Thermistors

6 Key excerpts on "Thermistors"

• 

  eBook - ePub

  High Temperature Experiments in Chemistry and Materials Science

  • Ketil Motzfeldt(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Two metal strips of different coefficients of linear expansion, welded or hot-rolled together, form a bimetallic strip. At a neutral temperature, most often 20 °C, the strip is flat. As the temperature is increased, the strip will bend towards the metal with the lower thermal expansion. The principle may be used to make rugged industrial thermometers of moderate sensitivity. More often it is used as the central element in simple thermostats.

  3.2.3 Semiconductor-Based Thermometers

  A semi-conductor is a piece of solid material characterized by a strongly positive coefficient of electric conductivity. Thus its electric resistance decreases at increasing temperature, in contrast to pure metals as in Figure 2.1 (p. 16). The whole field of solid-state electronics took off when Bardeen, Brattain and Shockley at the Bell Telephone Laboratories clarified the physics of semiconductors and came up with the first transistor in 1948.

  The field of electronics is definitely outside the scope of the present text. It may only be mentioned that a thermistor is a semi-conducting two-terminal component which in principle acts as a temperature-dependent resistor. Today Thermistors may be made from a variety of metallic or ceramic materials to almost any specification. In order to be used as a thermometer or a thermostat it needs a power supply (in contrast to a thermocouple).

  Temperature measuring devices based on Thermistors and transistors usually have operating ranges to a maximum of some 350 °C. Hence they are strictly not of interest for high temperature measurements, but definitely useful in auxiliary equipment. The technology is discussed by Meijer and van Herwaarden (1994).

  3.2.4 Resistance Thermometers

  The semiconductor-based temperature sensors of the preceding paragraph are based on the variation of electric resistance with temperature, and thus could be classified as resistance thermometers. Traditionally, however, the term is restricted to wire-wound resistors, usually of pure platinum, as originally developed by Callendar (1899) and others for precision measurements. A suitable length of fine platinum wire is wound on an insulating support of silica glass or ceramics, and held in place by a thin coating of the same material. The quality and purity of the platinum itself was a crucial point in early development and still is. As mentioned above, the platinum resistance thermometer is now the preferred reference standard in the ITS-90 all the way from the triple point of hydrogen to the freezing point of silver. The ITS-90 gives detailed relations between the resistivity of the Pt wire and the relevant temperature, in order to achieve a maximum accuracy. In most high temperature work, however, this extreme accuracy is not needed, and a thermocouple is more expedient.

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  eBook - ePub

  Basic Process Measurements

  • Cecil L. Smith(Author)
  • 2011(Publication Date)
  • Wiley-AIChE

    (Publisher)

  Compared to platinum RTDs, the resistance of Thermistors is much greater and is more sensitive to temperature. The nature of the sensitivity is the basis for the following classifications:

  Negative temperature coefficient (NTC) . The resistance of the thermistor decreases with temperature. The Thermistors used for industrial temperature measurement are of this type.

  Positive temperature coefficient (PTC) . These are sometimes referred to as “switching PTC Thermistors” because their resistance increases abruptly at a certain temperature. This makes them ideally suited for initiating actions (such as a shutdown) to avoid equipment damage due to elevated temperatures. One application is to protect the windings in electric motors from thermal damage. However, they have no application to process temperature measurement.

  The DIN 43760 standard for the 100-Ω platinum RTD contributed to the industrial acceptance of RTDs for temperature measurement. Unfortunately, no such standard has appeared for Thermistors. Thermistors are differentiated by their zero-power resistance, which is the DC resistance at a specified temperature (usually 25 °C) with negligible self-heating.

  Resistance-Temperature Characteristic

  Figure 2.25 shows a plot of the thermistor resistance as a function of temperature for a specific commercial product (this one was chosen only because the resistance-temperature data could be downloaded over the Internet). This graph of resistance as a function of temperature clearly shows that

  Figure 2.25 Resistance of a commercial thermistor as a function of temperature (R vs. T ).

  • Thermistor resistance at 25 °C is much greater than 100 Ω. There is an advantage for this: Lead wire resistance is insignificant in comparison. The resistance of the thermistor can be determined using two lead wires and a current source.
  • The resistance decreases rapidly with temperature. Thermistors are capable of detecting temperature changes on the order of 0.001 °C. But with such large resistance changes, temperature spans have to be narrow (like 20 °C).
  • The relationship is highly nonlinear. This presents great difficulties for analog systems but not for digital systems. The nonlinear problem is often overemphasized. For this reason, we will examine some formulations that very effectively address the nonlinear issues. These will involve logarithms. While this can lead to cardiac arrest for the designers of analog circuits, designers of digital systems barely take notice.

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  eBook - ePub

  Smart Sensors and MEMS

  Intelligent Sensing Devices and Microsystems for Industrial Applications

  • S Nihtianov, A. Luque(Authors)
  • 2018(Publication Date)
  • Woodhead Publishing

    (Publisher)

  Thermopiles can be fabricated with IC technology and are very suited for application in thermal sensors. In thermal sensors, physical quantities are measured by transducing the physical signals into temperature differences first, and then transducing this temperature difference into a thermopile voltage. Usually, in such sensors a reference temperature is also measured, for instance, with a bipolar transistor or a temperature-sensitive resistor. IR sensors, including the popular clinical ear thermometers, are examples of thermal sensors, in which radiation is absorbed at a cantilever beam (Herwaarden van, 2008), which causes a temperature difference that is measured with a thermopile. Measuring absolute temperature with a thermocouple or an IR sensor also requires the use of an absolute temperature sensor, for instance, a thermistor or a transistor, to measure a reference temperature. In industrial systems, often discrete temperature-sensing elements are used,because of their high accuracy and excellent long-term stability. The most commonly used elements are Pt resistors, thermocouples, and Thermistors. Because of their stability, platinum resistors are listed in the 1990 International Temperature Scale as interpolating temperature standard in the − 259.4°C to 961.9°C temperature range (Michalski et al., 2001). For higher temperatures, other types of sensors, such as certain types of thermocouples, are used. Because of their low cost and high reliability, discrete thermocouples are widely used in industrial applications, where different types are available for different temperature ranges. Thermistors are very sensitive but not as stable as Pt resistors. They are widely applied for the temperature range of about − 80°C to 180°C. In addition to their high sensitivity, Thermistors offer the advantages of being small in size and inexpensive. However, their strong nonlinearity complicates the processing of the thermistor signal

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  eBook - ePub

  Principles of Measurement and Transduction of Biomedical Variables

  • Vera Button(Author)
  • 2015(Publication Date)
  • Academic Press

    (Publisher)

  0 are determined by the proportions and quality control of the components used, sintering process, etc. Resistance of Thermistors can be affected by oxides composition, quality control of components, thermal treatment, thickness and diameter (disc type), distance between wire leads (bead type), impurity doping (silicon single crystal type), substrate material (thin film type), heterogeneity in diffusion of the contact material in the case of Thermistors with metallized surface (chip type), etc. The resistance tolerance of Thermistors is reduced producing them in “matched units pair,” which consists of an arrangement of two Thermistors (drop type, chip or thin film) connected in series or in parallel that results in better tolerance for a given temperature range. Accurate temperature sensing within wide temperature range can be obtained by mounting both high and low resistance Thermistors in the same casing.

  Thermistors, mainly NTC type, of different formats, sizes, and casings are used in medical electronic thermometers for monitoring the patient temperature with skin probes, for central temperature measurement (e.g., esophageal and rectal) with internal probes, and for direct temperature measurement (e.g., inside blood vessels and heart chambers) with invasive probes with mini and microsensors inserted in catheters. Thermistors are also used for liquid immersion temperature measurements (blood, saline, drug solutions, etc.).

  The main advantage of the use of Thermistors as temperature transducers is their high sensibility to small temperature changes. Thermistors are less expensive than RTD, copper or nickel extension wires can be used and they become more stable with use. Some disadvantages may also be mentioned, such as their limited working temperature range, the initial accuracy drift, the lack of standards for replacement, and the possibility of loss of calibration, if they are used beyond temperature ratings.

  4.2.2.2 Thermoelectric temperature transducers

  4.2.2.2.1 Thermocouple

  A thermocouple is a temperature sensor that consists of two wires of different metallic materials put in thermal contact (Figure 4.19

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  eBook - ePub

  Handbook of Measurement in Science and Engineering, Volume 1

  • Myer Kutz, Myer Kutz(Authors)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  Equation (14.25) (Wood et al., 1978).

  (14.25)

  where R0 is the resistance at T0 and B a constant for the particular thermistor material.

  The resistance characteristic of a thermistor expressed by Equation (14.25) is negative and nonlinear. This can be offset if desired using two or more matched Thermistors packaged in a single device so that the nonlinearities of each device negate each other (see Beakley, 1951; Sapoff and Oppenheim, 1964; Sapoff, 1980). Thermistors are usually designated by their resistance at 25°C, with common resistances ranging from 470Ω to 100kΩ. The high resistivity of Thermistors normally negates the need for a four-wire bridge circuit.

  In order to produce a lower uncertainty fit for the resistance temperature characteristic of a thermistor, a polynomial can be used with the degree used depending on the temperature range and the type of thermistor. Equation (14.26) is adequate for most systems.

  (14.26)

  Here A0 , A1 , A2 , and A3 are constants.

  Alternatively Equation (14.27) can be used where the second order term has been neglected.

  (14.27)

  where b0 , b1 , and b3 are constants.

  14.3.6 Semiconductor Devices

  A forward voltage across a transducer junction can be used to generate a temperature sensing output proportional to absolute temperature. Temperature sensors based on simple transistor circuits can be readily incorporated as a part of an integrated circuit to provide on-board diagnostic or control capability. The majority of semiconductor junction sensors use a diode connected bipolar transistor. If the base of the transistor is shorted to the collector then a constant current flowing in the remaining p-n junction (base to emitter) produces a forward voltage that is proportional to absolute temperature. This can be modeled by Equation (14.28)

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  eBook - ePub

  Real-Time Environmental Monitoring

  Sensors and Systems - Textbook

  • Miguel F. Acevedo(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  active, i.e., require energy to operate.

  Examples of resistive sensors are potentiometers, resistive temperature detectors, LDR, Thermistors, liquid level sensors, strain gages, resistive gas sensors, liquid conductivity sensors, and resistive hygrometers.

  Thermistors: Temperature Response

  Thermistors can be of two types: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). Those of NTC type are made from semiconductor material and resistance decreases gradually with temperature. In opposite fashion, a PTC, made from ceramic, will have a resistance that increases quickly with temperature.

  We will focus on NTC Thermistors for which the resistance decreases non-linearly with temperature. A general model is the B parameter equation

  R =

  R 0

  exp

  (

  B

  (

  T

  − 1

  −

  T 0

  − 1

  )

  )

  (3.16)

  where T is the temperature in K, R is the thermistor resistance in Ω, T0 is the nominal value of T (25°C = 298 K), R0 is the nominal value of R at T0 , and B is a parameter in K (Rudtsch and von Rohden 2015 ). Figure 3.15 shows an example of the model response for B = 4100 K, T0  = 25°C, and R0  = 10 kΩ.

  FIGURE 3.15 Thermistor resistance vs. temperature using the B parameter model.

  The B parameter equation can be inverted to calculate temperature given resistance by rearranging terms and taking logarithm of both sides of the equation

  1 T

  =

  1

  T 0

  +

  1 B

  ln

  (

  R

  R 0

  )

  (3.17)

  Denoting

  a 0

  = 1   /  

  T 0

  and

  a 1

  = 1   /   B

  , we obtain

  1 T

  =

  a 0

  +

  a 1

  ln

  (

  R

  R 0

  )

  (3.18)

  This equation can be considered the first-order approximation n = 

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