There are three main temperature sensor types: Thermocouples, Resistance Temperature Detectors (RTDs) and Thermistors. Temperature sensors are vital instruments used each day in a wide range of applications, including but not limited to:
- Building & Construction
- Green Technologies
- Industrial Electronics
- Water Systems
- Testing Facilities
- Commercial Food Service Equipment
- Medical and Scientific Laboratories
- HVAC Units
- Packaging Machinery
- Consumer Products
Each application may require unique temperature sensing parameters such as what is being measured (air, mass, or liquid), where it is being measured (inside or outside), and the range of temperature being measured. Understanding the advantages and disadvantages of the temperature sensor types will help select the right tool for the right application.
Resistance Temperature Detectors (RTDs):
The basic concept is that the resistance in metal reflects changes in temperature and that difference in resistance is what RTDs measure. An RTD is a resistor with well defined resistance vs. temperature properties. Platinum is the most common and accurate material used to develop RTDs.
The advantages of RTDs stem from their stability, accuracy, repeatability and fairly wide temperature range. The stability and accuracy derive from their almost linear response to temperature changes. RTDs are often preferred for precision applications because of their accuracy and repeatability.
Disadvantages include a higher thermal mass which causes them to respond more slowly to changes in temperature than thermocouples. They also require an excitation current to flow through the RTD. Other temperature sensors without this requirement are therefore better options for certain applications.
Configurations include two, three, and four wire options. The two-wire option is useful when lead length is short enough that resistance doesn’t significantly affect measurement accuracy. A three-wire adds an RTD probe that carries the excitation current. This provides a way to cancel wire resistance. Four-wire is the most accurate, as separate force and sense leads eliminate the effect of wire resistance.
Thermocouples are the workhorse temperature sensor type. They are used in many different applications. The advantages are numerous: thermocouples are self-powered, require no excitation, can operate over a wide temperature range, and have quick response times.
Thermocouples are made by joining two dissimilar metal wires together. This causes a Seebeck Effect. The Seebeck Effect is a phenomenon in which a temperature difference of two dissimilar conductors produces a voltage difference between the two substances. It is this voltage difference that can be measured and used to calculate the temperature.
There are several types of thermocouples that are made from a variety of different material, which allows for different temperature ranges and different sensitivities. The different types are differentiated by designated letters. The most commonly used is the K type.
Disadvantages of thermocouples include that their limited output voltage can make measuring temperature harder since it requires precise amplification, external noise issues over long wires, and cold junction compensation. Cold junction is where thermocouple wires meet copper traces of the signal circuitry. This creates another Seebeck Effect which needs to be compensated for in the measurements.
Thermistors, like RTDs, measure temperature changes caused by measurable resistance changes. Most Thermistors are made from polymer or ceramic material. In most cases, thermistors are cheaper but are also less accurate than RTDs. Most thermistors are available in two wire configurations.
The Negative Temperature Coefficient (NTC) thermistor is the most common thermistor type for temperature measurement. The NTC thermistor’s resistance decreases as the temperature increases, thus producing a non linear temperature resistance relationship. This requires a significant correction to interpret the data correctly.
These temperature sensor types, Thermocouples, RTDs and Thermistors, remain the common types of temperature sensors used today. Thermocouples are inexpensive, durable, and can measure a wide range of temperatures. RTDs offer a wide range of temperature measurements and provide accurate and repeatable measurements, but they are slower, require an excitation current, and require signal conditioning. Thermistors are durable and small, but they are less accurate than RTDs and require more data corrections to calculate temperature.
While there are other temperature sensor options available, these three options should offer a design solution for most applications.
Please contact us for any of your temperature sensor, thermocouple, thermistor or RTD requirements and needs.