Any height of material beyond the level of the source/detector arrangement will attenuate the strength of radiation reaching the detector. Composed of a radioactive source material and a radiation detector, the two are mounted across the diameter of a storage vessel for either solid or liquid material. Nuclear Level Switch Figure 9.12 Nuclear level switch.Īnother type of level switch, suitable for liquid or solid material detection, is the nuclear switch. Other flow switches are constructed as differential pressure switches, measuring the pressure drop across a restriction built into the pipe. Inserted into a pipe, a flow switch will detect any gas or liquid flow rate in excess of a certain threshold, usually with a small paddle or vane which is pushed by the flow. When the bin fills to that height, the solid material dampens the vibration of the fork, the change in vibration amplitude and/or frequency detected by the electronic circuit. The fork is vibrated at its resonant frequency by an electronic circuit and magnet/electromagnet coil assembly. Another design uses a “tuning fork” shaped metal prong, inserted into the bin from the outside at the desired height. The torque response of the small motor than trips the switch mechanism. When the solid material fills the bin to that height, the material prevents the paddle wheel from turning. A common design for this application is a small paddle wheel, inserted into the bin at the desired height, which is slowly turned by a small electric motor. Level switches can also be designed to detect the level of solid materials such as wood chips, grain, coal, or animal feed in a storage silo, bin, or hopper. In most cases it is impractical and dangerous to switch the full load current of the circuit through a liquid. The conductivity technique is usually implemented with a special design of relay triggered by a small amount of current through the conductive liquid. If the liquid is electrically conductive, the liquid itself can be used as a conductor to bridge between two metal probes inserted into the tank at the required depth. Speed SwitchesĪ floating object can be used to actuate a switch mechanism when the liquid level in an tank rises past a certain point. There are many different types of process switches. Such switches can be used to sound alarms, indicating that a process variable has exceeded normal parameters, or they can be used to shut down processes or equipment if those variables have reached dangerous or destructive levels. In many industrial processes, it is necessary to monitor various physical quantities with switches. Optical switches are also useful in safety applications, where beams of light can be used to detect personnel entry into a dangerous area. Machine position is detected by either the interruption or reflection of a light beam. Another form of proximity switch is the optical switch, comprised of a light source and photocell. A non-electronic proximity switch would use the same symbol as the lever-actuated limit switch. The symbol shown here for the proximity switch is of the electronic variety, as indicated by the diamond-shaped box surrounding the switch. If a metallic part (not necessarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring circuit. More complex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency current, and electronically monitoring the magnitude of that current. Simple proximity switches use a permanent magnet to actuate a sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). Proximity switches sense the approach of a metallic machine part either by a magnetic or high-frequency electromagnetic field. Any switch designed to be operated by a person is generally called a hand switch, and they are manufactured in several varieties: Toggle Switches In any case, the final output of any switch will be (at least) a pair of wire-connection terminals that will either be connected together by the switch’s internal contact mechanism (“closed”), or not connected together (“open”). Other switches are more complex, containing electronic circuits able to turn on or off depending on some physical stimulus (such as light or magnetic field) sensed. The simplest type of switch is one where two electrical conductors are brought in contact with each other by the motion of an actuating mechanism. There are many different types of switches, and we will explore some of these types in this chapter. Switches are essentially binary devices: they are either completely on (“closed”) or completely off (“open”). An electrical switch is any device used to interrupt the flow of electrons in a circuit.
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