A resistance thermometer is a tool for measuring temperature based on changes in conductor resistance. It is an accurate temperature sensor and is frequently referred to as a resistance temperature detector (RTD). For measuring dynamic temperature, RTD is not used.

Working Principle of Resistance Thermometer

As is common knowledge, a resistance thermometer depends on temperature variation to determine the conductor’s resistance. The atomic nuclei of the metal experience an increase in vibrational amplitude as its temperature rises. As a result, there is a higher chance that free electrons will collide with bound ions. As a result, resistance rises as a result of the electron’s motion being interrupted. Consequently, the temperature that goes along with it rises.

This is how RTD works.

Nickel, platinum, copper, or tungsten are the most common materials used to make resistance temperature detectors. However, due to its chemical inertness and use as a primary element in such precise temperature sensors, platinum. in order to lessen the possibility of oxidation when used in harsh environments.

In a metal, the relationship between resistance change and temperature is as follows:

So R_t = R_o (1 + αt + βt² + ϒt³ ———)  

: R_o = resistance at 0⁰ C

R_t = resistance at t⁰ C

α, β, ϒ etc are constants here.

Construction of Resistance Thermometer

Figure below depicts a platinum RTD’s structural layout.

It consists of a mica frame that has been crossed inside of which is a platinum coil. The entire setup is put inside a stainless steel evacuated tube. When temperatures rise, the coil form arrangement produces the least strain. With an increase in strain, tension also rises. Therefore, this will result in an unfavorable change in the wire’s resistance. When mica is inserted between the evacuated tube and platinum coil, we can achieve better electrical insulation. It is important to note that the material must be sufficiently pure in this case to yield accurate results.

The purity of platinum can be checked by the measurement of R₁₀₀ /R_o. As for pure platinum material, the value of the ratio should be higher than 1.390

Basic equation for RTD

As we know,

=> R_t = R_o (1 + αt + βt² + ϒt³ ———) 

R_t from the above equation can be approximated as:

R_t = R_o (1 + αt + βt² )

When the element is pure platinum,

α = 3.94 Χ 10^-3 /⁰C

β = – 5.8 Χ 10^-7 /(⁰C)²

The above equation can be rewritten as:

R_t = R_o (1 + C t_pt)

: C = mean resistance temperature coefficient between 0 ⁰C and 100 ⁰C.

t_pt = platinum temperature coefficient

and is given by

: R_t, R_o, R₁₀₀ are the resistance at t ⁰C, 0 ⁰C, 100 ⁰C

The fundamental interval of the thermometer is denoted by R₁₀₀ – R_o
The equation given below shows the difference true temperature ‘t’ and platinum temperature ‘t_pt’

: δ = constant

The range of’s value is 1.488 to 1.498. Considering that a lower value of denotes a high level of purity. The effective range of an RTD depends on the type of wire used. And The platinum RTD has a temperature range of 100 0C to 650 0C.

Characteristics of materials used in RTD

The resistance temperature characteristic of various types of materials used in RTDs is shown in the figure below.

Silver and gold are hardly ever used in RTD construction because they are less resistive materials. Although tungsten has a high resistivity when it comes to resistivity, it cannot be used in high-temperature applications. Copper is another material used in RTD construction because of its low linearity but low resistivity. As a result, platinum is chosen over all other elements.

Resistance Thermometer Circuit

RTD circuits are essentially Wheatstone Bridge circuits, but it is important to note that they are a modified version rather than a standard one. RTD can be connected in one of the Wheatstone bridge’s arms as seen in the following figure:

Here, R₁ and R₂ are 2 fixed resistances, R₃  is a variable resistance and R_t is the detector resistance.

At balance condition,

When R₁ = R₂

R_t = R₃

Here, R3 is a potentiometer with variable resistance. The resistors used in the circuit are made of manganin to prevent all the effect that increases with temperature changes. Manganin has the lowest temperature coefficient of resistance, which explains why.

Circuit designing considerations in RTD

In RTD certain things are to be kept in mind while designing the circuit. These are as follows:

1. The RTD must be wired into the circuit using lead wires of the proper length. Therefore, if the temperature changes, the resistance in the bridge circuit will also change. Therefore, a suitable distance must be maintained between the measuring point and the location where the RTD must be installed.

2. The heating effect in the circuit is caused by the current flowing through the RTD. The RTD sensor’s temperature rises as a result of the heat produced.

We cannot prevent this self-heating effect from occurring. The only thing we can do is compromise on the instrument’s sensitivity. The rate at which heat is produced will undoubtedly decrease with a reduction in current through the RTD, but so will the device’s sensitivity. However, with the right amplification, it can be enhanced.

The increase in temperature of the device due to self-heating effect can be given as:

: ∆T = rise in temperature in ⁰C
P = power dissipated in RTD in watts

P_d = Dissipation constant of RTD in W/ ⁰C

3 Wire Resistance Thermometer

The circuit for a 3-wire RTD is depicted in the figure below.

The three wire method is used to account for the impact of variable lead wire resistance. In order to have equal resistance, copper lead wire of similar length and diameter is typically used. So RTDs with three wires are most frequently used in industries.

Advantages

1. RTD provides a vast operating range.
2. It provides highly accurate results.
3. RTD is used in all such applications where precise results are required due to its high accuracy.

Disadvantages

1. RTD possess slower response time.
2. The platinum RTD’s sensitivity to even a slight temperature change is very low.

So A quotient or cross coil measuring device was one of the most frequently used RTD indicators in the past. Despite being inexpensive and durable, it has been replaced by bridge type RTDs with digital indication.