Temperature Transducer: Resistance Temperature Detector

What is a Temperature Transducer?

A Temperature Transducer is a device that converts the thermal quantity into any physical quantity such as mechanical energy, pressure and electrical signals etc. For example, in a thermocouple the electrical potential difference is produced due to temperature difference across its terminals. So, thermocouple is an temperature transducer.

Main Features of Temperature Transducers

• The input to them are always the thermal quantities
• They generally converts the thermal quantity into electrical quantity
• They are usually used for the measurement of the temperature and heat flow

Basic Scheme of Temperature Transducers

The basic scheme of temperature transducers is given below in following steps
Sensing Element.

The sensing element in the temperature transducers is the element whose properties change with change in temperature. As the temperature changes the corresponding change occurs in certain property of the element.

Example – In a Resistance Temperture Detector (RTD) the sensing element is the Platinum metal.

Desirable Conditions for Choosing the Sensing Element are as

• Change per unit resistance of material per unit change in temperature should be large
• The material should have a high resistivity so that minimum volume of material is used for its construction
• The material should have continuous and stable relationship with temperature
• Transduction Element
  It is the element that transforms the output of the sensing element into electrical quantity. The change in the property of the of sensing element acts as the output for it. It measures the change in the property of sensing element. The output is of transduction element is then calibrated to give output which represents the change in the thermal quantity.

Example- In the thermocouple the potential difference produced across the two terminal is being measured by voltmeter and magnitude of voltage produced after calibration gives idea of change in temperature.

Types of Temperature Transducers

Contact Temperature Sensor Types

In these the sensing element is in direct contact with the thermal source. They use the conduction for transfer of thermal energy.

Non-contact Temperature Sensor Types

In a non-contact temperature sensor, the element is not in direct contact with the thermal source (analogous to a non contact voltage tester or voltage pen). Non-contact temperature sensors use principle of convection for heat flow. 

What is a Thermistor?

A thermistor (or thermal resistor) is defined as a type of resistor whose electrical resistance varies with changes in temperature. Although all resistors’ resistance will fluctuate slightly with temperature, a thermistor is particularly sensitive to temperature changes.

Thermistors act as a passive component in a circuit. They are an accurate, cheap, and robust way to measure temperature.

While thermistors do not work well in extremely hot or cold temperatures, they are the sensor of choice for many different applications.

Thermistors are ideal when a precise temperature reading is required. The circuit symbol for a thermistor is shown below:

Uses of Thermistors

Thermistors have a variety of applications. They are widely used as a way to measure temperature as a thermistor thermometer in many different liquid and ambient air environments. Some of the most common uses of thermistors include:

• Digital thermometers (thermostats)
• Automotive applications (to measure oil and coolant temperatures in cars & trucks)
• Household appliances (like microwaves, fridges, and ovens)
• Circuit protection (i.e. surge protection)
• Rechargeable batteries (ensure the correct battery temperature is maintained)
• To measure the thermal conductivity of electrical materials
• Useful in many basic electronic circuits (e.g. as part of a beginner Arduino starter kit)
• Temperature compensation (i.e. maintain resistance to compensate for effects caused by changes in temperature in another part of the circuit)
• Used in wheatstone bridge circuits

How Does a Thermistor Work

The working principle of a thermistor is that its resistance is dependent on its temperature. We can measure the resistance of a thermistor using an ohmmeter.

If we know the exact relationship between how changes in the temperature will affect the resistance of the thermistor – then by measuring the thermistor’s resistance we can derive its temperature.

How much the resistance changes depends on the type of material used in the thermistor. The relationship between a thermistor’s temperature and resistance is non-linear. A typical thermistor graph is shown below:

If we had a thermistor with the above temperature graph, we could simply line up the resistance measured by the ohmmeter with the temperature indicated on the graph.

By drawing a horizontal line across from the resistance on the y-axis, and drawing a vertical line down from where this horizontal line intersects with the graph, we can hence derive the temperature of the thermistor.

Thermistor Types

There are two types of thermistors:

• Negative Temperature Coefficient (NTC) Thermistor
• Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor

In an NTC thermistor, when the temperature increases, resistance decreases. And when temperature decreases, resistance increases. Hence in an NTC thermistor temperature and resistance are inversely proportional. These are the most common type of themistor.

The relationship between resistance and temperature in an NTC thermistor is governed by the following expression:

Where:

• RT is the resistance at temperature T (K)
• R0 is the resistance at temperature T0 (K)
• T0 is the reference temperature (normally 25oC)
• β is a constant, its value is dependant on the characteristics of the material. The nominal value is taken as 4000.

If the value of β is high, then the resistor–temperature relationship will be very good. A higher value of β means a higher variation in resistance for the same rise in temperature – hence you have increased the sensitivity (and hence accuracy) of the thermistor.

From the expression (1), we can obtain the resistance temperature co-efficient. This is nothing but the expression for the sensitivity of the thermistor.

Above we can clearly see that the αT has a negative sign. This negative sign indicates the negative resistance-temperature characteristics of the NTC thermistor.

If β = 4000 K and T = 298 K, then the αT = –0.0045/oK. This is much higher than the sensitivity of platinum RTD. This would be able to measure the very small changes in the temperature.

However, alternative forms of heavily doped thermistors are now available (at high cost) that have a positive temperature co-efficient.

The expression (1) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistors is very definitely a non-linear sensor.

PTC Thermistor

A PTC thermistor has the reverse relationship between temperature and resistance. When temperature increases, the resistance increases.

And when temperature decreases, resistance decreases. Hence in a PTC thermistor temperature and resistance are inversely proportional.

Although PTC thermistors are not as common as NTC thermistors, they are frequently used as a form of circuit protection. Similar to the function of fuses, PTC thermistors can act as current-limiting device.

When current passes through a device it will cause a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings then the device heats up.

In a PTC thermistor, this heating up will also cause its resistance will increase. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current. In this way, it acts as a current limiting device – protecting the circuit.

Thermistor Characteristics

The relationship governing the characteristics of a thermistor is given below as:

Where:

• R1 = resistance of the thermistor at absolute temperature T1[oK]
• R2 = resistance of the thermistor at temperature T2 [oK]
• β = constant depending upon the material of the transducer (e.g. an oscillator transducer)

We can see in the equation above that the relationship between temperature and resistance is highly nonlinear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.05/oC.

Thermistor Construction

To make a thermistor, two or more semiconductor powders made of metallic oxides are mixed with a binder to form a slurry.

Small drops of this slurry are formed over the lead wires. For drying purposes, we have to put it into a sintering furnace.

During this process, the slurry will shrink onto the lead wires to make an electrical connection.

This processed metallic oxide is sealed by putting a glass coating on it. This glass coating gives a waterproof property to the thermistors – helping to improve their stability.

There are different shapes and sizes of thermistors available in the market. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters.

Thermistors may also be in the form of disks and washers made by pressing the thermistor material under high pressure into flat cylindrical shapes with a diameter from 3 millimeters to 25 millimeters.

Types of Temperature Sensors

The typical size of a thermistor is 0.125mm to 1.5 mm. Commercially available thermistors have nominal values of 1K, 2K, 10K, 20K, 100K, etc. This value indicates the resistance value at a temperature of 25oC.

Thermistors are available in different models: bead type, rod type, disc type, etc. The major advantages of thermistors are their small size and relatively low cost.

This size advantage means that the time constant of thermistors operated in sheaths is small, although the size reduction also decreases its heat dissipation capability and so makes the self-heating effect greater. This effect can permanently damage the thermistor.

To prevent this, thermistors have to be operated at low levels of electric current compared to resistance thermometer – resulting in lower measurement sensitivity.

Thermistor vs Thermocouple

The main differences between a thermistor and a thermocouple are:

Thermistors:

• A more narrow range of sensing (55 to +150oC – although this varies depending on the brand)
• Sensing parameter = Resistance
• Nonlinear relationship between the sensing parameter (resistance) and temperature
• NTC thermistors have a roughly exponential decrease in resistance with increasing temperature
• Good for sensing small changes in temperature (it’s hard to use a thermistor accurately and with high resolution over more than a 50oC range).
• The sensing circuit is simple and doesn’t need amplification & is very simple
• Accuracy is usually hard to get better than 1oC without calibration

Thermocouples:

• Have a wide range of temperature sensing (Type T = -200-350oC; Type J = 95-760°C; Type K = 95-1260°C; other types go to even higher temperatures)
• Can be very accurate
• Sensing parameter = voltage generated by junctions at different temperatures
• Thermocouple voltage is relatively low
• Have a linear relationship between the sensing parameter (voltage) and temperature

Thermistor vs RTD

Resistance Temperature Detectors (also known as RTD sensors) are very similar to thermistors. Both RTDs and thermistors have varying resistance dependent on the temperature.

The main difference between the two is the type of material that they are made of. Thermistors are commonly made with ceramic or polymer materials while RTDs are made of pure metals. In terms of performance, thermistors win in almost all aspects.

Thermistors are more accurate, cheaper, and have faster response times than RTDs. The only real disadvantage of a thermistor vs an RTD is when it comes to temperature range. RTDs can measure temperature over a wider range than a thermistor.

Aside from this, there is no reason to use a thermistor over an RTD.

What is an RTD (Resistance Temperature Detector)?

A Resistance Temperature Detector (also known as a Resistance Thermometer or RTD) is an electronic device used to determine the temperature by measuring the resistance of an electrical wire. This wire is referred to as a temperature sensor. If we want to measure temperature with high accuracy, an RTD is the ideal solution, as it has good linear characteristics over a wide range of temperatures. Other common electronics devices used to measure temperature include a thermocouple or a thermistor.

The variation of resistance of the metal with the variation of the temperature is given as,

Where, Rt and R0 are the resistance values at toC and t0oC temperatures. α and β are the constants depends on the metals.

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

In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics.

Platinum has the temperature range of 650oC, and then the Copper and Nickel have 120oC and 300oC respectively. The figure-1 shows the resistance-temperature characteristics curve of the three different metals. For Platinum, its resistance changes by approximately 0.4 ohms per degree Celsius of temperature.

The purity of the platinum is checked by measuring R100 / R0. Because, whatever the materials actually we are using for making the RTD that should be pure. If it will not pure, it will deviate from the conventional resistance-temperature graph. So, α and β values will change depending upon the metals.

Construction of Resistance Temperature Detector or RTD

The construction is typically such that the wire is wound on a form (in a coil) on notched mica cross frame to achieve small size, improving the thermal conductivity to decrease the response time and a high rate of heat transfer is obtained. In the industrial RTD’s, the coil is protected by a stainless steel sheath or a protective tube.

So that, the physical strain is negligible as the wire expands and increase the length of wire with the temperature change. If the strain on the wire is increasing, then the tension increases. Due to that, the resistance of the wire will change which is undesirable.So, we don’t want to change the resistance of wire by any other unwanted changes except the temperature changes.
This is also useful to RTD maintenance while the plant is in operation. Mica is placed in between the steel sheath and resistance wire for better electrical insulation. Due less strain in resistance wire, it should be carefully wound over mica sheet. The fig.2 shows the structural view of an Industrial Resistance Temperature Detector.

Signal Conditioning of RTD

We can get this RTD in market. But we must know the procedure how to use it and how to make the signal conditioning circuitry. So that, the lead wire errors and other calibration errors can be minimized. In this RTD, the change in resistance value is very small with respect to the temperature.

So, the RTD value is measured by using a bridge circuit. By supplying the constant electric current to the bridge circuit and measuring the resulting voltage drop across the resistor, the RTD resistance can be calculated. Thereby, the temperature can be also determined. This temperature is determined by converting the RTD resistance value using a calibration expression. The different modules of RTD are shown in below figures.



In two wires RTD Bridge, the dummy wire is absent. The output taken from the remaining two ends as shown in fig.3. But the extension wire resistances are very important to be considered, because the impedance of the extension wires may affect the temperature reading. This effect is minimizing in three wires RTD bridge circuit by connecting a dummy wire C.

If wires A and B are matched properly in terms of length and cross section area, then their impedance effects will cancel because each wire is in opposite position. So that, the dummy wire C acts as a sense lead to measure the voltage drop across the RTD resistance and it carries no current. In these circuits, the output voltage is directly proportional to the temperature. So, we need one calibration equation to find the temperature.

Expressions for a Three Wires RTD Circuit

If we know the values of VS and VO, we can find Rg and then we can find the temperature value using calibration equation. Now, assume R1 = R2:

If R3 = Rg; then VO = 0 and 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 3 to get the expression for Rg.

This expression assumes, when the lead resistance RL = 0. Suppose, if RL is present in a situation, then the expression of Rg becomes,

So, there is an error in the RTD resistance value because of the RL resistance. That is why we need to compensated the RL resistance as we discussed already by connecting one dummy line ‘C’ as shown in fig.4.

Video Presentation on Resistance Temperature Detector or RTD

Limitations of RTD

In the RTD resistance, there will be an I2R power dissipation by the device itself that causes a slight heating effect. This is called as self-heating in RTD. This may also cause an erroneous reading. Thus, the electric current through the RTD resistance must be kept sufficiently low and constant to avoid self-heating.

Thermocouple

Definition: The thermocouple is a temperature measuring device. It uses for measuring the temperature at one particular point. In other words, it is a type of sensor used for measuring the temperature in the form of an electric current or the EMF.

The thermocouple consists two wires of different metals which are welded together at the ends. The welded portion was creating the junction where the temperature is used to be measured. The variation in temperature of the wire induces the voltages.

Working Principle of Thermocouple

The working principle of the thermocouple depends on the three effects.

See back Effect – The See back effect occurs between two different metals. When the heat provides to any one of the metal, the electrons start flowing from hot metal to cold metal. Thus, direct current induces in the circuit.

In short, it is a phenomenon in which the temperature difference between the two different metals induces the potential differences between them. The See beck effect produces small voltages for per Kelvin of temperature.

Peltier Effect – The Peltier effect is the inverse of the Seebeck effect. The Peltier effect state that the temperature difference can be created between any two different conductors by applying the potential difference between them.

Thompson Effect – The Thompson effect state that when two dissimilar metals join together and if they create two junctions then the voltage induces the entire length of the conductor because of the temperature gradient. The temperature gradient is a physical term which shows the direction and rate of change of temperature at a particular location.

Construction of Thermocouple

The thermocouple consists two dissimilar metals. These metals are welded together at the junction point. This junction considers as the measuring point. The junction point categorises into three types.

1. Ungrounded Junction – In ungrounded junction, the conductors are entirely isolated from the protective sheath. It is used for high-pressure application works. The major advantage of using such type of junction is that it reduces the effect of the stray magnetic field.
2. Grounded Junction – In such type of junction the metals and protective sheath are welded together. The grounded junction use for measuring the temperature in the corrosive environment. This junction provides resistance to the noise.
3. Exposed Junction – Such type of junction uses in the places where fast response requires. The exposed junction is used for measuring the temperature of the gas.

The material uses for making the thermocouple depends on the measuring range of temperature.

Working of Thermocouple

The circuit of the thermocouple is shown in the figure below. The circuit consists two dissimilar metals. These metals are joined together in such a manner that they are creating two junctions. The metals are bounded to the junction through welding.

Let the P and Q are the two junctions of the thermocouples. The T1 and T2 are the temperatures at the junctions. As the temperature of the junctions is different from each other, the EMF generates in the circuit.

If the temperature at the junction becomes equal, the equal and opposite EMF generates in the circuit, and the zero current flows through it. If the temperatures of the junction become unequal, the potential difference induces in the circuit. The magnitude of the EMF induces in the circuit depends on the types of material used for making the thermocouple. The total current flowing through the circuit is measured through the measuring devices.

The EMF induces in the thermocouple circuit is given by the equationWhere Δθ – temperature difference between the hot thermocouple junction and the reference thermocouple junction.
a, b – constants

Measurement of Thermocouple Output

The output EMF obtained from the thermocouples can be measured through the following methods.

1. Multimeter – It is a simpler method of measuring the output EMF of the thermocouple. The multimeter is connected to the cold junctions of the thermocouple. The deflection of the multimeter pointer is equal to the current flowing through the meter.
2. Potentiometer – The output of the thermocouple can also be measured with the help of the DC potentiometer.
3. Amplifier with Output Devices – The output obtains from the thermocouples is amplified through an amplifier and then feed to the recording or indicating instrument.

Advantages of Thermocouple

The following are the advantages of the thermocouples.

1. The thermocouple is cheaper than the other temperature measuring devices.
2. The thermocouple has the fast response time.
3. It has a wide temperature range.

Disadvantages of the Thermocouples

1. The thermocouple has low accuracy.
2. The recalibration of the thermocouple is difficult.

Nickel-alloy, platinum/rhodium alloy, Tungsten/rhenium-alloy, chromel-gold, iron-alloy are the name of the alloys used for making the thermocouple.