Thermocouples and RTDs are the most common types of temperature sensors used in commercial and
industrial applications. The following dialog provides an overview of these sensors with some basic knowledge
to get you started in choosing the correct sensor for your application.
What Are Thermocouples and RTDs?
How do you make the choice between using a thermocouple or an RTD? Each uses a distinct method to measure temperature, but each has its own strengths and weaknesses. You must first begin by knowing the exact conditions of your process including the extremes of what the sensor may see, including max and min temperature extremes. As most of these sensors will be installed in a Thermowell (covered in another one of our videos), you will need to obtain process flow and pressure information, along with piping class and connection type. Now that you have that information, let’s talk in more detail about each sensor.
What is an RTD?
An RTD (Resistance Temperature Detector) is a temperature sensor with an incorporated platinum thin-film resistor designed to generate a resistance signal in Ohms directly correlating with the temperature of the process in which it is exposed. There are different resistor standards, but the most common is a PT100 which references it 100 Ohm resistance at 32 degrees F / 0 degrees C. Sensors can contain 1 (simplex) for common applications, or 2 (duplex) RTD’s for more critical applications.
The wire itself, especially in long runs in excess of 100’ or more, carries a measurable resistance which if left uncorrected would create a temperature error when connected to an RTD resistor. To negate this error, RTD sensors use a 3 wire or 4 wire configuration allowing a current to be imposed on the resistance signal “driving” the resistance output. Additionally, by keeping wire lengths under 20 feet when wiring back to a control device, or using a 2-wire transmitter to convert the signal to digital or analog 4-20mA is considered best.
Because an RTD sensor’s primary measuring component is a thin film resistor, it is only capable of measuring processes up to 600 degrees F, and is not recommended for high vibration applications. Temperatures in excess of 600 F, and high vibration, destroy the thin film resistor rendering it inoperative.
What is a Thermocouple?
Thermocouple sensors work differently than RTD’s. A thermocouple is an electrical device consisting of two dissimilar electrical conductors (wires) typically welded together to form an electrical “junction”. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect created by the junction, and this voltage can be interpreted to measure temperature. This effect is typically non-linear and only accurate in a certain range, so every thermocouple has a specific operating range it is designed for. When referring to the types of metal comprising the thermocouple junction, you will hear terms like Type J, Type K, Type R, each referring to its metallurgical make-up and operating range. Like an RTD a thermocouple can have more than 1 point of measurement, but due to the thermocouple’s unique construction, the number of measurement points along the length of the sensor is almost unlimited. This offers you the advantage of measuring many points along the entire sensor length, which is optimal for applications such as reactors and vessels where laminar temperature balance is optimal for the process.
Temperature Sensor Comparison Breakdown
The following chart should offer a brief general comparison between RTD’s and Thermocouples.
What’s Best for My Project?
We have provided a brief overview and comparison above, but there are many more considerations when choosing the correct sensor for your application. With temperature sensors and transmitter technology continually evolving, it is always best to evaluate your sensor requirements even when simply replacing an existing sensor.
Sagacity has an applications staff willing and ready to answer your temperature questions, and help you decide the best sensor for your given process conditions. Whether it’s a replacement RTD, or a new Thermocouple, we can help you get the sensor you need quickly.