RTD vs Thermocouple and Applications

Resistance Temperature Detectors (RTD) and Thermocouples are the most frequent sensor kinds used to measure temperatures. 

Everyday objects like thermometers, home water heaters, microwaves, and refrigerators all employ temperature sensors. Temperature sensors often have several uses, with the geotechnical monitoring industry being one of them.

Temperature sensors are simple devices that sense how hot or cold something is and turns that measurement into a readable quantity. Have you ever wondered how the temperatures of the ground, boreholes, enormous concrete dams, or structures are measured? Well, some of the specialized temperature sensors help do this.

Concrete constructions, bridges, railroad tracks, dirt, and other things are all monitored regularly by temperature sensors.

An instrument that offers temperature measurement in a usable form through an electrical signal is referred to as a temperature sensor. This device is commonly a thermocouple or RTD resistance temperature detector.

RTD and Thermocouple

Resistance Temperature Detectors, or RTDs, are a popular type of sensor used to monitor the temperature. RTD and thermocouples are both often used for monitoring temperature range. Due to their simplicity of use, these two sensor types are preferred for measurements over alternative approaches. These also do away with the tedious conversion procedure while processing temperature readings. It’s crucial to comprehend some differences between RTD and thermocouples, though. 

What is an RTD sensor, and how does it work? 

A resistive temperature detector RTD is a probe or device used in industrial applications to monitor and control temperature. A resistor’s value alters in response to temperature variations. Therefore, the measurement of temperature is relative rather than absolute.

Across industries, this relative temperature measurement and management are necessary since even small temperature fluctuations can affect the process. RTD sensors are renowned for providing precise readings and being built to maintain stability under challenging conditions. They are, therefore, resistant to high heat, vibration, shock, and other conditions.

Resistance and temperature often have a strong link that is well-known and expected to continue. It is unable to produce an output entirely on its own.

RTDs, or Resistance Temperature Detectors, are devices with a specific type of film made of wire coil, or frequently, pure metal. Since many industrial components are extremely resistant to the electrical commotion, it is suitable for measuring temperatures in such parts. Textile manufacturing and air conditioning applications are only two examples of RTD.

RTD also has several advantages. One of them is that measuring a wide range of temperatures, especially in harsh conditions, is quite helpful.

What Materials are Used in RTD Temperature Sensors? 

A tiny wire coil is wrapped around a glass or ceramic core, and the wire is coiled by RTD components in this resistive temperature sensor. Platinum is used to make this thin wire element because it is resistant to hostile conditions, corrosion, and oxidation. Copper and nickel are occasionally used. These RTDs might contain more than one wire. However, platinum is the greatest option because it provides several perks. Here are some of the advantages of platinum RTDs:

• Platinum is chemically inert.
• It offers a stable and almost linear temperature-resistance equation.
• Platinum, as a metal, has a good enough temperature coefficient to allow RTDs to detect rapid resistance changes.

Generally, 2- and 3-wire RTDs are frequently used, primarily dependent on the cabling; RTDs often have two, three, or four wires. 2-wire RTDs are widely employed in non-critical applications where the temperature change has little effect on the process or simply requires an estimated temperature change value. 3-wire RTDs, on the other hand, are widely used in industry.

To some extent, an RTD’s accuracy depends on the number of wires. The RTD provides higher precision as the number of wires increases. RTDs come in various designs based on temperature or resistance values and may be tailored to specific industrial uses.

RTD Applications

Here are some examples of RTD applications:

• It is used to measure the temperature of earth components such as air, liquid, and gaseous substances due to its exceptional sensitivity rate over a wide range.
• It is utilized in home basics, including stoves, air conditioning, refrigeration, grills, and more.
• It is also used in textile manufacturing services.

Thermocouples

A thermocouple, like an RTD, is a type of sensor that aids in temperature measurement. These are voltage devices that monitor temperature by changing the voltage. The output voltage of the thermocouple rises as the temperature rises, albeit not always linearly.

 It is frequently housed behind a metal or ceramic shield that protects it from various conditions. It usually contains two different metal kinds linked together at one end. 

Normally, when the junction of either metal is observed to be heated, a visible voltage generation occurs that has a conventional connection with the temperature. These are generally used in various applications, including gas turbine exhausts.

As a result of the continual impact of the Seebeck effect, it often creates a temperature-dependent voltage. Furthermore, the voltage is found to be transformed into a temperature form.

Metal-sheathed thermocouples are also available with various exterior coatings, including Teflon, for use in acids and strong acidic solutions.

Thermocouple Applications

Here are some common uses of thermocouples,

• These are commonly found in thermometers, particularly those used in hospitals and pharmacies.
• It is also a thermostat sensor to regulate and read the temperature.
• It is also often found in water heaters.

Types of RTDs 

There are two main types of resistance temperature detectors – wire-wound and thin-film.

• Wire-Wound RTDs

Wire wound RTDs are made from a minimal diameter wire made of platinum, wound into a coil, and packaged inside a ceramic insulator.

• Thin Film RTD Elements

Thin film RTDs are made by depositing a thin layer of resistive material, typically platinum film, onto a ceramic substrate.

Types of Thermocouples

These are classified into eight types: B, E, J, N, K, R, T, and S.

• E-Type Thermocouple

Chromel and Constantan alloys combine to make an E-type thermocouple. The temperature range is 0 to 870 degrees Celsius. This is not concerned with atmospheric oxidation and may be utilised in an inert environment. They must, however, be shielded from the sulphurous atmosphere. They are frequently seen in power plants.

• J-Type Thermocouple

Iron and Constantan are used to create a J-type thermocouple. Its temperature range is 0 to 760 °C. The thermocouple’s life duration decreases in high temperatures due to its low-temperature range. J thermocouples work best in vacuum and inert environments. Injection moulding is one of the most prevalent uses for thermocouples of this sort.

• K-Type Thermocouple

A K-type thermocouple is formed of Chromel and Alumel. The temperature range is 95 to 1260 °C. These are best suited to a neutral or oxidising environment. Due to hysteresis, it produces an EMF fluctuation below 1800°F, limiting its employment in an inert and oxidising environment below this temperature. They are typically found in refineries.

Different Situations Where You Prefer RTD Over Thermocouple

• Combined with Transmitters: It is simple to pair RTDs with transmitters that allow temperature data to be sent to a remote control site. It also allows users to view the local temperature display.
• Laboratories and Industrial Purposes: When we compare the precision of RTD vs. thermocouple, RTD is far more exact than thermocouple and hence preferable for laboratory and industrial reasons.
• Automotive Industries: When the temperature range is between -200 and 500 degrees Celsius, industrial RTDs outperform thermocouples. 
• Medical Electronics: Regarding a low number of shifts for RTD vs. thermocouples, RTD has extremely strong stability and linear display. However, these are unstable and hence less suitable for industrial applications.

 The illustration demonstrates the use of thermocouples in measuring a broad range of temperatures. RTD is helpful for low-temperature readings, though.

RTD vs Thermocouple 

RTD	Thermocouple
A RTD measures temperatures in the range of -200 °C to 500 °C.	A RTD measures temperatures between 180 and 2,320 degrees Celsius.
Its purpose is to measure lower temperatures.	Its purpose is to measure higher temperatures.
RTD is noted for its firm stability and low shifts.	These are known for their poor stability, resulting in less repeats over time.
RTD is well-known for its accuracy.	It is usually exhibit inaccurate and substandard results.
RTDs, due to their manufacturing benefits, are quite more expensive than thermocouples.	Thermocouples, however, are usually very cheap, as opposed to an RTD
RTD has a good response time.	The reaction time of a thermocouple is faster than that of an RTD.
The output presented by an RTD is linear.	The output of a thermocouple is non-linear.
Good interchangeability	Excellent interchangeability

RTD sensors quickly replace thermocouples in industrial applications because of their precision and stability. RTD can provide data with greater precision. Unlike the thermocouple, which is only stable for a short time after usage, RTD can remain stable for many years. Our everyday appliances, including coffee makers and cellphones, contain RTD.

Frequently Asked Questions

1. What does RTD Stand for? 

RTD stands for Resistance Temperature Detector

2. Is a Thermocouple AC or DC? 

The thermocouple voltage is a direct current signal and removing AC noise through filtering is advantageous; additionally, These produce voltages in the tens of millivolts range, necessitating amplification.

 3. What is the function of a thermocouple? 

It is a type of temperature sensor. It is constructed of two different metals that are joined at one end. When the two metals’ junction is heated or cooled, a voltage is produced that can be related to the temperature. 

4. What are K type and T type of thermocouple? 

The temperature ranges of various thermocouple types differ. Type T, for example, has a maximum temperature of 370C or 700F with its Copper leg. Type K, on the other hand, can withstand temperatures of up to 1260°C (2300°F).