How do RTD Temperature Probes Work?
Temperature sensing is an essential part of industrial and manufacturing processes, but it’s easy to gloss over how this data is collected. Measured temperatures do not magically show up on monitors or appear as inputs for controllers to act upon. Hence there are many types of specialized sensors out there gathering and relaying this critical data. One specialized type within the RTD sensor family is RTD temperature probes. A section that makes up a large percentage of sensors used throughout the world. Don’t take for granted this incredible instrument that plays such an important role. Now let’s see what RTD temperature probes are all about.
Figure 1: A RTD probe
Where are RTDs used?
In all sorts of processes, RTD temperature probes are present; take any type of manufactured good for example, like plastics, textiles, or even some foods. Their production all require the monitoring and management of temperature. Have a look around you. The few examples listed above cover all sorts of products that you use every day, and that’s just the start. There are countless products out there that use RTDs for temperature measurement throughout the manufacturing process.
RTDs usages aren’t restricted to just the manufacturing of goods you can touch, wear, eat, or use. For instance, homes heated by anything other than a wood fireplace will have some sort of automated system that needs to sense the temperature of a room. An RTD helps call for heat if it’s too cold or turns off the heat when the interior air reaches the desired temperature. Other intangible processes like electricity generation require temperature management as well. It is a truly universal requirement, and RTDs play an enormous role in this field.
How do RTD probes work?
RTD is short for Resistance Temperature Detector, and resistance is the key to how these probes sense temperature. An electrical current flows into the RTD probe through a circuit. The electrical current flows through the RTD probe in a predictable manner, differing only with a change in temperature. As an RTD probe gets warmer, the electrical current flowing through it is resisted, and that change is measured.
Since an RTD probe reacts in a predictable manner every time, the resistance measured can, therefore, be equated to a respective temperature. It’s kind of like squeezing a garden hose tight; the more resistance put on the hose, the less water will come out the end. When it comes to resistance, there is a lot to talk about, and you’ll find more information over at our blogs, “What is a Thermistor?” and “Ohm’s Law.” The fundamental concept to grasp, for now, is that RTD probes work by the resistance of an electrical current being measured and correlated to a respective temperature.
What does an RTD probe look like?
Many different components make up an RTD probe. Some are vital to the basic operation of the probe, and some are optional, depending on its location and usage. Let’s look at the parts of an RTD probe a little closer and see what exactly makes up this type of temperature sensor.
At the heart of an RTD probe resides the element; an electrical component made from a combination of metal wire with specific resistive properties and insulative materials. They work a lot like thermistors, but with some crucial differences, be sure to check out the link to the thermistor blog above for more information. We devote another whole discussion to RTD elements over at “What are RTD elements?” so have a look there for more details on this critical piece to RTD probes.
Figure 2: A 100Ω platinum RTD element
A stainless steel sheath surrounds the RTD element and is a necessary part of an RTD probe. The sheath provides a protective layer around the fragile element, allowing the temperature of the measured process to permeate to the element. While adding protection from any corrosive effects of the environment and physical abuse from the RTD probe being moved. The “immersion” refers to the part of the RTD probe that will be submerged in the process.
Figure 3: Open (left) and welded (right) RTD probe sheathing
An easily overlooked component of an RTD is the lead wire. In the end, an RTD element must be connected to a process control that digests the temperature information being sensed. It does so by sending the signal electrically through wires, sort of like an extension cord. There are some considerations when choosing RTD probe lead wire; for example, the lead wire itself will inherently have some sort of resistive properties. To obtain accurate readings from the RTD, the wire resistance must be taken into account. The composition of the wire and if there is some sort of protective coating are also aspects that require review depending on the job and the operating environment of the RTD probe. For example, if the ambient temperature doesn’t exceed 100℃ then PVC wire could be used.
Figure 4: Coils of PVC wire
Finally, an RTD probe can have several optional features. Such as, the addition of a threaded fitting, which allows the probe to be securely mounted into the process.
Another option is complete assemblies, where heads are located at the end of RTD probes. The heads can contain just the wires, a terminal block or a temperature transmitter. Terminal blocks make it easy to swap RTD probes for replacement without having to run new lengths of lead wire. Temperature transmitters read the resistance of the RTD element and transmit it to a process control, negating the need for lead wire and calculating the variables necessary with that material.
Figure 5: An RTD probe with a head, transmitter, and process fitting
So now you know a little more about RTDs, how they sense temperature, and how they relay that data to a centralized hub or process control. The requirement for temperature measurement is so widespread. Yet it’s easy to forget about the instruments that actually sense this information for us and how much our modern processes rely on them. If you want to learn more about the other temperature sensors that are employed to sense this fundamental variable, be sure to read through “What is a Thermocouple?” You’ll find out a lot more about this hidden world of temperature sensing. And why stop at temperature? Be sure to read through our other blogs covering topics such as pressure, electricity, and many more exciting subjects.
