1.2.5 Precision Temperature Sensing Resistor PRC100

Length: 13.5mm (excl. leads)Diameter: 5mmRS stock number 341-452

A low cost alternative to the wire-wound platinum resistance sensor, the PCR100 exhibits a base zero resistance of 100Ω, and has a linear output with an average sensitivity of 0.00385 Ω/Ω/°C over the temperature range -40°C to 150°C.

The resistor is made from wire of an alloy of nickel copper, maganese and iron, wound around an epoxy former and encapsulated in a moisture and solvent resistant epoxy resin.

Technical SpecificationResistance at 0°C 100ΩResistance at 100°C 138.5ΩTemperature coefficient 0.00385Ω/Ω/°COperating temperature -40°C to +150°CStability < ±0.038%Power rating 0.25 W (at 125°C)

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1.3 Thermistors

The name thermistor is derived from thermally sensitive resistor, since the resistance of a thermistor varies as a function of temperature. Although the fundamental principles of the thermistor were established several decades ago, industrial and commercial utilization of thermistors in temperature measurement systems was very slow in developing. Since the early 1980s a proliferation of relatively low-cost, usually portable thermistor thermometers has become available. However, the thermistor to date has not been widely applied in the traditional industrial and process control temperature measurement field because of several inherent problems. A thermistor is an electrical device made of a solid semiconductor with a high temperature coefficient of resistivity which would exhibit a linear voltage-current characteristic if its temperature were held constant. When a thermistor is used as a temperature sensing element, the relationship between resistance and temperature is of primary concern. The approximate relationship applying to most thermistors is:

where R0 = resistance value at reference temperature T0 K, ΩRT = resistance at temperature T0 K, ΩB = constant over temperature range, dependent on manufacturing

process and construction characteristics (specified by supplier)

A second form of the approximate resistance-temperature relationship is:

where R∞ is the thermistor resistance as the temperature approaches infinity, in ohms.

These equations are only best approximations and, therefore, are of limited use in making highly accurate temperature measurements. However, they do serve to compare thermistor characteristics and thermistor types.

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1.3.1 Thermistors configurations

Various configurations of thermistors are shown in Fig 17.

(a) Beads may be made by forming small ellipsoids of material suspended on two fine leadwires. The wires may be approximately 2.5 mm (0.1 inch) apart or even closer. If the material is sintered at an elevated temperature, the leadwires become tightly embedded within the bead, making electric contact with the thermistor material. For more rugged applications, the bead thermistor may be encapsulated or placed within a suitable metal sheath.

(b) Disk configurations are manufactured by pressing the semiconductor material into a round die to produce a flat, circular probe. After sintering, the pieces may be silvered on the two flat surfaces. Thermistor disks may range from 2.5 to 25 mm (0.1 to 1 inch) or less in diameter, and from 0.5 to 13 mm (0.02 to 0.5 inch) or less in thickness. The disk configuration is well suited where a moderate degree of power dissipation is needed.

(c) Washer-type thermistors are like the disk types, but provide a hole for bolt mounting. Often they are applied where high power dissipation is a primary requirement.

(d ) Rod-type thermistors are extruded through dies, resulting in long, cylindrical probes. Rod configurations generally are of high thermal resistance and are applied wherever power dissipation is not a major concern.

FIGURE-17 Various configurations of thermistors.

See: Thermistors RS Data Sheet 232-4538.pdf

Testing and Calibrations of Thermistors.pdf

RS Sensors Thermistors.doc

Thermistor Types.doc

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1.3.2 Thermistor Performance

The evaluation of thermistor performance characteristics is in many cases similar to that of resistance thermometers. Figure 18 shows the logarithm of the specific-resistance-versus-temperature relationship for three thermistor materials as compared with platinum metal. The specific resistance of the thermistor represented by curve 1 decreases by a factor of 50 as the temperature is increased from 0 to 100.C. Over the same temperature range, the resistivity of platinum will increase by a factor of approximately 1.39. Thermistors range in terminal resistance at room temperature from about 1 ohm to the order of 108 ohms, depending on composition, shape, and size. Within a given type, they commonly vary 10 to 30 percent in resistance from the nominal at the reference temperature. Advanced manufacturing techniques and careful selection of suitable units can bring resistance values within closer limits..

FIG. 18 Resistance-temperature characteristics of 3 thermistors compared with platinum

Choosing between Thermocouples, Platinum RTD and Thermistors.See: Guide for choosing temperature sensors.pdf.

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1.4 IC Temperature Sensors

See: temp sensors Complete Temperature Sensor Family Selection Table.pdf

1.4.1 Temperature Sensor IC LM35

Provides a linear voltage output of 10mV/°C in a TO-92 plastic package.

Technical specifications

Type LM35CZ LM35DZ

Temperature range -40 to +110°C 0 to +100°C

Accuracy @ 25°C ±0·4°C (typ) ±0·9°C (typ)

Operating voltage +30V to +4V

Absolute max. voltage +35V -0·2V

National Semiconductors Ref.

RS stock no. Price each

1-9 10-24

LM35 CZ 317-960 £4.16 £3.33

LM35 DZ 317-954 £1.39 £1.11

See: LM35A Data Sheet.pdf.

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1.4.2 Temperature Sensor IC 590kH

A high impedance constant current regulator providing an output of 1µA/°C.

Technical specifications

Operating voltage range 4 V to 30 V d.c.

Nominal current output 298.2 µA at +25°C

Nominal temperature coefficient 1µA/°C

Calibration error at 25°C ±2.5°C max.

Operating temperature range -55°C to +150°C

Philips Ref.RS stock no. Price each

1-24 25-99

IC 590kH 308-809 £9.94 £7.20

See: AD590 Termperature Transducer Data Sheet.pdf.

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1.4.3 IC Programmable Temperature Controller

a) The TMP01FP

Features

TTL and CMOS compatible. 8-Pin mini-dip package. High/low trip points set by external

resistors.

Generates a voltage output proportional to temperature, and a control signal from 1 of 2 outputs when set to either above or below, a specific temperature range.

Technical specifications

Supply range 4·5V to +25V

Quiescent current (max.) 400µA

Temperature range -55 to +150°C

Accuracy (typ.) ±0·6%

Temperature coefficients (typ.) 5mV/°K (1·49V @ 25°C)

Trip points output current (typ.) 20mA

Operating temperature range -55 to +150°C

All parameters measured at V = +5V

Analog Devices Ref.RS stock no. Price each

1-24 25-99

TMP01FP 310-874 £4.57 £4.11

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b) The LM3911N

An accurate temperature measurement and/or control system fabricated on a single chip and including a temperature sensor, stable reference and an operational amplifier. The output voltage is directly proportional to temperature at 10mV/°C. Using the internal op-amp with external resistors any temperature scale factor can be obtained. The op-amp can also be configured as a comparator which will switch at a set point making the device useful as an ON-OFF temperature controller. An active shunt regulator should be connected in order to provide a stable 6.8V reference. This allows the use of any power supply voltage with suitable external resistors. Typical applications for the LM3911N are in driving lamps and relays up to a maximum supply voltage of 35V. The LM3911N is available in 8-pin d.i.l. package.

Technical specifications

Sensor Output voltage (max) 3.08V

Long term stability (typ) 0.3%

Voltage ref.Output voltage 6.8V

Temperature stability (max) 85mV

Op-ampVoltage gain 15000 V/V

Output sink current (min) 2mA

General device

Supply current (max) 10mA

Output collector voltage (max) 36V

Operating temperature -25°C to +85°C

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1.4.5 Thermal Switches

a) Solid State Thermal Switches

Features:

Thermal sensors encapsulated in TO18 type packages with electrically isolated mounting tabs.

Choices of 57°C or 75°C temperature centers.

Resistance changes from 100kΩ to 100Ω at center temperature for a 10°C change (approx) (with small changes outside this transition range).

TypeRS stock no. Price each

1-24 25-99

57°C 307-929 £2.63 £2.44

75°C 307-935 £2.63 £2.44

See: Thermal Switch RS Data Sheet.pdf.

b) Thermostats and Bi-Metallic Switches

See: Industrial Thermostats.pdf.

Thermostat BiMetalic.doc.

Capillary Thermostat.doc.

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1.5 RADIATION THERMOMETERS

Radiation thermometry makes it possible to measure the temperature of an object without making physical contact with it. Although these instruments are widely used because of the contactless feature, this is by no means the exclusive advantage of the method. Another important advantage is the wide useful temperature range; from subzero temperatures to extremely high, virtually unlimited values. Carefully constructed instruments have been used for many years to maintain laboratory primary and secondary standards above the gold point (1064.43°C) within accuracy of ±0.01°C. Representative industrial designs generally have a precision of ±0.5 to ±1 percent, even under rather adverse conditions. Radiation thermometers, although much improved during the past decade or two through the incorporation of technological advancements in electronic circuitry and optics, have been used for many decades. Originally called radiation pyrometers, then radiation thermometers, and, more recently, infrared (IR) thermometers, the initial applications of the method usually were in those processing and manufacturing applications where great amounts of heat were required, often in small spaces, thus creating v ery high temperatures. Often such materials were moving and could not be contacted by a temperature detector. Thus early applications generally involved monitoring such operations as glass, metal, chemical, cement, lime, and refractory materials. During the past few decades, radiation thermometry has extended into lower temperature regions, including subzero measurements, as encountered in the foods, electronics, paper, pharmaceutical, plastics, rubber, and textile industries, among others. Portable IR thermometers also find wide application in industrial energy conservation strategies for checking large lines, vessels, steam traps, and so on, for faulty operation. Relatively recent increased usage of IR thermometers now gives it well over 10 percent of the industrial temperature measurement field.

1.5.1 IR Thermometry PrinciplesThe energy radiated by an object as a result of its temperature is quantitatively expressed in terms of a perfect radiating body, which is traditionally designated a blackbody. This concept has been described in several ways, such as a body that absorbs all the radiation it intercepts and a body that radiates more thermal energy for all wavelength intervals than any other body of the same area, at the same temperature.

Stefan-Boltzmann Law: The total power, R, of radiant flux off all wavelengths emitted per second by a blackbody of area A at temperature T in Kelvin to surroundings at To is:

R = σ A (T 4 - To 4)

Where σ is the Stefan-Boltzmann constant. This equation applies to perfectly blackbodies with an emissivity ε of unity. ‘Real’ objects have non-unity emissivities and a correction must be made for this. Hence,

R = σ ε A (T 4 - To 4)

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1.5.2 Hand-Held Infra-red Thermometer

Supplied to RS by Digitron.

A hand-held infrared thermometer with separate sensing heads for measurement of hard-to-access surfaces such as photocopiers etc. The instrument uses a precision fresnel lens focused onto a thermopile. Available in models (D202ASC) with fixed emissivity of 0.95, or models (D202ES) with adjustable emissivity, display hold and max./min. mode. Useful for measuring temperatures of hazardous hard-to-reach or moving materials and without risk of contamination to instrument, process or user.

Features :• Robust ergonomic case.• Separate sensing head for awkward spots.• Backlit LCD display.• Auto shut-off.

Technical specifications (D202ASC)

Range -20 to +250°C

Resolution 1°C

Accuracy (at 20°C emissivity 0.95) 1% (± 2°C)

Battery type 9V PP3

Battery life 500 hours

Spectral response 6 to 14 microns

Dimensions H. 140 D. 28 W. 70 (excl. sensor)

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Advanced Application Examples

1- Temperature Sensors in the Food Industry

Fig. 7.5 Low cost portable infra-red thermometers.

See: 7. Practical Aspects of Infra-red Remote Thermometry.pdf

2- Temperature Sensors in Home Appliances

Check this Washing Machines example for an advanced appliance system.

See: Infineon Technologies AG - Solutions -~Washing Machines.htm

Infineon Technologies AG - Products-Temperature Sensors--~KT and KTY Series.htm

3- Temperature Sensors in Automotive Industry

Check this Gasoline Engine Management example.

See: Infineon Technologies AG - Solutions -~Gasoline Engine Management.htm

Infineon Technologies AG - Products-Temperature Sensors--~KT and KTY Series.htm

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