RTD sensors, also known as resistance temperature detectors or resistance thermometers, are a type of heat sensor, or heat transducer, used to measure temperature. They typically do so using coiled wire elements spun around glass or ceramic, which react to temperature and temperature changes with measurable resistance. The relationship between this resistance and temperature change is highly accurate, making RTD sensors very reliable devices.
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RTD Sensor Design
RTD sensors can be divided into two main groups: immersion sensors and surface. Immersion sensors are installed directly into the heart of an application, where they experience direct contact with the process medium, typically a liquid or gas. Surface sensors are used at those times when total immersion is not possible. To work with the same level of accuracy as immersion sensors, they require further insulation. RTD sensors are further organized into classes, per their accuracy, construction and applications. They are: Standard Platinum Resistance Thermometers (SPRTs), Secondary-Standard Platinum Resistance Thermometers (SSPRTs) and Industrial Platinum Resistance Thermometers (Industrial PRTs).
Types of RTD Sensors
There are five main element types with which RTD sensors may be made. They are: wire-wound elements, carbon resistor elements, strain-free elements, thin-film elements and coiled elements.
- Elements are wrapped around an insulating core or mandrel. The types of wire available in the creation of wire-wound elements include copper, iron, platinum and others. Of these, platinum offers the strongest resistance-temperature relationship. Unfortunately, it is also the most expensive wire, so if their application will not suffer a detriment from their choice, many manufacturers opt for a less expensive alternative. Wire-wound elements are valued for the expansion allowance they offer, which is done with minimal strain.
- Carbon Resistor
- Very economical and offer repeatable results at low temperatures. They are only appropriate for use with very low temperature applications.
- Also use wire coils, but instead of being wrapped around glass or ceramic, this wire is wrapped around a sealed housing filled with inert gas. They only use platinum wire, which must only be loosely coiled, so that the element can expand and contract freely per temperature change. The main disadvantage of this element type is the fact that it is not well-supported and is therefore susceptible to vibration and shock.
- Composed of sensing elements coated with a protective and strain-relieving glass or epoxy. The protected element is formed from the deposit of an extremely thin layer of resistive material, such as platinum, on a ceramic substrate. While useful for some applications, ultimately, thin-film elements are not as stable or as widely applicable as their coiled and wire-wound cousins.
- Quite similar to wire-wound elements, except that they function without any constriction. They are the most popular type of RTD sensing element. As mentioned, in this context, wire is wrapped around glass or ceramic. Note that, in order to work, this glass or ceramic must be kept contaminant-free.
Things to Consider When Choosing RTD Sensors
If manufacturers are constructing one or more RTD sensors to be immersed in or surrounded by cold temperatures, they will typically equip them with attached insulated leads. Such insulated leads are most often made from materials like PTFE (polytetrafluorethylene), PVC (polyvinyl chloride) or silicone rubber. In addition, manufacturers usually house fragile sensors in protective, chemically inert metal alloy sleeves. The selection of these housings, or sheaths, should be done with care, as they must be able to shield the sensors from the damage of chemicals, physical elements like wind and rain or whatever else they may encounter.
As was first mentioned, RTD sensors are incredibly reliable. In fact, they are so reliable, as well as stable and prone to repeatability, that they have begun to replace thermocouples. In practice, the main difference between RTDs and thermocouples is the fact that thermocouples run and generate voltage using the Seebeck effect, while RTDs require a power source to run and generate voltage using electrical resistance. The resistance they use more closely models the Callendar-Van Dusen equation. The choice of which sensor type to select usually comes down to four main factors: accuracy and stability requirements, response time, temperature and size. To find out more about RTD sensors and to find your best fit, reach out to a knowledgeable sensor engineer today.