Thermocouple Junction and Type: Basic Guide
Thermocouples are incredibly versatile instruments; they are rugged, dependable, and of simple construction. When it comes to designing a thermocouple, however, there are many choices such as thermocouple junction; thermocouple type. It can be hard to know what options are available and what they do. If you’re still not quite as familiar with this sensor as you would like then be sure to read through “What is a Thermocouple?” There you’ll find a lot of general information and links to other more focused discussions about this temperature sensor. With that said, read on to get the details and you’ll know the basics to design a thermocouple in no time.
Figure 1: A thermocouple threaded into a thermowell at Enercorp
What Thermocouple Type should I pick?
When choosing a thermocouple, the very first place to start is deciding what type is best. There is a good handful of standard thermocouple types that are identified by a letter of the alphabet. These standard thermocouples are pairs of dissimilar metals; that, throughout two centuries of scientific tests and practical applications, have been discovered to work well together. There are a handful of reasons to opt for one type over another but the most important concern is the range, so be sure to know the expected lower and upper limits of what temperature you will be measuring.
Another consideration could be the cost and availability of materials. Platinum, for example, is one of the key metals in Type R thermocouples and can measure between 0…1400*C. However, platinum is literally worth it’s weight in gold so other alternatives should be explored first. Type K and J cover a wide spectrum of temperature ranges and the metals are both cheap and convenient to find, so you’ll see lots of these thermocouples in use. If you’re still not too sure what thermocouple type you should start out with we’ve got a whole blog specifically discussing the topic. Head over to “What are the Different Thermocouple Types?” for a more refined blog post on this subject.
Figure 2: Thermocouple temperature range chart
What Measurement Junction should I pick?
A topic that is rarely addressed in the thermocouple world is the measurement junction type. This is the place where our two dissimilar metals connect to each other. This is also known as the hot junction and is the part of the thermocouple that is placed within the process where you want to measure the temperature. Most are housed within a stainless steel sheath but it is not necessary; arguably the most common type of measurement junctions are ungrounded stainless steel probes, but there are other options out there. Here are the three different types of measurement junctions to choose from:
Thermocouple Junction: Ungrounded
In this type, the junction is suspended within the sheath, surrounded by insulation, and not touching the edges. This helps protect the sensor and provides a steady and accurate temperature measurement reading. That, however, comes at the cost of response time as any temperature changes must permeate through to the enveloped junction.
Figure 3: An ungrounded thermocouple measurement junction
Thermocouple Junction: Grounded
A grounded measurement junction is the union of two dissimilar metals that also touches the outside protective sheath. Any temperature changes will be picked up much quicker than an ungrounded probe with this direct physical connection. This rapid response comes with a risk though. By connecting the hot junction of the thermocouple to the protective sheath there is the potential for a ground loop to be created. This is another (undesirable) path for current to travel through and will interfere with the signals of your thermocouple. This interference will lead to inaccurate readings.
Figure 4: A grounded thermocouple measurement junction
Thermocouple Junction: Exposed
Making most direct contact with the process, exposed measurement junctions are the quickest responding thermocouple junction type. These may have some sort of protective layer but ultimately the union of the two dissimilar metals is bare. The drawback to this type is the direct wear on the junction and either the acceptance of often replacing the instrument or restricting its use in harsh environments.
Figure 5: An exposed measurement junction
What is the best Thermocouple Wire?
There are many options when it comes to thermocouple wire and it can be rather daunting trying to choose the best way to integrate this type of temperature sensor within a process control system. The working environment where one uses a thermocouple will be a determining factor in many of the design decisions after choosing a thermocouple type and measurement junction. However, considerations like the cost of materials and thermocouple lifespan before replacement are aspects that will carry some design weight as well.
From deciding between thermocouple grade or extension grade wire; connectors for coupling two lengths of wire; protective heads; or even transmitters to change the thermocouple measurement output; there are an incredible amount of available options to customize a thermocouple. We’ve dedicated a whole separate blog to these additional considerations so head over to our “Basic Guide on choosing Thermocouple Wire“ and we’ll get you squared away with those aspects of thermocouple design.
Figure 6: Teflon-coated J-type wire (left) and stainless steel braided fibreglass-coated K-type wire (right)
At Enercorp, we will custom build you a thermocouple that is the best match to help you accomplish your temperature sensing needs. With the information provided in these online articles; you’ll be able to give us a sense of your most important requirements; from which we’ll be able to produce an instrument of top quality at an agreeable price. Keep in mind, there are also a couple of other temperature sensors that you might consider before opting for a thermocouple, so visit “What is Thermistor?” and “How do RTD Temperature Probes Work?” for more on the other go-to options when it comes to temperature measurement. We custom build everything in house to your specifications; if you need it, Enercorp can make it.
