The Temperature Sensors, there are several occasions when the ability to detect temperature is critical. The industrial electrician will come across devices that adjust a set of contacts in response to temperature changes, as well as devices that sense the quantity of temperature. The approach utilized is heavily influenced by the circuit’s uses and the quantity of temperature that must be measured.
Temperature Sensors
Expansion of Metal in Temperature Sensors
The expansion of metal is a highly popular and reliable way for measuring temperature. Metal expands when heated, as has long been known. Two things influence the quantity of expansion:
1. The metal kind utilized.
2. The degree of warmth.
Take a look at the metal bar in Figure 1-1.
When the bar is heated, it grows in length. When the metal is allowed to cool, it contracts. Despite the fact that the amount of movement caused by contractions and expansions is tiny, a simple mechanical concept may be employed to enhance the amount of movement (Figure 1-2).
Figure 1–1 Metal expands when heated.
One end of the metal bar is mechanically held. This allows the quantity of growth to be limited to one direction alone. When heated, the metal expands and presses against the mechanical arm. A tiny movement of the bar creates a large movement of the mechanical arm. This increased movement in the arm may be used to indicate the temperature of the bar by connecting a pointer and scale, or it can be utilized to control the switch indicated. It is important to understand that drawings are utilized to express a concept. In practice, the spring-loaded switch indicated in Figure 1-2 would provide a “snap” action for the contacts.
Figure 1–2 Expanding metal operates a set of contacts.
Electrical connections should never be allowed to open or shut slowly. This results in low contact pressure, which causes the contacts to burn or causes inconsistent performance of the device they are supposed to regulate.
Hot-wire Starting Relay
In the refrigeration business, a popular device that exploits the idea of expanding metal to activate a series of contacts is the hotwire starting relay.
The hot-wire relay gets its name from the fact that it detects motor current by connecting a length of resistive wire in series with the motor. Figure 1-3 depicts a schematic of this sort of relay.
Figure 1–3 Hot-wire relay connection.
When the thermostat contact closes, current can flow from line L1 to the relay’s terminal L. The current then travels to the run and start windings through the resistive wire, the moveable arm, and the usually closed contacts. The temperature of the resistive wire rises as current runs across it. As the temperature rises, the wire expands in length. The moveable arm is driven downward as the length rises.
This downward pressure puts strain on both contacts’ springs. The relay is built in such a way that the start contact opens first, separating the motor start winding from the circuit. If the motor current is not too high, the wire will never grow hot enough to open the overload contact.
However, if the motor current becomes too high, the temperature of the resistive wire rises to the point where it expands, causing the overload contact to break open and detach the motor run winding from the circuit.
The Mercury Thermometer
The mercury thermometer is another handy gadget that operates on the idea of metal contraction and expansion. Mercury is a metal that, at ambient temperature, remains liquid. If the mercury is contained in a glass tube, as illustrated in Figure 1-4, it will climb up the tube as the temperature rises. If the tube is appropriately calibrated, it offers an accurate temperature measurement.
Figure 1–4 A mercury thermometer operates by the expansion
of metal.
The Bimetal Stripe
Another device that works through metal expansion is the bimetal strip. It is most likely the most often utilized heat detecting device in the manufacture of room thermostats and thermometers. The bimetal strip is created by joining two different metals (Figure 1-5).
Figure 1–5 A bimetal strip.
Because these two metals are not identical, their growth rates differ. When heated, this causes the strip to bend or warp (Figure 1-6).
Figure 1–6 A bimetal strip warps with a change of temperature.
As illustrated in Figure 1-7, a bimetal strip is frequently fashioned into a spiral shape. The spiral allows for the employment of a longer bimetal strip in a short space. A lengthy bimetal strip is preferable since it moves more when the temperature changes.
Figure 1–7 A bimetal strip used as a thermometer.
A change in temperature will cause the pointer to revolve if one end of the strip is mechanically held and a pointer is attached to the center of the spiral. When a calibrated scale is put beneath the pointer, it transforms into a thermometer. When the Centre of the spiral is kept in place and a contact is connected to the end of the bimetal strip, it transforms into a thermostat. To generate a snap action for the connections, a tiny permanent magnet is employed (Figure 1-8)
Figure 1–8 A bimetal strip used to operate a set of contacts.
. When the moving contact approaches the stationary contact, the magnet draws the metal strip, causing the contacts to shut abruptly. The bimetal strip pulls away from the magnet as it cools.
When the force of the bimetal strip grows sufficiently enough, it overcomes the force of the magnet, and the contacts pop open.
Thermocouples
A German scientist called Seebeck discovered in 1822 that when two different metals are linked at one end and heated, a voltage is formed (Figure 1-9).
Figure 1–9 Thermocouple.
This is referred to as the Seebeck effect. A thermocouple is a device formed by the connecting of two different metals for the purpose of creating electricity with heat. The amount of voltage produced by a thermocouple is governed by the following factors: 1. The materials utilized to make the thermocouple.
2. The difference in temperature between the two junctions.
Figure 1–10 Thermocouple chart.
Figure 1-10 depicts the most prevalent types of thermocouples. The various metals used in thermocouple fabrication are depicted, as well as their usual temperature ranges. A thermocouple produces a little amount of electricity, often in the millivolt range (1 millivolt = 0.001 volt). The temperature determines the polarity of the voltage of some thermocouples.
A type “J” thermocouple, for example, generates zero volts at around 32 degrees Fahrenheit. When temperatures rise over 32 degrees Fahrenheit, the iron wire becomes positive and the constantan wire becomes negative. When temperatures fall below 32 degrees Fahrenheit, the iron wire turns negative and the constantan wire becomes positive. A type “J” thermocouple produces a voltage of roughly 7.9 millivolts at 300 degrees Fahrenheit.
At 300 degrees Fahrenheit, it produces a voltage of roughly 7.9 millivolts.
Figure 1–11 Thermopile.
Because thermocouples produce such low voltages, they are frequently coupled in series, as seen in Figure 1-11. This is known as a thermopile connection.
Thermocouples and thermopiles are commonly used to monitor temperature and are occasionally used to detect the existence of a pilot light in natural gas appliances. The pilot light heats the thermocouple. The thermocouple’s current is utilised to generate a magnetic field that keeps a gas valve open and allows gas to flow to the main burner. If the pilot light fails, the thermocouple stops producing current and the valve closes (Figure 1-12).
Figure 1–12 A thermocouple provides power to the safety cut-off valve.
Resistance Temperature Detectors
Platinum wire is used to make the resistance temperature detector (RTD). Platinum’s resistance varies dramatically with temperature. When platinum is heated, its resistance rises at a predictable pace, making the RTD a perfect instrument for monitoring temperature precisely. RTDs are used to measure temperatures ranging from 328 to 1166 degrees Fahrenheit (200 to 630 degrees Celsius). RTDs are created in many styles to serve various duties. Figure 1-13 depicts a standard RTD being used as a probe. A tiny coil of platinum wire is wrapped within a copper tip.
Figure 1–13 Resistance temperature detector.
To achieve good thermal contact, copper is employed. This enables the probe to be extremely fast-acting. Figure 1-14 depicts resistance vs temperature for a standard RTD probe. The temperature is expressed in degrees Celsius, while the resistance is expressed in ohms.
Figure 1–14 Temperature and resistance for a typical RTD.
Figure 1-15 depicts RTDs in two distinct casing types.
Figure 1–15 RTDs in different case styles.
Thermistors
The name thermistor comes from the phrase “thermal resistor.” Thermistors are semiconductor devices that are thermally sensitive. Thermistors are classified into two types: those with a negative temperature coefficient (NTC) and those with a positive temperature coefficient (PTC). A thermistor with a negative temperature coefficient will lose resistance as the temperature rises. When the temperature rises, a thermistor with a positive temperature coefficient increases its resistance. The most common kind is the NTC thermistor.
Thermistors are extremely nonlinear electronic devices. As a result, they are difficult to utilise for temperature measurement.
Devices that employ a thermistor to monitor temperature must be calibrated for the specific type of thermistor used. If the thermistor is ever changed, it must be an identical replacement otherwise the circuit will not function properly. Because of their nonlinear features, thermistors are frequently utilised as set point detectors rather than temperature sensors.
When the temperature reaches a specified level, a set point detector triggers some process or circuit.
Assume a thermistor is installed within the stator winding of a motor. If the motor becomes overheated, the windings may be badly damaged or destroyed. The thermistor can measure the temperature of the windings.When the temperature reaches a specific degree, the thermistor’s resistance value changes sufficiently to force the starting coil to drop out and disconnect the motor from the line.
Thermistors can be used in temperatures ranging from 100 to 300 degrees Fahrenheit.
Figure 1–16 Solid-state starting relay.
Thermistors are commonly utilized in solid-state starting relays used with tiny refrigeration compressors (Figure 1-16). With hermetically sealed motors, starting relays are used to separate the start windings from the circuit when the motor achieves around 75% of its full speed. Thermistors can be utilized for this purpose since they alter resistance incredibly quickly with temperature. Figure 1-17 depicts a schematic design of a solid-state relay connection.
When electricity is given to the circuit for the first time, the thermistor is cold and has a low resistance.
Figure 1–17 Connection of solid-state starting relay.
This allows electricity to pass through the motor’s start and run windings. Because of the current passing through it, the temperature of the thermistor rises. The resistance changes from a very low value of 3 or 4 ohms to several thousand ohms as the temperature rises. This abrupt rise in resistance has the consequence of opening a series of contacts linked to the start winding. Although the start winding is never totally detached from the power line, the current flowing through it is very little, generally 0.03 to 0.05 amps, and has no effect on the motor’s performance. This tiny amount of leakage current keeps the thermistor temperature stable and prevents it from reverting to a low resistance state. After turning off the electricity to the motor, it should be allowed to cool down for about 2 minutes before resuming it. This cooling interval is required for the thermistor to revert to a low resistance value.
The PN Junction
The PN junction, often known as a diode, is another device that can monitor temperature. Because it is precise and linear, the diode is quickly becoming a popular technology for detecting temperature.
A continuous current is sent through a silicon diode when it is employed as a temperature sensor.
This sort of circuit is seen in Figure 1-18.
Figure 1–18 Constant current generator.
Resistor R1 in this circuit restricts the current flow via the transistor and sensor diode. The amount of current that travels through the diode is similarly determined by R1. Diode D1 is a 5.1 volt zener used to provide a consistent voltage drop between the PNP transistor’s base and emitter. The amount of current that flows through the zener diode and the transistor base is limited by resistor R2. D1 is a typical silicon diode. It serves as the temperature sensor in the circuit. A voltage drop between 0.8 and 0 volts may be noticed when a digital voltmeter is put across the diode. The temperature of the diode determines the amount of voltage loss.
Figure 1–19 Field effect transistor used to produce a constant
current generator.
Figure 1-19 depicts another circuit that may be utilized as a constant current generator. A field effect transistor (FET) is utilized to create a current generator in this circuit. The amount of current that flows through the diode is determined by resistor R1. The temperature sensor is diode D1.
When the diode is exposed to a lower temperature, such as by contacting it with ice, the voltage loss across the diode increases. Because the diode has a negative temperature coefficient, increasing the diode temperature reduces the voltage loss. The voltage drop decreases as the temperature rises.
Figure 1–20 Solid-state thermostat using diodes as heat sensors.
In Figure 1-20, an electronic thermostat is built using two diodes linked in series. To enhance the amount of voltage loss when the temperature varies, two diodes are employed. To give a steady current to the two diodes used as heat sensors, a field effect transistor and resistor are utilized. When the temperature changes, an operational amplifier turns a solid-state relay on or off. The circuit will function as a heating thermostat in the case depicted. When the temperature drops sufficiently, the amplifier’s output will switch on. By reversing the connections of the amplifier’s inverting and noninverting inputs, the circuit may be changed to a cooling thermostat.
Expansion Due to Pressure
The rise in pressure of certain compounds is another typical means of sensing a change in temperature.
When the temperature of a refrigerant in a sealed container rises, the pressure in the container rises as well. When a basic bellows is linked to a refrigerant line (Figure 1-21), the bellows will expand as the pressure inside the sealed system rises.
Figure 1–21 Bellows contracts and expands with a change of
refrigerant pressure.
When the temperature of the surrounding air drops, the pressure inside the system drops and the bellows contracts. When the temperature of the air rises, so does the pressure, and the bellows expands. A bellows type thermostat is formed when the bellows regulates a series of contacts. Figure 1-22 depicts a bellows thermostat and the conventional NEMA symbols used to denote a temperature-controlled switch.
Figure 1–22 Industrial temperature switch.
Smart Temperature Transmitters
To show the temperature, standard temperature transmitters typically send a 4 to 20 milliampere signal.
They are calibrated for a certain temperature range, such as 0 to 100 degrees. Standard transmitters are designed to work with a single type of sensor, such as an RTD or a thermocouple. Any modifications to the settings necessitate a unit recalibration.
Figure 1–23 Cut-away view of a smart temperature transmitter.
Smart transmitters have an integrated microprocessor that may be calibrated remotely by sending a signal to the transmitter. It is also feasible to troubleshoot the transmitter from a remote place.
Figure 1-23 depicts a cutaway view of a smart temperature transmitter. The HART (Highway Addressable Remote Transducer) protocol is used by the transmitter seen in Figure 1-23. RTD, differential RTD, thermocouple, ohm, and millivolt inputs are all supported by this transmitter. Figure 1-24 depicts a smart temperature transmitter with a meter.
Figure 1–24 Smart temperature transmitter with meter.
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