Bi-metal Temperature Sensors (Thermometers)

What is the thermal expansion of a metal?

Solids tend to expand when heated and depend on the material it is made of, and the amount of temperature rise.

The following formula relates linear expansion to temperature change:

As the values of the linear coefficient are quite small, the amount of expansion (or contraction) for temperature changes is almost too small to see unless the sample size is huge.

One way to amplify the motion resulting from thermal expansion is to bond two strips of dissimilar metals together, such as copper and iron. If we were to take two equally-sized strips of copper and iron, lay them side-by-side, and then heat both of them to a higher temperature, we would see the copper strip lengthen slightly more than the iron strip:

If we bond these two strips of metal together, this differential growth will result in a bending motion greatly exceeding the linear expansion. This device is called a bi-metal strip:

How does a bimetallic thermometer work?

A bimetallic thermometer is a temperature measurement device. It converts the media’s temperature into mechanical displacement using a bimetallic strip. The bending motion of the bimetallic strip is significant enough to drive a pointer mechanism, activate an electromechanical switch, or perform any number of other mechanical tasks, making this a quite simple and useful primary sensing element for temperature. Older home thermostats often used this principle to indicate room temperature and actuate electrical switches for controlling room temperature. Electric hot water heater units still use this type of device (usually in the form of a convex bi-metal disk) to sense over-temperature conditions and automatically shut off power to the heater if the water temperature exceeds a pre-set limit.

If a bi-metallic strip is twisted over a long length, it will tend to un-twist as it heats up. This twisting motion may be used to directly drive the needle of a temperature gauge.

Types of bimetallic thermometer:

Helix strip bimetallic thermometer

In this helix-shaped bimetallic strip is used to measure the temperature. The pointer is connected through the shaft at the free end of the strip. As the temperature increases, the helical strip senses the temperature change. The stripped metal with a higher coefficient of thermal expansion expands and winds up along the stem, rotating the shaft. This rotation causes the pointer to move its position in the dial which indicates the media’s temperature. As the temperature decreases, the metal with a lower coefficient of thermal expansion shrinks and rotates the shaft. The pointer then reads the lower temperature in the dial.

Spiral strip bimetallic thermometer

In this spiral-shaped strip is used to measure the temperature. As the temperature rises, the two metal strips expand differently. This creates a bending effect and the strip coils in such a way that the metal with a higher thermal coefficient forms the outer side of the arc. As the temperature decreases, the metal with a lower thermal coefficient forms the inner layer of the arc. The pointer and dial attached to the spiral read this deformation which indicates the media’s temperature.

Advantages:

• Simple and robust design
• Less expensive than other thermometers
• Fully mechanical and does not require any power source to operate.
• Easy installation and maintenance
• Nearly linear response to temperature change
• Suitable for wide temperature ranges

Disadvantages:

• Not used for very high temperatures
• It may require frequent calibration
• May not give an accurate reading for low temperature

List of Prominent Manufacturers: Afrisco, Ametek, Ashcroft, Badotherm, Budenberg, Delta-Mobrey, Dwyer, Elcometer, GeOrigin, Labfacility, New-Flow, Noshok, Omega, PCI Instruments, Prisma, Riels, Rototherm, Sensotec, Sika, Tameson, Trafag, WIKA, Winters

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Flow Switch

What is a flow switch and how it works?

A flow switch is one detecting the flow of some fluid through a pipe. Flow switches often use “paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts.

A flow switch will be in its “normal” status when it senses minimum flow (i.e. no fluid moving through the pipe). For a flow switch, “normal” status is any fluid flow rate below the trip threshold of the switch.

A simple paddle placed in the midst of a fluid stream generates a mechanical force that may be used to actuate a switch mechanism, as shown in the following photograph:

Like all other process switches, flow switches exhibit deadband (also called differential) in their switching action. A flow switch that trips at 15 GPM rising, for example, will not re-set at 15 GPM falling. That switch would more likely reset at some lower flow rate such as 14 GPM. With the “trip” and “reset” points being different, the switch avoids unnecessary cycling if the process flow rate hovers near one critical value.

List of Prominent Manufacturers: ABB, Anderson-Negele, Barksdale, Burkert, Comeco, Connetech, Danfoss, DropsA, Dwyer, Elettrotec, Envea, FineTek, Flowline, GEMS, GTE, Krohne, Lefoo, Magnetrol, Mecon, Meister, New-Flow, Omega, Riels, Sensirion, Sierra, Sika, Sitron, Soway, Tecfluid, Val.co, WIKA, Watts

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Temperature Switch

What is a temperature switch and how it works?

The switch detects the temperature of an object. It uses bimetallic strips as the temperature-sensing element. When the switch senses minimum temperature it will be referred to as its “normal” status that any sensed temperature below the trip threshold of the switch.

The temperature switches exhibit a certain amount of trip point or deadband in their switching action like all other process switches and reset at a lower temperature below the trip point. With mechanical switch designs, some amount of Deadband is inevitable due to friction inside the mechanism. To understand the mechanism, consider the process variable is at or very near the trip point. Any further rise in temperature will trip the switch and sound the alarm. With trip point or deadband, the switch will immediately re-set when the temperature falls back down. The switch will “cycle” back and forth between its trip and reset states with just a minute change in process temperature. If the switch is activating an alarm every time it trips, it will create a series of alarm events to repeatedly acknowledge the operator and sound the alarm. It is mandatory for the switch to trip at rising and remain in that tripped state until the temperature falls below the trip point. In this manner, it sounds alarmed only once rather than multiple alarm events for each process temperature excursion.

The mechanical switches come equipped with a separate adjustment for trip points or deadband. Setting this dead band adjustment in a mechanical temperature switch requires the technician to repeatedly subject the sensing element to a rising and falling temperature, to check and verify that the switch trips and resets at the proper set point. When adjusting the “zero” and “span” settings of an analog transmitter we have to repeat the process variable back and forth, checking to see that the transmitter repeatedly outputs a 0% signal at the lower range value (LRV) and a 100% signal at the upper range value (URV).

For discrete temperature-sensing applications demanding high accuracy and repeatability, electronic temperature switch circuits using thermocouples, RTDs or thermistors may be used instead of a mechanical (bi-metallic or filled bulb) sensing element.

Moore Industries model SPA (“Site Programmable Alarm”) is an example of an electronic temperature switch module shown below. This temperature switch is capable of directly interpreting both RTD and thermocouple signals and input 4-20 mA loop current signals as well.

With electronic temperature switches, the adjustment of trip points or Deadband is both precise and flexible. Electronic switching circuits may be precisely set for any trip and reset points along with its measurement range, remaining very stable over time, unlike mechanical switches where a trip point or Deadband is primarily a function of friction, and therefore liable to change over time.

List of Prominent Manufacturers: Ashcroft, Barksdale, Danfoss, Delta-Mobrey, Dwyer, Elettrotec , Engler, Fisher, Georigin, GHM Group, Hydac, Jumo, Labfacility, Neo-Dyn, New-Flow, Omega, PCI Instruments, Prisma, United Electric Controls, WIKA 

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Level Switch

What is a level switch and how it work?

The switch detects the level of liquid or solid (powder or granules) in a vessel. One or more switch contacts are actuated by the motion of the level sensing element using float. When the switch senses a minimum level (an empty vessel), it will be in its “normal” status. Any fluid level below the trip threshold of the switch is referred to as the “normal” status of the level switch.

The electrical schematic symbol is shown below for a level switch based on the mechanism with a round ball float drawn as the actuating element.

1. Float-type level switches

The level switches use float to sense the liquid surface level, actuating an electrical switch by the motion of the float. This mechanism uses a mercury tilt bulb, tilted by a magnet’s attraction to a steel rod lifted into position by a float. By positioning the steel rod either closer to or farther away from the magnet, the float directly senses liquid level. The mercury bottle tilts to change the switch’s electrical status if the rod comes close enough to the magnet.

A feature of this design is complete isolation between the electrical components and the “wet” components sensing the liquid level. The steel rod moves inside a non-magnetic metal tube, with the tube sealing process fluid pressure from escape while still allowing the magnetic tilt switch to sense float position.

For direct installation in open (vented) process vessels, resembling the float valve assembly on a water-flush toilet is an example of simpler float switch designs. To sense the float’s position in an environment where no isolation needs to exist between the switch and the process fluid(s), the “limit” style switching element will use here, including inductive proximity switches.

2. Tuning fork level switches

In this type of switch, a metal tuning fork structure is used to detect the presence of a liquid or solid (powder or granules) in a vessel as shown in the below photograph.

An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. The resonant frequency of the structure dramatically decreases when the prongs of the fork contact anything with substantial mass. The circuit detects this change and indicates the presence of material contacting the fork.

3. Paddle-wheel level switches

A more primitive variation on the theme of a “tuning fork” level switch is the rotating paddle switch, used to detect the level of powder or granular solid material. This level switch uses an electric motor to slowly rotate a metal paddle inside the process vessel. If solid material rises to the level of the paddle, the material’s bulk will place a mechanical load on the paddle. A torque-sensitive switch mechanically linked to the motor actuates when enough torsional effort is detected on the part of the motor.

A “Bindicator” style (so-called because they detected the level of solid material in storage bins) of the level switch used to detect the presence of soda ash powder in a hopper at a water treatment plant.

4. Ultrasonic level switches

Some of the electronic levels, switches use ultrasonic sound waves to detect the presence of process material (either solid or liquid) at one point:

Sound waves pass back and forth within the gap of the probe, sent, and received by piezoelectric transducers. The presence of any substance other than gas within that gap affects the received audio power, thus signaling to the electronic circuit within the bulkier portion of the device that the process level has reached the detection point. Due to the lack of moving parts the probe is quite reliable.

5. Capacitive level switches

Another electronic liquid level switch technology is a capacitive type, sensing level by changes in electrical capacitance between the switch and the liquid. Below the photograph shows a couple of capacitive switches sensing the presence of water in a plastic storage vessel.

6. Conductive level switches

This type of switch only works with granular solids and liquids that are electrically conductive (dirty water, acids, caustics, food liquids, coal, metal powders) and not with nonconducting materials (ultra-pure water, oils, ceramic powders).

A special design for conductive level switches is shown below, using a special transformer/relay to generate an isolated AC probe voltage and sense the presence of liquid.

120 VAC line voltage energizes the primary coil, sending a magnetic field through the laminated iron core of the relay. This magnetic field easily passes through the center of the secondary coil when the secondary circuit is open (no liquid closing the probe circuit), thus completing the magnetic “circuit” in the core. The armature will not be attracted to the core even if the magnetic circuit is completed. However, when a circuit is completed by liquid level rising to contact both probes, the secondary coil’s resulting current “bucks” the magnetic flux through its center, causing the more magnetic flux to bypass to the end poles where it attracts the iron armature toward the core frame. This physical attraction actuates switch contacts which then signal the presence of liquid level at the probes.

The following pair of illustrations shows the two conditions of this level switch, with the magnetic lines of flux, highlighted as dashed lines through the core.

The conductive level switch has the “transformer” design that provides electrical isolation between the probes and the energizing (120 VAC) circuit. It also enables a wide range of detection voltages to be generated for the probes just by altering the number of wires “turns” in the secondary coil.

More modern variations on the same design theme use much lower AC voltages to energize the probes, employing sensitive semiconductor amplifier circuits to detect probe current and signal liquid level.

List of Prominent Manufacturers: Aalborg, ABB, Aeco, Afriso, Anderson Negele, Aplisens, Barksdale, Baumer, Berthold, Bieri, Burkert, Comus, Comeco, Condor, Connetech, Danfoss, Divatec, Drexelbrook, DropsA, Dwyer, Elobau, Elettrotec, Emas, Emco Controls, Elmess, Endress+Hauser, Engler, FineTek, Fisher, Flowtech, Flowline, GEMS, Hydac, Indumart, Introtek, Jacob, Jiwei, Krohne, Madison, Magnetrol, Minco, Monitor, New-Flow, Nivelco, Omega, Omnitrol, Pepperl+Fuchs, Prisma, Pulsar, Riels, Seli, Sensotec, Siemens, Sierra, Sonotec, Soway, Tecfluid, Telemecanique, Trafag, VEGA, Walchem, Weka, WIKA, Woerner

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Pressure Switch

What is a pressure switch and how it works?

It is the switch that detects the presence of fluid pressure. It uses diaphragms or bellows as the pressure-sensing element and its motion actuates one or more switch contacts.

When the pressure switch senses minimum pressure, it will be referred to as in its “normal” status. Any fluid pressure below the trip threshold of the switch is referred to as the “normal” status of the pressure switch.

Two pressure switches as shown in the below photograph sensing the same fluid pressure:

A special design of pressure switch uses a bourdon tube as the pressure-sensing element, and a glass bulb partially filled with mercury as the electrical switching element. When the pressure is applied to the bourdon tube to flex sufficiently, the glass bulb tilts to cause the mercury to fall against a pair of electrodes, thus completing an electrical circuit.

Below the photograph is a close-up view of one of these pressure switches. The bourdon tube is grey in color inside the case, and almost as wide in diameter as the circular switch housing. Yellow-colored plastic caps covering up their external electrical contacts are mercury tilt switch bottles.

The next set of photographs shows a mercury tilt switch removed from the switch mechanism, the switch in two different states (contact open on the left and closed on the right).

The mercury tilt switches include immunity to switch contact degradation from harmful atmospheres (oil mist, dirt, dust, corrosion) as well as safety in explosive atmospheres (since a spark contained within a hermetically sealed glass bulb cannot touch off an explosion in the surrounding atmosphere) is its advantage. Including the possibility of intermittent electrical contact resulting from mechanical vibration, as well as sensitivity to mounting angle (i.e., you would not want to use this kind of switch aboard a moving vehicle!) is its disadvantage.

The below photograph shows a pressure switch manufactured by the Danfoss corporation:

The force generated by a pressure-sensing element against mechanical spring balances by the pressure switch. Tension on the spring and the trip point of this switch is adjustable.

There is one setting known as the dead band or differential pressure setting shown in the lower window on the switch. After the switch tripped, we have to reset the setting which determines the amount of pressure change required to bring it normal state.

List of Prominent Manufacturers: Afrisco, Aircom, Aircomp, Airlogic, Airpress, Airtec, Airwork, Ashcroft, Aventics, Barksdale, BCM Sensor, BD Sensor, Bieri Hydraulics, Cameron, Comitronic, Comus, Danfoss, Delta-Mobrey, Doepke, DropsA, Dwyer, Eaton, Elettrotec, Engler, Festo, FineTek, Fratelli Ghiotto, Gems, Georigin, GHM Group, Herga, HK Instruments, Honeywell, Huba Control, Hydac, Hydropa, Indumart, Inelta, Jiucheny, Keller, Layher, Leeg Sensor, Lefoo, Leybold, Mico, Neo-Dyn, New-Flow, Nivelco, Norgren, Noshok, Pascal, PCI Instruments, Piezus, Prisma Instruments, Rexroth, Riels, Roemheld, Rototherm, Sauermann, Sauter, Schischeck, Schmalz, Selco, Sensata, Setra, SMC, United Electric Controls, Telemechanique, Trafag, Val.co, Wasinsx, WIKA, Yuanben

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Proximity Switch

What is a proximity switch and how it works?

It is the switch that detects the closeness/ proximity of an object. The switches are non-contact type sensors that use magnetic, electric, or optical means to sense the proximity of objects.

The below schematic diagram symbol for a proximity switch with mechanical contacts is the same as for a mechanical limit switch, except the switch symbol is enclosed by a diamond shape, indicating a powered or active device.

The proximity switch will be in its “normal” condition/status when it is distant from any actuating object that we have to detect. Due to the non-contact type in nature, it is often used instead of direct-contact limit switches for the same purpose of detecting the position of a machine part, with the advantage of never wearing out over a long time due to repeated physical contact. However, the greater complexity and cost of a proximity switch over a mechanical limit switch shows their use in applications where lack of physical contact yields tangible benefits.

What are different types of proximity switches?

Most of the proximity, switches are active in design. They have a powered electronic circuit inside them to detect the proximity of an object.

• Inductive proximity switches detect the presence of metallic objects with the help of a high-frequency magnetic field.
• Capacitive proximity switches detect the presence of non-metallic objects with the help of a high-frequency electric field.
• Optical proximity switches detect the interruption of a light beam by an object when it passes through the beam.
• Ultrasonic proximity switches detect the presence of dense matter by the reflection of sound waves by sending the sample waves.

Most of the proximity, switches have no “dry contact” outputs. Instead of the contacts output elements are transistors configured as either source current or sink current. Both “sourcing” and “sinking” will be understood by visualizing electric current in the direction of electron flow instead of conventional flow. The below schematic diagrams show the direction of current (conventional flow notation). The load being driven by each proximity switch is a light-emitting diode (LED) in both the examples:

What is sinking and sourcing? or How the switch connected as “NPN” type and “PNP type?

An NPN switch uses a transistor in its output referred to as an electronic switch designed to sink current through its signal wire. Similarly, a PNP switch is referred to as an electronic switch designed to source current through its signal wire. To understand this, recognize the emitter terminal of the output transistor is always the one connected to the power supply rail. The emitter must connect to the negative rail, necessitating an NPN transistor to do the switching, for a sinking switch. And the emitter connects to the positive rail for a PNP transistor will suffice known as a sourcing switch.

Sometimes sourcing and sinking transistor switches are referred to as high-side switches and low-side switches, respectively. The emitter terminal is attached directly to the “high” rail (+) of the DC power supply known as sourcing transistor (PNP). And the emitter terminal is attached directly to the “low” rail (-) of the DC power supply known as the sinking transistor (NPN).

There are two different styles of electronic proximity switch shown below:

Most of the industries that use proximity switches have built-in LED indicator lamps to help technicians diagnose circuit problems by directly indicating the switch status.

The below photograph shows a proximity switch detecting the passing of teeth on a chain sprocket, generating a slow square-wave electrical signal as the sprocket rotates. Those switches may be used as a rotational speed sensor (sprocket speed proportional to signal frequency) or as a broken chain sensor (when sensing the rotation of the driven sprocket instead of the drive sprocket):

Proximity switches come in both “normally open” (NO) and “normally closed” (NC) types. Normally-open proximity switches that are sinking (NPN) as well as normally-open proximity switches that are sourcing (PNP), and normally-closed proximity switches in either sinking or sourcing designs as well. The proximity switch detection range is usually a fixed parameter rather than being adjustable.

List of Prominent Manufacturers: Adtek, Aeco, Aignep, Airmar, Autonics, Azbil, Balluff, Baumer, Bernstein, Broadcom, Comus, Crouzet, Datalogic, Di-soric, DropsA, Electro Sensors, Elen, Etatron D.S., Fargo Controls Inc, Festo, Fipa, Fiama, Gimatic, Hasco, Honeywell, IPF Electronic, JPC Connectivity, Klaschka, Littlefuse, MaxBotix, Melexis, Microsonic, Micro Detectors, Migatron Corp, Monarch, Nidec, Nivelco, Norelem, Omron, Optex, Parker, Pepperl+Fuchs, Proxitron, RS Pro, Schmersal, Sensopart, Soway, Stahl, Telemecanique, Univer, Waycon, Weber, Wick Sensors, Yaskawa, Zimmer

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Limit Switch

What is a limit switch?

A limit switch detects the physical motion of an object by direct contact with that object. It will be in its “normal” status when it is not in contact with anything (i.e. not touching the switch actuator).

An example of a limit switch is a switch detecting the open position of a door or an elevator, automatically energizing the cabin light when the door opens.

Limit switches find many uses in industry, generally in robotic control and CNC (Computer Numerical Control) machine tool systems. In many motion-control systems, the moving elements have “normal” positions where the computer assigns a position value of zero. These normal positions are detected by means of limit switches. The computer gives the command to each servo motor to travel fully in one direction until a limit switch on each axis trips. The position counter for each axis resets to zero as soon as the respective limit switch detects that the normal position has been reached.

A typical limit switch design uses a roller-tipped lever to contact the moving part. Screw terminals on the switch provide connection points with the NC and NO contacts inside the switch. Most limit switches of this design share a “common” terminal between the NC and NO contacts like shown below:

This switch contact arrangement is referred to as a form-C contact set since it incorporates both a form-A contact (normally-open) as well as a Form-B contact (normally-closed) inside it.

A close-up view of several limit switches shows the arrangement of connection terminals for form-C contacts. As shown above, each limit switch has its own “NO” (normally-open), “NC” (normally-closed), and “C” (common) screw terminal for wires.

As shown in the photograph above a limit switch assembly attached to the stem of a rotary valve used to detect the fully-closed and fully-open positions of the valve.

List of Prominent Manufacturers:  ABB, Allen Bradely, Ametek, Auspicious, Azbil, B-Command, Bartec, Berstein, Craig & Derricott, Crouzet, Divatec, Eaton, Elen, Elfin, Emass, Euchner, Festo, Gessmann, Greegoo, HASCO, Honeywell, IMO, Mayr, Metso, Micronor, Omron, Otto, Panasonic, Prisma, Schmersal, Telemechanique, Topworx, Vatroc,

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