Thermocouples and RTDs are temperature sensing devices that play and extremely important role in a wide range of measurement and control applications. Though both thermocouples and RTDs
share many similarities including a wide measuring range; good accuracy; and simple, reliable design, they are very different devices that rely on very different technologies . RTDs
and/or thermocouples are found in cars, homes, offices, commercial spaces, and industrial settings—anywhere accurate, affordable temperature readings are required.
Thermocouples
A thermocouple is a temperature sensing device consisting of two dissimilar metals joined together at one end. This configuration produces a voltage proportional to the difference between the
temperature at the sensing end of the thermocouple and a reference temperature. Thermocouples are one of the most widely used temperature sensors available. They are very common in measurement
and control applications in industrial and commercial settings and are also found in thermostats and flame sensors in residential applications.
The popularity of thermocouples stems, in part, from their simplicity, adaptability and cost. Thermocouples are a fraction the cost of other temperature sensors and can be fashioned into nearly
any length or diameter. They are supplied with standard connectors making them very interchangeable. In contrast to most other methods of temperature measurement, thermocouples are self-powered
and require no external power supply. The main limitation with thermocouples is accuracy, system errors of less than one degree Celsius (°C) can be difficult to achieve.
How Thermocouples Work
When any conductor is subjected to a thermal gradient it will generate a voltage, a condition known as the Thermoelectric Effect or Seebeck Effect. To correlate
that voltage to a specific temperature, we can’t simply measure the voltage. To do so it would be necessary to attach another conductor to the “hot” end. If that conductor were of the same material
as the first, as it experienced the temperature gradient , it would produce its own voltage, opposing the first.
Fortunately, different metals generate different voltages when exposed to a thermal gradient. Using a dissimilar metal to complete the circuit creates two legs which generate different voltages,
leaving a small difference in voltage available for measurement. That difference increases with temperature.
Creating a junction of any two dissimilar metals will produce an electric potential related to temperature. Thermocouples, designed for accurate temperature measurement in a variety of environments,
use junctions of specific metals and alloys which have a highly predictable and repeatable relationship between voltage and temperature. Different combinations of metals and alloys are used for different
temperature ranges.
The junction created by joining together the two dissimilar metals is known as the measurement or thermocouple junction. This is the “hot” end of the thermocouple. The other end, where the thermocouple
ties into the system (often a controller or display) is called the reference or cold junction. A thermocouple generates a voltage proportional to the temperature difference between the measurement
junction and the reference junction. This is a relative temperature measurement. In order get more useful absolute temperature readings, there need to be a stable reference temperature. This is
accomplished through cold junction compensation.
Depending upon the measurement and control system, cold junction compensation can be accomplished in a number of ways. In highly sophisticated calibration systems, the cold reference is maintained
at a controlled temperature, often 0˚C. Though useful in a laboratory, controlling the temperature of the reference junction isn’t practical for most applications. There are also other ways to
compensate. One is to add a small temperature sensor to monitor the cold junction temperature. A second is to have a diode simulate a specific temperature within the cold junction. The electronics
to which the thermocouple is attached can quickly apply corrections based upon the cold junction compensation and provide an accurate absolute temperature.
Most thermocouples include sheathing for protection from their environment. Thermocouples that are sheathed have options grounding their measurement junctions. A grounded junction,
the most common type, physically connects the thermocouple wires to the metal sheathing. This configuration provides a faster response time as it improves heat transfer from the outside. Grounded
junctions are more prone to electrical interference, however, which can cause measurement errors.
Ungrounded junctions include an insulating layer between the measurement junction and the metal sheathing. Response time is slower than the grounded style, but the ungrounded junction
offers electrical isolation.
Thermocouples without sheathing are referred to as exposed thermocouples or bare-wire thermocouples. This style exposed the measurement junction which provides the fastest response
time but forgoes any protection the sheathing offers limiting their use to dry, noncorrosive and non-pressurized applications.
Thermocouple Types
Although thermocouples can be made from joining together any two different metals, science and experience have established that certain combinations of metals produce reliable temperature measurements
suitable to particular environments. These combinations of metal are referred to as thermocouple types.
Since different thermocouple provide different temperature ranges and physical capabilities, it’s important to select the correct type based upon the application in which it will be installed.
The desired temperature range, chemical resistance, abrasion and vibration resistance, and installation requirements are all factors to consider when selecting a thermocouple type.
Some of the more common thermocouple types include:
Type J Thermocouple: Type J thermocouples are the most common of all types. They include an Iron leg and a Constantan leg. They are suitable for temperatures from -346 to 1,400˚F
(-210 to 760˚C) and have a basic accuracy of ±2.2˚C or ±0.75%. Type J thermocouples can be used in vacuum, reducing, oxidizing and inert atmospheres.
Type K Thermocouple: Type K thermocouples consists of a Chromel and an Alumel leg. They are suitable for temperatures from -454 to 2300˚F (-270 to 1260˚C) and have a basic accuracy
of ±2.2˚C or ±0.75%. Type K thermocouples are recommended for oxidizing or inert atmospheres. Cycling above and below 1800˚F is not recommended due to EMF alteration from hysteresis.
Type N Thermocouple: Type N thermocouples consist of a Nicrosil and a Nisil leg. They are suitable for temperatures from -454 to 2300˚F (-270 to 1260˚C) and have a basic accuracy
of ±2.2˚C or ±0.75%. Considered an improved Type K, Type N thermocouples have better resistance to degradation due to temperature cycling, green rot and hysteresis than the Type K.
Type T Thermocouple: Type T thermocouples consist of a Copper and a Constantan leg. They are suitable for vacuum, oxidizing, reducing and inert atmospheres. Type T thermocouples are
suitable for temperatures from -454 to 700˚F (-270 to 370˚C) and have a basic accuracy of ±1.0˚C or ±0.75%. They maintain good resistance to corrosion in most atmospheres and high stability at sub-zero
temperature making them an ideal option for very cold temperatures.
Type E Thermocouple: Type E thermocouples consist of one Chromel leg and one Constantan leg. They are not subject to corrosion in most atmospheres. The Type E also has the highest EMF
per degree of any standard thermocouple type. However, this thermocouple must be protected from sulfurous atmospheres. They are suitable for temperatures from -454 to 1600˚F (-270 to 870˚C) and have a
basic accuracy of ±1.7˚C or ±0.5%.
Type S Thermocouple: Type S thermocouples consist of a Platinum-10% Rhodium leg and a Platinum leg. They are recommended for use in oxidizing or inert atmospheres. Reducing atmospheres
may cause excessive grain growth and drift in calibration. They are used in very high temperature applications and are suitable for temperatures from -58 to 2700˚F (-50 to 1480˚C) and have a basic accuracy
of ±1.5˚C or ±0.25%.
Type R Thermocouple: Type R thermocouples consist of a Platinum-13% Rhodium leg and a Platinum leg. They are recommended for use in oxidizing or inert atmospheres. Reducing atmospheres
may cause excessive grain growth and drift in calibration. They are good in very high temperature applications and are suitable for temperatures from -58 to 2700˚F (-50 to 1480˚C) and have a basic accuracy
of ±1.5˚C or ±0.25%.
Type B Thermocouple: Type B thermocouples consist of a Platinum-30% Rhodium leg and a Platinum leg. They are recommended for use in oxidizing or inert atmospheres. Reducing atmospheres
may cause excessive grain growth and drift in calibration. They are used in extremely high temperature applications and are suitable for temperatures from 32 to 3100˚F (0 to 1700˚C) and have a basic
accuracy of ±0.5%.
Type C Thermocouple: Type C thermocouples consist of Tungsten and Rhenium legs. They are recommended for use in vacuum, high purity hydrogen or pure inert atmospheres. They are used
for extremely high temperatures and are suitable for temperatures from 32 to 4208˚F (0 to 2320˚C) and have a basic accuracy of ±1.0%. Type C thermocouples are inherently brittle.
Advantages and Disadvantages of Thermocouples
Thermocouples have a number of in inherent advantages and disadvantages when compared to other temperature sensors, particularly RTDs.
Advantages:
- Low cost
- High temperature range
- Very simple design
- Rugged
- Fast response
- Temperature sensing is at the tip of a thermocouple
Disadvantages:
- Low sensitivity
- Moderate accuracy
- Non-linear
- Low voltage
- Low stability/repeatability
Resistance Temperature Detectors (RTDs)
Resistance temperature detectors (RTDs), often referred to as resistance thermometers, are temperature sensing devices that work by correlating the resistance of a highly pure conductor to temperature.
RTDs are commonly used in a wide range of process and measurement applications. For applications below 600°C, RTDs have generally supplanted thermocouples as the temperature sensor of choice.
RTDs are generally considered to be among the most accurate temperature sensors available. In addition to offering very good accuracy, they provide excellent stability and repeatability. RTDs also
feature high immunity to electrical noise and are, therefore, well suited for applications in process and industrial automation environments, especially around motors, generators and other high
voltage equipment.
How RTDs Work
RTDs work on the well-known principle that the resistivity of a conductor increases as the temperature increases and decreases as the temperature decreases. In practice, a small electrical current
is passed through a conductor, which serves as the RTD element. The resistance to that electrical current is then measured and correlated to a specific temperature based upon the known resistance
characteristics of the material that makes up the RTD element.
RTD elements are available in different styles but all consist of a highly pure conductive metal such as platinum, copper, aluminum, or nickel; inside of a sheathed probe for protection. These materials,
are used for their highly predictable resistance versus temperature relationship and operating temperature range.
Platinum is the preferred material for RTD elements. As a noble metal, platinum doesn’t react with other materials making it highly stable with a very linear and repeatable resistance-temperature
relationship over an effective temperature range of -272.5 to 961.78°C. Platinum RTDs, which are often called PRTs (Platinum Resistance Thermometers) , are accurate enough to serve
as the sensors that define the International Temperature Standard, ITS-90. RTDs of the highest accuracy are called SPRTs (Standard Platinum Resistance Thermometers) which can achieve
an accuracy of up to ±0.001°C.
RTD elements are classified according to their resistance in ohms at 0°C. For example, the most common classification, Pt100, demonstrates 100Ω of resistance at 0°C. The resistance to temperature
coefficient depends on the classification of the RTD element with Pt100 sensors at 0.385 Ohm/°C and Pt1000 sensors at 3.85 Ohm/°C. Pt1000 sensors, therefore makes possible a higher resolution n while
Pt100 sensors provide a wider temperature range.
RTDs also differ in their wiring configurations. A simple rule of thumb is that the more wires an RTD has the more accurate it is. Since the lead wires, usually copper, have their own resistance value
different from that of the platinum element, they can impact the accuracy of the RTD. Two-wire RTDs do not have a practical means for accounting for the resistance associated with the copper lead
wires and are therefore the least accurate configuration. Three-wire RTDs, the most common configuration, use a Wheatstone bridge to compensate for the lead wire resistance. Four-wire RTDs are the most
accurate because they are able to completely compensate for the resistance of the wires without having to pay attention to the physical properties of them.
RTD Styles
Thin Film: Thin film elements consist of a ceramic substrate to which a very thin (1 to 10 nanometers) layer of resistive material, normally platinum, has been deposited. A layer of
epoxy or glass provides a protective coating over the platinum. Thin film elements provide a faster response than other construction styles though at a cost of lower stability. The size and shape of
the element makes it easy to mount on flat surfaces, especially in tight spaces.
These elements work with temperatures up to 300°C though, if encapsulated with ceramic or glass, can operate up to 500°C. Thin film elements can only operate over a narrow temperature range since
different expansion rates for the ceramic substrate the platinum resistive layer can affect the resistive temperature coefficient.
Wire Wound: Wire-wound elements consist of a sensing wire wrapped around an insulating core. The coil diameter provides a compromise between mechanical stability and allowing expansion
of the wire to minimize strain and consequential drift. The materials chosen as the sensing wire, core, and lead wires are selected to minimize EMF that would otherwise distort readings. Wire wound
elements provide higher accuracy than other types of RTDs, especially over a wide temperature range. Wire wound elements are suitable for temperatures to 660°C.
Coil Element: Coiled elements utilize a wire coil held in place by a mechanical support. This produces a strain-free design which allows the coil to expand and contract freely as
the temperature changes without any influence from other materials. It is the most accurate and stable of RTD styles and has largely supplanted wire wound elements. Coil elements are suitable for
temperatures up to 850°C.
Advantages and Disadvantages of RTDs
RTDs have a number of in inherent advantages and disadvantages when compared to other temperature sensors, particularly thermocouples.
Advantages:
- Good stability
- Excellent accuracy
- Contaminant resistant
- Good linearity
- Highly repeatable
- Area temperature sensing
Disadvantages:
- Higher cost
- Current source required
- Slow response time
- Lower temperature range
- Self-heating
- Medium sensitivity to small temperature changes
Things to Consider When Selecting an RTD or Thermocouple:
- What is the required temperature range?
- What level of accuracy is needed?
- Is response time a factor?
- Is a power supply available?
- Will the sensor be exposed to abrasion or vibration resistance?
- Is chemical resistance required?
- How far will the sensors be from their controller or display?
If you have any questions regarding thermocouples or RTDs please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
