Temperature Measurement

Resistance Temperature Detectors Working Principle

What does RTD stands for?

RTD is acronym for Resistance Temperature Detectors.   

What is meant by thermo-resistive temperature measuring devices?

The principle of device in which electrical resistance of material changes as a function of temperature is thermo-resistive temperature device. 

List few thermo-resistive temperature measuring devices

  • Resistance Temperature Detectors
  • Thermistors

State the Principle of RTD

An RTD operates on a fundamental principle of when metal’s temperature rises, so does its electrical resistance. The resistance element in the sensor is used to measure the resistance of the current being passed through it when an electrical current is being carried through it.

Components of RTD

1. RTD platinum resistance element: The RTD’s actual temperature sensor component is located here. Elements have lengths ranging from 1/8″ to 3″. There are numerous choices. Ohm is equal to 100 for the standard resistance.

2. RTD Outside diameter:  The exterior diameter with the greatest usage is 6mm (.236″). Outside diameters preferred in non-US applications is range from.063″ to.500″

3. Tubing Material in RTD: Up to 500°Fahrenheit, 316 stainless steel is frequently used for assembly. It is advised to utilise Inconel 600 over 500°F.

4. RTD Process Connection: All commonly used thermocouple fittings are included in the category of process connection fittings (i.e. compression, welded, spring-loaded, etc.).”

5. Wire Configuration in RTD: RTDs come with 2, 3, or 4-wire configurations. The most typical wire layouts for industrial applications are three. The two most common types of wire insulation are Teflon and fiberglass. Teflon can withstand 400°Fahrenheit and is moisture-resistant. Up to 10,000°F fiberglass can be utilized.
6. Cold end termination in RTD: RTDs can be terminated at the cold end using plugs, bare wires, terminal heads, or any of the thermocouple reference junctions.

Construction of Resistance Temperature Detector or RTD 

The wire is commonly coiled on a form (in a coil) on a notched mica cross frame to achieve small size, improve thermal conductivity to shorten response time, and achieve a high rate of heat transfer. A stainless steel sheath or a protective tube encases the coil in industrial RTDs. 

As a result, the minimal physical strain is caused by the wire’s expansion and lengthening due to temperature changes. The tension rises if the pressure on the wire does as well. This will cause the wire’s resistance to alter, which is not ideal. Therefore, other than temperature variations, we do not wish to alter the resistance of the wire. 

Construction of Resistance Temperature Detector or RTD 

Additionally, this is helpful for RTD maintenance while the plant is in use. Mica is inserted between the resistance wire and steel sheath to improve electrical insulation. Resistance wire requires less stress, thus it should be gently wound over a mica sheet. An industrial resistance temperature detector’s structural view is depicted in below figure.

Construction of Resistance Temperature Detector or RTD  1

How does an Resistance Temperature Detector (RTD) Work?

RTDs rely on a fundamental relationship between temperature and metals. The resistance of a metal to the flow of electricity rises as the temperature of the metal does. Similar to this, the electrical resistance, expressed in ohms (Ω), rises as the temperature of the RTD resistance element does. RTD elements are frequently described in terms of their resistance measured in ohms at absolute zero.

It is possible to measure the RTD resistance directly to determine the temperature if the temperature variations are significant or if accuracy is not crucial. 

An electrical circuit is constructed to measure a change in the resistance of the RTD, which is then utilized to calculate a change in temperature, in cases where the temperature changes are minimal and/or high precision is required. 

Why the platinum material is frequently used for RTD?

  1. Chemical inertness
  2. Nearly linear temperature versus resistance relationship;
  3. Temperature coefficient of resistance that is large enough to give easily measurable resistance changes with temperature
  4. Stability (in that its temperature resistance does not significantly change over time).

Elements of RTDs are commonly arranged in one of these three configurations: 

  • A platinum or metal glass slurry film deposited or screened onto a small flat ceramic substrate, which is referred to as a “thin film” RTD element; and
  •  A platinum or metal wire wound on a glass or ceramic bobbin and sealed with a coating of molten glass, which is referred to as a “wire wound” RTD element. Both types of RTD elements are known as “resistive temperature detectors.” 
  • A coiled element that is partially supported and consists of a short coil of wire that is put into a hole in a ceramic insulator and attached along one side of that hole. The thin film RTD element is the most durable of the three RTD components, and it has maintained or even improved its accuracy over time.

Platinum is the conductor that is most frequently utilized in industrial RTDs, but copper and silver conductors are also employed. The Platinum 100 RTD, often known as the Pt-100 RTD, is the most widely used element in the industry. At the reference temperature of 0° C (32° F), the RTD has a resistance of 100 ohms. The conductor is typically enclosed in a 6mm stainless steel tube and twisted into a coil. The probe is the name given to the entire assembly.

Elements of RTDs

RTD Materials 

  • RTD types are generally categorized based on the many sensing components used. Among sensing elements, platinum, nickel, and copper are most frequently utilized. 
  • Platinum-type RTD is renowned for having superior interchangeability with copper and nickel. Also, it offers the best time stability.

Types of RTDs and their specifications

ParameterPlatinumCopperNickelMolybdenum
Range, °C-200 to +850-200 to 260-80 to +320-200 to +200
Temperature Coefficient,α at 0°C0.003850.004270.006720.003786
Resistance ranges at 0°C25Ω to 2k Ω (50,100,200,500,1K)10Ω (20°C)50,100,120Ω100Ωto 2kΩ(100, 200, 500, 1K, 2K)
Resistivity at 20°C, μΩ.m10.61.6736.8445.7

Characteristics of RTD

A resistance thermometer, also known as a resistance temperature detector, measures the resistance of a clean electrical wire to determine the temperature. RTD is the one main option available in industries if we want to measure temperature accurately. It exhibits good linear properties over a broad temperature range.

The formula for how metal resistance varies with temperature is as follows:

formula for how metal resistance varies with temperature

      where,

  • Rt is the resistance values at toC temperatures
  • and R0 is the resistance values at t0oC temperatures.
  • α, β = constants depends on the metals. 

This expression is for huge range of temperature. For small range of temperature, the expression can be, 

formula for how metal resistance varies with temperature 1

The nominal resistance at zero degrees Celsius, the tolerance classes, and the temperature coefficient of resistance (TCR) are key features of an RTD. The relation between resistance and temperature is determined by the TCR.

Characteristics of RTD 1

RTD Applications

  • In automobiles, RTD sensors are utilized as intake air temperature sensors, oil level sensors, and engine temperature sensors. 
  • In instrumentation and communication for monitoring the temperature of transistor gain amplifiers, etc.
  • Medical research & Food service processing
  • Petrochemical and plastic processing
  • Air conditioning, Refrigeration & Furnace servicing
RTD Applications

Benefits of RTD Sensor

  • High repeatability
  • High accuracy and consistency
  • Capacity to provide accurate measurement even under harsh conditions. 
  • Long-term steadiness. 
  • Higher temperature ranges are suitable for platinum RTDs. 
  • Most reliable and consistent over time (when compared to thermocouples and thermistors)

RTD Disadvantages

  • Response time is high
  • Cost is high 

Limitations of RTD 

There will be a small heating impact on the RTD resistance due to an I2R power dissipation by the device. In RTD, this is referred to as self-heating. This could result in a misreading as well. In order to prevent self-heating, the electric current through the RTD resistance must be kept low and consistent.

Signal Conditioning of RTD 

This RTD should be used with the signal conditioning circuitry. In order to reduce lead wire mistakes and other calibration issues. 

As a result, a bridge circuit is used to measure the RTD value. The RTD resistance can be measured by applying a steady electric current to the bridge circuit and observing the voltage drop that occurs across the resistor. Thus, the temperature can also be established. By translating the RTD resistance value using a calibration expression, this temperature is ascertained. The figures below display the various RTD modules.

Two-wire Bridge configuration

Two-wire Bridge configuration

The basic model of resistance thermometer configuration is two wire and causes measurement inaccuracies, it is only used when high accuracy is not necessary. With this setup, 100 meters of cable can be used. Both fixed bridge systems and balanced bridges are covered by this.

The dummy wire is not present in the RTD Bridge’s two wires. The output obtained from the final two ends is depicted in the figure. But because the impedance of the extension wires may alter the temperature reading, it is crucial to take the resistances of the extension wires into account. By connecting a dummy wire C, this impact is minimized in the three wire RTD bridge circuit. 

Three-wire configuration

Three-wire configuration

The dummy wire is used in three wire configuration for minimizing the effect of the lead resistances.

The output obtained from the final two ends is depicted in figure. But because the impedance of the extension wires may alter the temperature reading, it is crucial to take the resistances of the extension wires into account. By connecting a dummy wire C, this impact is minimized in the three wire RTD bridge circuit. The impedance effects of wires A and B will cancel out if their lengths and cross sections are suitably matched since each wire is in an opposing position. Because it carries no current, the dummy wire C serves as a sense lead to monitor the voltage drop across the RTD resistance. The output voltage in these circuits is inversely proportional to the temperature. So, to determine the temperature, we only need one calibration equation.

Expressions for a Three Wires RTD Circuit 

Knowing the values of VS and VO allows us to calculate Rg, which allows us to use the calibration equation to determine the temperature value. Assume R1 = R2

Expressions for a Three Wires RTD Circuit 
  • VO = 0 and the bridge is balanced if R3 = Rg
  • If we don’t want to perform a laborious computation, we may simply solve equation to obtain the expression for Rg.
  • VObecome 0 when R3 = Rg, the bridge is balanced. This can be done manually, but if we don’t want to do a manual calculation, we can just solve the equation to get the expression for Rg
Expressions for a Three Wires RTD Circuit  1
  • When the lead resistance RL = 0, this formula assumes that. 
  • If RL is present in a circumstance, then let’s say that the expression for Rg is.
Expressions for a Three Wires RTD Circuit  3

As a result, the RL resistance has caused an inaccuracy in the RTD resistance value. This is why, as we previously explained, we need to compensate for the RL resistance by connecting one dummy line “C,” as illustrated in figure.

3-wire Bridge configuration 1

What is the need of RL Resistor?

  • RL is lead resistor which is a representation of the resistance of one of the lead wires, which connect the bridge to the RTD. 
  • RL is always constant in strain gauge circuits, lead resistance poses little of a problem, and we can easily zero the bridge by adjusting one of the other resistors. However, in RTD circuits, a portion of the lead wires is exposed to temperature variations.
  • RL fluctuates with T as a result of the resistance of metal wire changing with temperature, which can lead to measurement mistakes. Non-trivial changes in lead resistance may be mistaken for changes in RTD resistance in this error, leading to an inaccurate temperature reading.

Four Wire RTD Configuration 

Four Wire RTD Configuration 

Instead of two leads, the RTD has three leads in this circuit. RL2 solely serves as a potential lead while RL1and RL3carry the measurement current. When the bridge is balanced, no current passes through it. 

Resistance is eliminated since RL1 and RL3 are in different arms of the bridge. High impedance at ‘V0’ and close resistance matching between wires ‘RL2’ and RL3are presumptions made for this circuit.

Contact points near RTD element (second and third line) measure the voltage drop across the RTD element while first and last drive a precise measurement current. To stop current from flowing in the potential leads, ‘V’ needs to have a high impedance. A transmitter should be used in electrically loud areas even if 4-wire circuits could be longer-range than 3-wire.

These factors make a 4-wire RTD suitable for a wide range of applications. These are high-accuracy applications with remote sensors and receiving equipment as well as corrosive environments.

Why use a 4 Wire RTD?

The issues caused by extension wire length and lead resistance imbalance are reduced by the 4-wire arrangement. As a result, corrosion and the environment are less of an issue. 4-wire RTDs can use a smaller gauge wire because lead resistance is not a concern. 

In addition to cancelling lead wires, 4-wire RTD circuits also eliminate the effects of mismatched resistances.

How an RTD different from thermistor?

RTD has a positive temperature coefficient, while thermistor has a negative coefficient.

What is meant by Pt100?

Platinum 100, an element often seen in RTDs. Platinum exhibits a 100 ohm resistance at 0°Celsius, hence the name PT100.

How to identify an RTD and a thermocouple?

Connect a multimeter to the transmitter in resistance mode. It is an RTD if the reading is in ohms; otherwise, it is a thermocouple. Thermocouple always displays mV readings and RTD displays ohms readings.

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