Temperature-Sensing Devices and Sensors

Temperature is one of the most familiar engineering variables. Its measurement and control is one of the earliest known metrological achievements. Temperature measurement is based on one of the following principles.

1. Material expansion based on change in length, volume, or pressure.

2. Based on the change in electrical resistance.

3. Based on contact voltage between two dissimilar metals.

4. Based on changes in radiated energy.

An RTD is a length of wire whose resistance is a function of temperature. The design consists of a wire that is wound in the shape of a coil to achieve small size and improve thermal conductivity. In many cases the coil is protected from the environment by a protecting tube which inevitably increases response time, however, this enclosure is essential when RTDs are used in hostile environments.

Resistance relationships of most metals over a wide range of temperature are given by quadratic equations. A quadratic approximation to the R-T curve is a more accurate representation of the resistance variation over a span of temperatures. It includes both a linear term and a term that varies as the square of the temperature. An analytical approximation is represented as,

R = R_0 (1 + a(T – T_0)) R = Ro(1 + alpha (T – T_0) + beta (T – T_0)^2 + ... )

Here Ro is the resistance at absolute temperature T and alpha and beta are material constants which dependent on the purity of material used.

An examination of the resistance versus temperature curves of Ill. 77 shows that the curves are quite linear in short ranges. This observation is employed to develop approximate analytical equations for resistance versus temperature of a particular metal.

ILL. 77 RESISTANCE TRANSDUCER CHARACTERISTICS (OF PURE METALS)

Over a small temperature range of 0°C to 100°C , the linear relationship is written as,

Here alpha is the temperature coefficient of resistivity. Typical values of _ for three materials are Cu = 0.0043 /°C; Pt = 0.0039 /°C; Ni = 0.0068/°C.

An estimation of RTD sensitivity can be calculated from typical values of the linear fractional change in resistance with temperature, as shown in Ill. 76. The sensitivity for platinum is 0.004/°C and for nickel is 0.005/°C. Usually, a specification provides calibration information, either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined.

An RTD has a response time of 0.5 to 5 seconds or more. The speed of the response is governed by its thermal conductivity which governs the time required to bring the device into thermal equilibrium with its environment. The operating range of an RTD depends on the type of wire used as the active element, e.g., a typical platinum RTD has an operating range between -100 to 650°C, and an RTD constructed from nickel has a range in the vicinity of -180°C to 300°C.

Variation of the resistance in a sensing element is measured using some form of electrical bridge circuit. Such a circuit may employ either the deflection mode of operation or the null mode.

Resistance variations in a typical RTD tend to be quite small-in the vicinity of 0.4%. Because of these small fractional resistance changes with temperature, process-control applications require the use of a bridge circuit in which the null condition is accurately detected.

1 Thermistors

A thermistor is a temperature transducer whose operation relies on the principle of change in semiconductor resistance with change in temperature. The particular semiconductor materials used in a thermistor vary widely to accommodate temperature ranges, sensitivity, resistance ranges, and other factors. The characteristics depend on the peculiar behavior of semiconductor resistance versus temperature. When the temperature of the material is increased, the molecules begin to vibrate. Further increases in temperature cause the vibrations to increase, which in turn increase the volume occupied by the atoms in the metal lattice. Electron flow through the lattice becomes increasingly difficult, which causes electrons in the semiconductor to detach resulting in increased conductance. In summary, an increase in temperature decreases electrical resistance by improving conductance. The semiconductor becomes a better conductor of current as its temperature is increased. This behavior is just the opposite of a metal. An important distinction, however, is that the change in semiconductor resistance with respect to temperature is highly nonlinear.

Individual thermistor curves are approximated by the following nonlinear equation, 1/T=A + B ln R + C ( ln R)^3

where T = temperature in kelvins R = resistance of thermistor A, B, C = curve fitting constants The temperature range measured with a typical thermistor is between _250°C and 650°C. The high sensitivity of the thermistor is one of its significant advantages. Changes in resistance of 10% per degree Celsius are not uncommon.

Because a thermistor exhibits such a large change in resistance with respect to temperature, there are many possible circuits which can be used for their measurement. A bridge circuit with null detection is most frequently used because the nonlinear behavior of the thermistor makes it difficult to use as a primary measurement device. Thermistors using null detecting bridge circuits and proper signal conditioning provide extremely sensitive temperature measurements.

Since the thermistor is a bulk semiconductor, it can be fabricated in many forms including discs, beads, and rods varying in size from a bead of one millimeter in diameter to a disc several centimeters in diameter and several centimeters thick. By varying the manufacturing process and using different semiconducting materials, a manufacturer can provide a wide range of resistance values at any particular temperature.

The response time of a thermistor depends primarily on the quality and quantity of material present as well as the environment. When encapsulated for protection against a hostile environment, the time response is increased due to the protection from the environment.

2 Thermocouples

When two conductors of dissimilar material are joined to form a circuit the following effect is observed.

When the two junctions are at different temperatures, theta_1 , and theta_2 , small emf, e1 and e2, are produced at the junctions and the algebraic sum of these causes a current.

This effect is known as the Seebeck effect. The Peltier effect is the inverse of the Seebeck effect and described as follows.

When the two dissimilar conductors which are joined together have a current passed through them, the junction changes its temperature as heat is absorbed or generated.

Another effect, called the Thomson effect, predicts that, in addition to the Peltier emf, another emf occurs in each material of a thermocouple which is due to the longitudinal temperature gradient between its ends when it forms part of a conductor.

When a thermocouple is used to measure an unknown temperature, the temperature of the thermo-junction, called the reference junction, must be known by some independent means and maintained at constant temperature.

Ill. 78 shows a typical thermocouple circuit using a chromel constantan thermocouple, reference junction, and a potentiometric circuit to monitor the output voltage. Calibration of the thermocouple is performed by knowing the relationship between the output emf and the temperature of the measuring junction.

The standards for the production of thermocouples are provided by The National Institute of Standards and Technology (NIST). Table 3-5 presents standard thermocouple characteristics.

ILL. 78 SCHEMATIC OF THERMOCOUPLE CIRCUIT

TABLE 5 STANDARD THERMOCOUPLE CHARACTERISTICS

Type Material Operating Range Accuracy K Chromel/Alumel _200 to 1350 /_ 3°C J Iron/Constantan _200 to 800 /_ 3°C E Chromel/Constantan _200 to 1000 /_ 1.5°C R Platinum/Platinum Rhodium (10%) _50 to 1600 /_ 2°C S Platinum/Platinum Rhodium (13%) _50 to 1600 /_ 2°C T Copper/Constantan _200 to 400 /_ 2°C

Chromel is an alloy of nickel and chromium, alumel is an alloy of nickel, aluminum is an alloy of nickel, and constantan is an alloy of copper. Thermocouple materials are divided into two categories: base metal types and rare metal types using platinum, rhodium, and iridium.

The general requirements for industrial thermal transducers are

• High output electromotive force.

• Resistance to the chemical changes when it comes in the contact with the fluids.

• Stability of voltage developed.

• Mechanical strength in their temperature range.

• Linearity characteristics.

The resultant emf of a particular transducer may be increased by multiplying the number of hot and reference junctions. If there are three measuring junctions, the emf is enhanced appropriately.

If the thermocouples in this arrangement are at different temperatures, the resultant emf is a measure of the mean value.

Susceptibility to interference is an important consideration in any measurement application.

Temperatures measured in hostile environments; in the presence of strong electrical, magnetic, or electromagnetic fields; or near high voltages are susceptible to interference. Susceptibility can be reduced by using non-contact methods of temperature detection.

3 Radiative Temperature Sensing

Bodies at any temperature emit radiation and absorb radiation from other bodies. A body at a temperature greater than 0°K radiates electromagnetic energy in an amount that depends on its temperature and physical properties. A sensor for thermal radiation need not be in contact with the surface to be measured. Since the radiation emitted by an object is proportional to the fourth power of its temperature, the following relationship exists.

W = sT^4

Here W is the flux of energy radiated from an ideal surface and _ is the Stefan-Boltzmann constant.

Commercial radiation thermometers or radiometers vary in their complexity and accuracy. A schematic of a basic radiometer is shown in Ill. 79 schematic of thermocouple circuit.

ILL. 79 SCHEMATIC OF RADIATION THERMOMETER: Optical mirror; Optical component; Thermopile detector

The thermopile detector is subjected to radiation from a heat source whose temperature is to be detected. The resulting rise in temperature is recorded by measuring the thermoelectric power produced by a thermopile detector. A pyrometer is a device that measures the temperature of an object by measuring its radiated energy using an optical system. The radiation emitted by the object passes through the lens system and impacts the thermal sensor. The increase in temperature of the thermopile is a direct indication of the temperature of the radiation source.

An optical pyrometer identifies the temperature of a surface by the color of the radiation emit ted by the surface. Other methods of temperature detection include optical fiber thermometers, acoustic temperature sensors, interferometric sensors, and thermochromic solution sensors.

4 Temperature Sensing Using Fiber Optics

Several concepts of temperature monitoring using fiber optics have been investigated. Operating principles based on intensity modulation in the optical fibers while under the influence of temperature has been discussed in the fiber-optic section of this chapter. In one type of reflective sensor, the displacement of a bimetallic element under the influence of temperature is measured providing an indication of temperature variation. In another type of sensor, an active sensing material (such as a liquid crystal) is used which produces fluorescence. The spectral response of the material as it's placed in the path of temperature is calibrated to produce a temperature output. The concept of micro bending is also used for temperature measurement. Using the thermal expansion of component structure, the sensor can measure the temperature by altering the fiber bend radius with temperature.

5 Temperature Sensing Using Interferometrics

lnterferometric sensing is another method used for temperature measurement. it's based on the light intensity of interfering light beams. One is a reference beam, and the other, which travels through a temperature sensitive medium, is delayed. The length of the delay is a function of the temperature. The resulting phase shift between the two beams excites the interference signal.

Under extreme conditions temperature measurement may become a difficult task. Examples of such conditions include:

• Cryogenic temperature ranges such as high radiation levels inside nuclear reactors.

• Temperature measurement inside a sealed enclosure with a known medium, in which no contact sensors can be inserted and the enclosure is not transmissive for the infrared radiation.

In such unusual conditions, acoustic temperature sensors may be useful. The operating principle of this sensor is based on the relationship between temperature of the medium and the speed of sound.

SUMMARY Temperature Sensors

RTD is a length of wire whose resistance is a function of temperature. It consists of a wire that is wound in the shape of a coil to achieve small size and improve thermal conductivity.

Thermistors

A thermistor is a transducer whose operation relies on a change in semiconductor resistance with change in temperature. Increase in temperature decreases electrical resistance by improving conductance. A Semiconductor becomes a better conductor of current as its temperature is increased. Individual thermistor curves (Ill. 80) are approximated by the nonlinear equation,

1/T=A + B ln R + C ( ln R)^3

…where T = temperature in kelvins; R = resistance of thermistor; A, B, C = curve fitting constants Radiative Temperature Sensing:

The radiation emitted by an object is proportional to the fourth power of its temperature, W = sT ^4

..where W is the flux of energy radiated from an ideal surface, and _ is the Stefan-Boltzman constant. The thermopile detector (Ill. 81) is subjected to radiation from a heat source whose temperature is to be detected.

ILL. 81 SCHEMATIC OF RADIATIVE THERMOMETERS: Optical mirror, Thermopile detector

Pyrometers measure the temperature of an object by measuring its radiated energy using an optical sys tem. The radiation emitted by the object passes through the lens system and impacts the thermal sensor.

Increase in temperature of the thermopile is a direct indication of the temperature of the radiation source. An optical pyrometer identifies the temperature of a surface by the color of the radiation emitted by the surface.

Features Since the thermistor is a bulk semiconductor, it can be fabricated in many forms, including discs, beads, and rods varying in size from a bead of one millimeter in diameter to a disc several centimeters in diameter and thickness.

Other methods include optical-fiber thermometers, acoustic sensors, interferometric sensors, and thermo-chromic solution sensors.

Applications

• The operating range of an RTD depends on the type of wire used as the active element.

• Platinum RTD has an operating range between -100 to 650°C,

• Nickel RTD constructed from nickel has a range in the vicinity of -180°C to 300°C.

• Temperature range measured with a typical thermistor is between - 250°C and 650°C.

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