Which Temperature Device Uses Voltage To Register Temperature Change?

Nuclear plant instrumentation

Thomas W. Kerlin , Belle R. Upadhyaya , in Dynamics and Control of Nuclear Reactors, 2019

16.2.two.1 Resistance thermometers

RTDs have sensing elements made of metal, typically platinum. The platinum metal in some reactor RTDs is in the form of a wire wrapped around a mandrel (typically magnesium oxide) inside a stainless-steel tube with magnesium oxide insulator betwixt the mandrel and the inner wall of the sheath. See Fig. 16.six.

Another RTD design uses a platinum wire coil cemented to the inside wall of a hollow section of a metallic tube. This approach provides a very fast-responding temperature measurement considering the oestrus transfer resistance between the coil and the sheath is small. See Fig. 16.seven.

Platinum has a well-defined temperature resistance human relationship. Instrumentation measures the resistance and converts information technology to a temperature measurement using temperature vs. resistance calibration information. The resistance increases with temperature and the temperature-resistance relation is well-nigh linear. But the readout instrumentation accounts for the pocket-size non-linearity.

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Gas Processing Plant Automation

Saeid Mokhatab , ... John Y. Mak , in Handbook of Natural Gas Manual and Processing (Fourth Edition), 2019

20.4.two.i Resistance Temperature Detectors

An RTD is a passive excursion chemical element whose resistance is greater at higher temperature in a predictable fashion. The traditional RTD element is synthetic of a small coil of platinum, copper, or nickel wire wound to a precise resistance value around a ceramic or drinking glass bobbin. The winding is by and large of helix style for industrial use.

The most common RTD chemical element textile is platinum, as information technology is more accurate, reliable, chemically resistant, and stable material, making it less susceptible to environmental contagion and corrosion than other metals. It is also easy to industry and widely standardized with readily available platinum wire available in very pure form with excellent reproducibility of its electrical characteristics. Platinum besides has a higher melting bespeak, giving information technology a wide operating temperature range. For an RTD sensor, it is the wires, which connect to the sensing element and the wire insulation, which more often than not limit the maximum application temperature of the sensor.

Measuring the temperature requires accurate resistance measurement. To measure the resistance, it is necessary to convert resistance to a voltage, and utilise the voltage to drive a differential input amplifier. The use of a differential input amplifier is important as it volition decline the common mode noise on the leads of the RTD and provide the greatest voltage sensitivity.

The RTD signal is by and large measured by connecting the RTD element in i leg of a Wheatstone bridge excited either by a constant reference voltage or past running it in series with a precision current reference and measuring the corresponding intensity resistance (IR) voltage driblet. The latter method is generally preferred equally it has less dependence on the reference resistance of the RTD element.

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Sensors

Hank Zumbahlen , with the engineering science staff of Analog Devices, in Linear Excursion Design Handbook, 2008

Resistance Temperature Detectors

The resistance temperature detector, or the RTD, is a sensor whose resistance changes with temperature. Typically congenital of a platinum (Pt) wire wrapped around a ceramic bobbin, the RTD exhibits behavior which is more accurate and more linear over wide temperature ranges than that of a thermocouple. Figure 3-42 illustrates the TC of a 100 Ω RTD and the Seebeck coefficient of a Type S thermocouple. Over the entire range (approximately −200°C to + 850°C), the RTD is a more linear device. Hence, linearizing an RTD is less complex.

Unlike a thermocouple, however, an RTD is a passive sensor and requires current excitation to produce an output voltage. The RTD'southward low TC of 0.385%/°C requires similar high performance bespeak conditioning circuitry to that used by a thermocouple; even so, the voltage drop across an RTD is much larger than a thermocouple output voltage. A system designer may opt for large value RTDs with higher output, but large-valued RTDs exhibit dull response times. Furthermore, although the cost of RTDs is college than that of thermocouples, they use copper leads, and thermoelectric effects from terminating junctions do non affect their accuracy. And finally, because their resistance is a part of the absolute temperature, RTDs require no common cold-junction compensation.

Caution must be exercised using current excitation because the current through the RTD causes heating. This cocky-heating changes the temperature of the RTD and appears as a measurement error. Hence, careful attention must be paid to the pattern of the signal conditioning circuitry then that cocky-heating is kept below 0.five°C. Manufacturers specify self-heating errors for various RTD values and sizes in still and in moving air. To reduce the fault due to self-heating, the minimum current should be used for the required organisation resolution, and the largest RTD value called that results in acceptable response fourth dimension.

Another outcome that can produce measurement error is voltage drop in RTD lead wires. This is especially disquisitional with low value 2-wire RTDs because the TC and the absolute value of the RTD resistance are both small. If the RTD is located at a long distance from the indicate workout circuitry, then the atomic number 82 resistance can be ohms or tens of ohms, and a modest amount of lead resistance can contribute a significant mistake to the temperature measurement. To illustrate this point, let us assume that a 100 Ω platinum RTD with 30-guess copper leads is located about 100 feet from a controller's display panel. The resistance of 30-gauge copper wire is 0.105 Ω/feet, and the two leads of the RTD will contribute a full 21 Ω to the network which is shown in Figure 3-43. This boosted resistance volition produce a 55°C error in the measurement! The leads' TC can contribute an boosted, and possibly significant, error to the measurement. To eliminate the effect of the lead resistance, a four-wire technique is used.

In Figure three-44, a four-wire, or Kelvin, connection is made to the RTD. A constant current is applied though the Force leads of the RTD, and the voltage across the RTD itself is measured remotely via the SENSE leads. The measuring device tin can be a digital voltmeter (DVM) or an instrumentation amplifier, and high accuracy tin can be achieved provided that the measuring device exhibits high input impedance and/or low input bias electric current. Since the SENSE leads practice non acquit appreciable current, this technique is insensitive to lead wire length. Sources of errors are the stability of the constant current source and the input impedance and/or bias currents in the amplifier or DVM.

RTDs are by and large configured in a 4-resistor span circuit. The bridge output is amplified by an instrumentation amplifier for further processing. Withal, high resolution measurement ADCs such equally the AD77XX series allow the RTD output to be digitized direct. In this manner, linearization tin be performed digitally, thereby easing the analog circuit requirements.

Effigy 3-45 shows a 100 Ω Pt RTD driven with a 400 μA excitation electric current source. The output is digitized by one of the AD77XX serial ADCs. Note that the RTD excitation current source also generates the 2.five V reference voltage for the ADC via the 6.25 kΩ resistor. Variations in the excitation current practice non bear on the excursion accuracy, since both the input voltage and the reference voltage vary ratiometrically with the excitation current. Still, the 6.25 kΩ resistor must take a low TC to avoid errors in the measurement. The high resolution of the ADC and the input PGA (gain of 1–128) eliminates the need for additional conditioning circuits.

The ADT70 is a complete Pt RTD signal conditioner which provides an output voltage of 5 mV/°C when using a 1 kΩ RTD (meet Figure 3-46). The Pt RTD and the 1 kΩ reference resistor are both excited with 1 mA matched electric current sources. This allows temperature measurements to be made over a range of approximately −fifty°C to +800°C.

The ADT70 contains the ii matched electric current sources, a precision rail-to-track output instrumentation amplifier, a 2.v 5 reference, and an uncommitted rail-to-rail output op amp. The ADT71 is the aforementioned as the ADT70 except the internal voltage reference is omitted. A shutdown function is included for bombardment powered equipment that reduces the quiescent electric current from three mA to 10 μA. The gain or full-scale range for the Pt RTD and ADT701 system is set by a precision external resistor connected to the instrumentation amplifier. The uncommitted op amp may be used for scaling the internal voltage reference, providing a "Pt RTD open" signal or "over temperature" warning, providing a heater switching signal, or other external conditioning determined past the user. The ADT70 is specified for operation from −40°C to + 125°C and is bachelor in xx-pin DIP and SOIC packages.

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Full general Instruments

Swapan Basu , Ajay Kumar Debnath , in Ability Plant Instrumentation and Control Handbook (Second Edition), 2019

iii.ii.1 Transmitter (RTD Input)

The RTD is basically a coil element/wire-wound/thin-pic resistance with two end terminals. At that place are various types that utilise two-, three-, or four-wire measuring systems (Fig. 4.threeB–D) depending upon the accurateness level desired and the expected cost. The two-wire system has one wire from each end and the iii-wire system has i wire from each end and some other wire from whatsoever end that goes to the measuring circuit. A two-wire system does not provide correct output due to variation in ambient temperature, as the resistance of the pb wires (both sides) changes unpredictably. The three-wire system is a improve way to have intendance of this problem. The iv-wire arrangement has two wires from each end that go to the measuring excursion, which to a great extent eliminates the trouble of variation in ambience temperature. In all of the wire measuring systems, the RTD is ultimately connected to a span circuit to generate a voltage signal.

At that place is another four-wire measuring system that is more accurate, because it well-nigh completely eliminates the ambience temperature variations. The transmitter injects a precise and controlled current into the temperature sensor by ii wires, and with the other two wires, the resultant voltage drop across the temperature sensor is used to measure resistance in a circuit, which provides high input impedance so that minute current flows. With almost nothing level current, the lead resistance modify will introduce minimum effect, and voltage drop across the RTD will be read as an open circuit voltage, even at the remote location. The voltage is then converted into a digital format using an analog-to-digital converter provided past a microprocessor. The microprocessor converts the measured voltage into a digital value representative of temperature. The temperature transmitter generally includes housing and a temperature probe, which attaches to the housing. To monitor a procedure temperature, the transmitter includes a sensor, such as an RTD or a THC. An RTD changes resistance in response to a change in temperature. Past measuring the resistance of the RTD, the temperature can exist calculated.

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In vivo physiological measurements

J.Y. Hu , ... Grand.W. Yeung , in Clothing Biosensory Engineering, 2006

17.2.two Resistance thermometer detector (RTD)

The resistance temperature detector (RTD), is a thin film device made of platinum, which is used for measuring temperature. It has keen stability, accuracy and repeatability. The resistance tends to be almost linear with temperature – the higher the temperature, the larger the resistance.

Different other thermocouples, no special extension cables or cold junction compensations are required for RTD. Furthermore, the conductor resistance is related to its temperature. Commercial platinum grades are produced which exhibit a alter of resistance of 0.385 Ω/°C, co-ordinate to the European Central Interval. The sensor is ordinarily fabricated to have 100 Ω at 0 °C, which is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ω/°C. ³⁰ They are amid the well-nigh precise temperature sensors available with resolution ± 0.1 °C.

In society to determine the resistance, RTD requires a modest constant current to laissez passer through; notwithstanding, this may cause cocky-heating. Atomic number 82 wire resistance should exist considered and adopting iii and four wire connection strategies can issue in eliminating connectedness lead resistance effects from the measurements.

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TRANSDUCERS AND DATA ACQUISITION

RICHARD HATHAWAY , KAH WAH LONG , in Fatigue Testing and Analysis, 2005

1.10.3 ELECTRICAL RESISTANCE THERMOMETERS AND RESISTANCE-TEMPERATURE DETECTORS

The electrical resistance thermometer and resistance-temperature detectors (RTDs) are authentic methods of temperature measurement. The RTD relies on the change in resistance in the temperature-sensing material as an indicator of the thermal activity. Unlike thermistors, which are fabricated of semiconductor materials and take a negative temperature–resistance relationship, the RTD has a positive temperature–resistance relationship, although the sensitivity is lower than that of a thermistor. RTD temperature–resistance characteristics may also exist somewhat nonlinear. The RTD typically can be used over a higher temperature range than a thermistor, having temperature ranges of −250 to 1000°C. A constant-voltage bridge excursion, like to that used with strain gages, is usually used for sensing the resistance change that occurs.

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Thinning Films and Tribological Interfaces

Sergei B. Glavatskikh , in Tribology Series, 2000

2.2 Instrumentation

Since instrumentation blazon and position is of paramount importance for effectiveness of experiments, some comments need to exist fabricated on their choice.

Thermocouples and resistance temperature detectors (RTD) are the most common forms of temperature sensors used in rotating machinery. RTDs despite their loftier accuracy are unsuitable for monitoring thermal transients as they take very poor response time [ xiv]. Thermocouples have rapid response fourth dimension. But their accuracy depends on the ability to accurately determine the cold junction temperature, because a thermocouple measures the difference in temperature between its hot junction (the temperature to be measured) and its cold junction (the reference junction). The traditional method of cold junction compensation (CJC) uses a split temperature measuring device to monitor the temperature of the reference junction and from that measurement derive a correction to the readout. Only the necessity of using skid rings to transmit collar and shaft temperature information poses a serious problem as an additional junction, which temperature is changing with speed, is introduced. However, the difficulty can be eased by using thermistors (or thermally sensitive resistors). They have fast response time, small size and are cheap. No bounty for ambient temperature is needed. Negative temperature coefficient (NTC) blazon thermistors of 1.ix mm in diameter were chosen for this study. NTC thermistors give a relatively large output (alter of resistance) for a small temperature alter. The resistance-temperature relationship of NTC thermistors is negative and highly not-linear. Merely it is hands linearised by software. Unfortunately, such thermistors can't be flushed with the collar surface. 2 thermistors (T_75%, T_25%) were therefore installed 1,v mm from the collar face (figure 4a and 4b) and fixed in place past a thermally conductive adhesive.

The 3rd one (T_c) was placed in the shaft as shown in figure 4a and 4c.

Response time of the chosen thermistors in agitated oil is better than 1 2nd. The response time is divers, as the time required for the sensor change 63.2% of the total difference between its initial and concluding body temperature when subjected to a footstep change in temperature.

A piezoelectric transducer was chosen for pressure profile measurement, as high frequency response was required. Ascent fourth dimension of the transducer is amend than two μs. Transducer mounting is shown in effigy 4d. Recessed mounting allows the measuring area to be minimised and protects the sensor diaphragm from particle impingement. On the other mitt, recessed installation can have a limiting event on pressure pulse rise time. For the given length of the passage filled with oil the fastest pulse rise time is amend than one μs which is much above whatsoever pressure fluctuations. In that location is therefore no effect of such mounting on the pressure measurement.

Except the to a higher place-mentioned sensors a number of other sensors take also been used to measure parameters, which are important for decision-making the experimental weather condition. These are thermocouples for monitoring oil and water temperatures, a shaft speed meter, oil and water flow meters, etc. Uncertainty of the primal measurands are listed in Table two.

Table 2. Dubiety of measurands

Temperature, °C
Thermocouple (type T)	± 1
Thermistor	± 0.three
Friction torque, Nxm	± 0.2%FS
Begetting load, MPa	± ane%
Rotational speed, rpm	± 0.1%
Oil flick pressure, MPa	± 4%FS
Flow rate, 50/min	± 0.five%

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Temperature Transducers

Vera Lucia Da Silveira Nantes Push button , in Principles of Measurement and Transduction of Biomedical Variables, 2015

4.2.2.one.i.3 RTD casing types

To protect the RTD from damage caused by mechanical shock or from corrosion when the temperature measurement is made in environments with corrosive liquids, moist, or gases (e.g., sweat, blood, and exhaled air), the heat-sensitive resistive wire, filament, or film is covered by a protective case, usually made of ceramic, glass, synthetic resin, brass, or stainless steel.

The external case not only protects the transducer but likewise increases the final size and the response fourth dimension of the temperature sensor. Typical RTD sizes for applications in biomedical engineering science are around 1.5 mm diameter×25 mm length. Although not likewise large, in straight and invasive measurements of temperature in which the sensor is placed inside the trunk through a catheter, for case, this size becomes quite significant. Physical dimensions become less of import when dealing with the measurement in temperature command equipment such equally Bier oven, infant incubators, and muffle furnace.

The casing can cause issues ranging from an increment in the time abiding of the system to a decrease in the input impedance caused by the jacket in series association with the oestrus-sensitive filament. The fourth dimension constant of the RTDs used in biomedical engineering applications is in the range 0.5–1 southward (typical), while thermistors and thermocouples (type T) have time constant values typically round 0.one s. The response time of the RTD is a limitation of usage of this sensor compared to thermistors and thermocouples. Nonetheless, the fact that it is a robust element and requires relatively like shooting fish in a barrel measurement circuitry causes the RTD sensor to be of not bad importance, both in industry and medical applications.

RTDs are manufactured in the range of values from a few units Ω to tens of hundreds Ω. Higher values RTDs accept the advantage of presenting smaller errors in relation to the resistance of the lead wires. The nigh used resistive temperature transducers are PT100 and and so the PT1000, which is similar to the commencement, with the difference that its resistance in temperature 0°C is 1000 Ω instead of 100 Ω.

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Alarm direction systems

B.R. Mehta , Y.J. Reddy , in Industrial Process Automation Systems, 2015

21.2.1 Conventional alarm system

Sensors such every bit resistance temperature detectors (RTDs), bellows, pressure detectors, etc., are used to measure out various plant parameters such as temperature and pressure. The output signal of the sensors is processed electronically and sent to various circuits that serve equally controls, displays, and alarms. Figure 21.1 shows the inputs to a parameter display and to an warning bistable. Each warning circuit for a parameter has a setpoint and actuates the alarm display. The control room operators then make judgments about the plant state and the deportment to take based upon the parameter displays and procedures. The operators would also review other information sources (e.grand., admission other displays, contact institute personnel) and brand adjustments to the institute systems and components through plant controls. These adjustments would bear on plant processes and the results would be detected by the sensors and transmitted back to the alarm displays of HMI.

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Instrumentation Condom Implementation and Explosion Protection

Swapan Basu , in Plant Hazard Analysis and Prophylactic Instrumentation Systems, 2017

3.7.ii Thermocouple/Resistance Temperature Detector (RTD) Input

Discussions begin with thermocouple/RTD inputs. Since both belong to the unproblematic apparatus category they do not need certification by a notified torso. Polarity, rated nominal voltage, and internal resistance are major blueprint parameters to be considered. Both have similar requirements for these parameters.

•

Thermocouple input: A thermocouple is a simple device, so information technology is incapable of creating or storing plenty energy to ignite any mixture of volatile gases. When the free energy level of a typical thermocouple circuit is seen confronting the ignition curve (similar one shown in Fig. X/three.7.one-2) of the gas group it is seen that these are very much on the left and lower office of group A.

Now, if a fault occurs on the secondary device, say in the logic solver, so it could crusade excess free energy to reach the hazardous area, as seen in Fig. X/3.7.2-ane. To make sure that the circuit remains intrinsically prophylactic, that is, the fault does not achieve a hazardous area, a barrier limiting the energy is required equally shown in the bottom role of Fig. X/3.7.two-ane.

Design issues: A few pattern issues are discussed:

•: Polarity: A thermocouple has two wires with positive and negative polarity. 2 single-channel barriers, each with the proper polarity, could be used but the trouble comes when polarities are reversed by fault. To avoid polarity problems on the terminals, a double Ac barrier could exist used.
•: Rated voltage (V _n): A thermocouple produces a very small voltage. Since the thermocouple produces such a minor voltage, it makes sense to choose a double AC barrier with a higher rated nominal voltage (V _{due north} > one V) [34].
•: Internal resistance (R _i): Because the mV indicate has a very modest current and is going to a high-impedance voltmeter, the resistance of the bulwark will not bear upon functioning of circuit. However, it is wise to select a barrier with a low resistance (<110 Ω) [34].

•

Resistance temperature detector (RTD) input: Like the thermocouple, in the case of an RTD, IS barriers prevent excess energy from possible faults on the safe side, say from the logic solver, from reaching the hazardous expanse. A typical RTD input IS circuit is depicted in Fig. X/3.7.2-2.

In most cases, three-wire RTDs are used in the manufacture, so these will be considered here. Mainly, bridge circuits using a modified Wheatstone bridge are used. In these circuits, measured output voltage is a function of the RTD resistance. Requirements for RTDs are similar to thermocouples. Use of a double-aqueduct AC barrier with parameters discussed afterward volition be a cost-constructive solution.

•: Polarity: The current loop of the RTD has a positive and negative polarity. Hence i each of a standard DC barrier, one standard DC barrier (+ve and −ve), i double AC barrier are to be chosen. The last one is a better choice to avoid polarity issues.
•: Rated voltage (5 _n): The abiding current sent to the RTD is very small at the 10⁻⁶ level. RTDs are recommended for use in the range 600–700°C. Fifty-fifty if a very high temperature is considered, the maximum resistance of the RTD Pt100 is 390 Ω at 1560°C [34]. And so, the voltage drib across the RTD will be very low in the order of mV. Naturally, the V _n of the RTD loop is similar to the thermocouple, that is, V _n > 1 V.
•: Internal resistance (R _i): Any constant current source volition take a rated maximum load that it can drive. Considering a minimum standard load of 500 Ω and RTD value at loftier temperature every bit 390 Ω, the R _i to be chosen is less than 110 (500–390) Ω.

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