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A simple rugged device for monitoring temperature. The device is made by fastening two metal strips that have different coefficients of linear thermal expansion
Bimetallic strip thermometer
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A precision resistance-measuring device that is calibrated to give the proper temperature reading.
Bridge circuit
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The change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature
Coefficient of resistance
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A sensor (typically thermocouple) used to measure the surface temperature of components through physical contact with the component
Contact Pyrometer
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An indirect temperature measuring device constructed of pure metals or certain alloys that increase linearly in resistance as temperature increases and, conversely, decrease in resistance as temperature decreases
Resistance temperature detector (RTD)
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A measure of the average kinetic energy of the molecules of a substance
Temperature
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A temperature measuring device consisting of a pair of dissimilar metal wires joined at one end. A thermocouple is used to measure temperature by means of electrical currents developed when there is a temperature difference between the ends
Thermocouple
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A sensor (typically infrared) used to measure the surface temperature of components without physical contact
Optical Pyrometer
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An electrical device used for measuring electrical resistance. It consists of a conductor joining two branches of a circuit
Wheatstone bridge
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STATE the three basic functions of temperature detectors
Temperature detectors are used to provide three basic functions: indication, alarm, and control
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DESCRIBE the two alternate methods of determining temperature when the normal temperature sensing devices are inoperable.
The dual-element RTD has two sensing elements, of which only one is normally connected. If the element in use becomes faulty, the second element may be connected to provide temperature indication.
If an installed spare is not utilized, a contact pyrometer (portable thermocouple) may be used to obtain temperature readings on equipment or systems that are accessible.
If the malfunction is in the circuitry and the detector is still functional, it may be possible to obtain temperatures by connecting an external bridge circuit to the detector. Resistance readings may then be taken and a corresponding temperature obtained from the detector calibration curves.
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STATE the two environmental concerns that can affect the accuracy and reliability of temperature detection instrumentation
- Ambient temperature
- Humidity
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EXPLAIN the basic theory of operation of a bimetallic strip thermometer
- A bimetallic strip thermometer is a simple, rugged device for monitoring temperature. The element is made by
- fastening together two strips of metals with different coefficients of thermal expansion. The general range of
- bimetallic elements is from –200°F to 1,000°F
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DESCRIBE the basic construction of a thermocouple
- A thermocouple is constructed by joining two dissimilar metal wires at one end. The other end of each wire is
- connected to a measuring instrument
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EXPLAIN how a thermocouple provides an output representative of the measured temperature
- Thermocouples cause an electric current to flow in the attached circuit when subjected to changes in temperature. The amount of current produced depends on:
- The characteristics of the two metals used
- The temperature difference between the measurement and reference junction
- The characteristics of the attached circuit
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DESCRIBE the construction of a basic RTD
- The RTD uses pure metals or certain alloys.
- RTD elements are normally constructed of platinum, copper, or nickel
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EXPLAIN how RTD resistance varies for the following
As temperature increases the electrical resistance of the metal increases. Conversely, as temperature decreases, the electrical resistance of the metal decreases
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EXPLAIN how an RTD provides an output representative of the measured temperature
When placed in a liquid or gas medium, the Inconel sheath quickly reaches the temperature of the medium. Any change in the temperature of the medium will cause the platinum wire to change similarly. The temperature change results in a proportional change in resistance.
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Given a simplified schematic diagram of a basic bridge circuit, STATE the purpose of the following component
R1 and R2
Rx
Adjustable resistor
Sensitive ammeter
Resistors R1 and R2 are the ratio arms of the bridge. The resistors ratio the two variable resistances for current flow through the ammeter. R3 is a variable resistor known as the standard arm that is adjusted to match the unknown resistor. The sensing ammeter visually displays the current that is flowing through the bridge circuit. The value of Rx can be calculated for the bridge when the ammeter reads zero current condition
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DESCRIBE the bridge circuit conditions that create a balanced bridge
The balanced bridge circuit (Figure 2‑12) uses a galvanometer to compare the RTD resistance with that of a fixed resistor. The galvanometer uses a pointer that deflects on either side of zero when the resistance of the arms is not equal
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Identify the three basic components of a thermocoupler
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Identify the components of an RTD
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Given a block diagram of a basic temperature instrument detection and control system, STATE the purpose of the following blocks
RTD
Bridge circuit
DC-AC converter
Amplifier
Balancing motor/mechanical linkage
The detector is felt as resistance to the bridge network. The bridge network converts this resistance to a DC voltage signal.
An electronic instrument has been developed in which the DC voltage of the potentiometer, or the bridge, is converted to an AC voltage. The AC voltage is then amplified to a higher (usable) voltage that is used to drive a bi-directional balancing motor.
The bi-directional motor positions the slider on the slidewire to balance the circuit resistance
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DESCRIBE the temperature instrument indication(s) for the following circuit faults
Short circuit
Open circuit
If the RTD becomes open in either the unbalanced and balanced bridge circuits, the resistance will be infinite and the meter will indicate a very high temperature
If the RTD becomes shorted, resistance will be zero and the meter will indicate a very low temperature
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EXPLAIN the three methods of bridge circuit compensation for changes in ambient temperature
- The resistors that are used in the measuring circuitry are selected so that their resistance will remain constant over the range of temperature expected.
- Temperature compensation is also accomplished through the design of the electronic circuitry to compensate for ambient changes in the equipment cabinet.
- To compensate for this change, three and four wire RTD circuits are used. In this way, the same amount of lead wire is used in both branches of the bridge circuit, and the change in resistance will be felt on both branches, negating the effects of the change in temperature.
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- Temperature controlled to maintain a constant temperature for the reference junction
- Reference junction allows use of standard copper wire instead of T/C wire.
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