1: Different sizes of stainless steel sheathing to help protect a thermocouple
2. Thermocouple protection: Pre-packaged (and protected!) MI Cable
Mineral insulated cable, or MI cable for short; is a length of cable filled with some type of insulative mineral. There are a few different versions of MI cable but for thermocouples, it is typically comprised of three things: stainless steel sheathing; two differing thermocouple wires; and an insulating powder. Let’s investigate each of those three components a little further.
The stainless steel tubing is really just a sturdy container to keep the insulating powder and thermocouple wires together. Naturally, it also provides the thermocouple with all the protective measures that we highlighted in the previous paragraph.
Figure 2: Coils of Enercorp’s stock MI cable
The two differing thermocouple wires are a requirement to measure the two differing voltages that tell us the temperature of the process. K-type MI cable, for example, will have a Chromel and an Alumel wire running inside the length of the stainless steel tubing. Some MI cables can contain four wires, or two thermocouples, to provide more accuracy and redundancy. A dual J-type MI cable, for example, has two wires of iron, and two of Constantan. Each thermocouple pair provides its own voltage differences to be measured and interpreted. Some specialized MI cable might even have 3 pairs of wires within.
Figure 3: Exposed ends of MI cable, single (2-wire) and dual (4-wire)
The insulation within MI cable is usually comprised of magnesium oxide, or MgO, powder. One can use, Aluminum oxide as a cheaper alternative but it cannot withstand as high a temperature as MgO. When tightly packed around the wires within a sheath, the powder acts as an electrical resistive barrier, reducing the effects of moisture and letting the thermocouple read true. It also provides excellent thermal conductivity; any temperature changes will transmit quickly and allow for fast responses to a fluctuating process. A secondary effect of filling the stainless steel sheath with powder is that it makes the whole composition more rigid, adding to the durability of thermocouples made from MI cable.
Figure 4: Magnesium oxide powder to help protect a thermocouple
3. Thermocouple protection: Keep it tidy with a Vacuum
Just like during the manufacturing process, different elements can mix with the thermocouple wires over time and change their composition. These reactions will fundamentally change how the thermocouple reads. Therefore making the instrument more inaccurate over time and eventually requiring a replacement. To eliminate impurities around the metal wires you can seal a thermocouple within a vacuum. By encompassing them in a vacuum the risk of their electromagnetic characteristics being altered is lessened and they will report voltages true to their respective types for a longer lifespan.
4. Thermocouple protection: Keep it safe in a Thermowell
Thermowells are like having your cake and eating it too; the ultimate in thermocouple protection. Typically milled from a solid section of brass or stainless steel bar stock, they are a barrier that separates the instrument from a harsh process environment. That might sound like it does the same job as the stainless steel sheathing we mentioned earlier, except thermowells do it better. Under hot temperatures and pressures, conditions often found in thermocouple measuring processes, even stainless steel sheathing will wear and deform, making the instrument susceptible to rapid degradation.
Thermowells are stronger and sturdier than MI cable or a sheathed thermocouple. Specifically, designed to fit thermocouples that already have a stainless steel jacket. You get the best protection available by the MgO powder, a sheath of steel, and the thermowell barrier all in one. We’ve got lots more information on this incredibly helpful tool; check out “What is a Thermowell” for a more in-depth conversation on this armour for your sensor.
Figure 5: A cross-section view of a sheathed protected thermocouple inside a thermowell
As simple and reliable thermocouples are, they can be vulnerable to the harsh conditions of the process environments they are measuring. With the right type of protection, you can ensure they provide accurate temperature readings throughout a long lifespan. Every process is a little different but there is always an optimal balance; between instrument replacement cost and protective measures taken. Now you know what sort of options are available and can begin to customize your thermocouple to the standard that you specifically require. Contact an Enercorp sales representative to find out how we can help you design the perfect instrument for your need.
