RTDs & Thermocouples FAQs
RTDs
Product Overview
What is an RTD?
An RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its temperature changes. The resistance increases as the temperature of the sensor increases. The resistance versus temperature relationship is well known and is repeatable over time.
An RTD is a passive device. It does not produce an output on its own. External electronic devices are used to measure the resistance of the sensor by passing a small electrical current through the sensor to generate a voltage.
Thermocouples
Product Overview
What is a thermocouple?
A thermocouple is a device consisting of two different conductors (usually metal alloys) that produce a voltage proportional to a temperature difference between either ends of the pair of conductors. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation.
What are the differences between a thermocouple and an RTD sensor?
The main differences between thermocouples and RTDs are their temperature range capabilities, accuracy and stability, and response time.
| RTD | Thermocouple | |
|---|---|---|
| Temperature | Narrow temperature range: -200°C to 600° | Wider temperature range from -200°C to 2000°C |
| Accuracy & Stability | Capable of higher accuracy and can maintain stability for many years | Can be less accurate and can drift over time |
| Response Time | 1 to 7 seconds | Less than one second |
RTD Basics
What is an RTD element?
An element is the component, which actually provides the measurement. Based on the function it is going to perform and the technology employed in fabricating it, an element can have coils and/or different patterns to provide a signal, which is then used to measure the changes in different properties of energy.
What are the most common RTD element types?
The two most common types of RTD sensor elements are thin-film and glass wirewound elements. Wirewound elements typically come in two types – glass and ceramic. Wire-wound elements are labor intensive, they tend to be more expensive than thin-film elements.
A thin-film element is manufactured using a technology called photolithography. This technology enables sensing element to be more rugged, smaller in size, more accurate, and more cost effective than traditional wire-wound elements.
What is the advantage between using a 3-leadwire versus a 2-leadwire configuration in an RTD sensor?
Leadwires can introduce errors in resistance and temperature measurements. The longer the leadwires, the greater the increase in measured resistance, resulting in a higher temperature reading than the actual value. Incorporating a third leadwire can compensate for the effects of the first two, improving measurement accuracy.
Thermocouple Basics
What are the most common thermocouple types?
TE utilizes four standard thermocouple calibration types; J, K, T, and E. Selecting a thermocouple type would typically include requirements for materials, operating temperature ranges, tolerances, wire size, application environment, and construction.
What is are the differences between a grounded thermocouple and an ungrounded thermocouple?
A grounded thermocouple puts the junction in direct contact with the packaging - a protective metal sheath. Though this allows for a faster response time, a grounded tip can be susceptible to electromotive forces (EMF) in the environment, which might cause potential errors in the measurement. An ungrounded junction is one without direct contact with the metal case. It will have slower response time, but is less likely to provide erroneous readings.
What are cold junctions and hot junctions?
A junction is a point where two dissimilar metals are joined. In a thermocouple, a cold junction (also known as a reference junction) is held at a known and stable temperature. The standard temperature is 32°F (0°C). A hot junction (also known as a measuring junction) detects the temperature of an unknown object or environment.
Standard resistance chart for RTDs at 0°C based on element type, base resistance (Ohms), and temperature coefficient of resistance (TCR).
