Switches
A switch is a device that breaks the connection in an electrical circuit through disconnecting two contacts so that electricity cannot flow between them. The contacts are two pieces of metal, usually a material that is resistant to corrosion, can conduct electricity, resistant to abrasive wear, low cost and with low toxicity. Sometimes nonmetallic conductors, such as conductive plastic (a plastic resin that has been embedded with carbon powder or fibre), can be used instead.
The moving part of the switch that applies force to the contacts is called the actuator (or colloquially a toggle, dolly, rocker, etc). When force is applied to the actuator to separate the contacts (ie they are ‘open’), they are said to have an insulating air gap between them and no electricity can be conducted through them. Some switches are normally open (labelled n.o. switches), while some are normally closed (labelled n.c. switches). A switch that can switch on a circuit in one position and can switch to another circuit in the other is called a changeover switch (also ‘make-before-break’). By contrast, a switch that momentarily disconnects before activating the other circuit is called a ‘break-before-make’ changeover switch. One type of switch explained below is the level switch.
Level Measurement
Level switches are used to measure the level of liquid, solids or powders in a given container or tank. This can be done through many different types of level measurement, whether conductance, hydrostatic, radar, microwave, ultrasonic or sonic.
Semrad Pty Ltd offers level switches such as the NivoCONT R series Vibration Rod Level used for bulk solids or the NivoFloat used for liquid level sensing. Even in arduous conditions Semrad’s switches can provide reliable and accurate level measurements. For example, the ‘intelligent’ NivoSwitch combines frequency and amplitude sensing for excellent performance even on vibrating vessels and turbulent liquid surfaces. It can also be used for measurement of light granules.
Switches
A switch is a component of an electrical circuit used to interrupt the current, consisting of an electromechanical device with a set of contacts. When the contacts are touching (’closed’) then electricity can flow between them, when they are separated (’open’) electricity cannot be conducted between them. This binary characteristic is the common feature in all types of switches, whether in networks or telecommunications, meaning that they are either on or off, closed or open, connected or disconnected.
The most commonly known type of switch is the light switch, used to turn lights on and off in the home. Interestingly, light switches in the US must be moved up to be turned on, while in Europe the up position means the light will be off. In Japan and other countries where earthquakes frequently occur, switches are turned left and right to prevent things being turned on or off inadvertently by falling objects.
Level Switches
Level switches come in different forms for liquid, solids, and powders. One example of a level switch is the NivoFloat used to switch from drinking water to sewrage, which is specially designed with non-toxic materials and mercury-free microswitch so that it can be used in drinking water applications.
Another example is the NivoCONT R series vibration rod level switches for bulk solids. The vibration rod is energised and kept in resonance by an electronic circuit. When covered by material the damping of the vibration is detected by the electronics which, after a built-in time delay, initiates the switching of the output relay.
The NivoSwitch on the other hand is a very versatile vibration fork that can be used for both liquids and powders, and is suitable for arduous conditions due to the combination of frequency and amplitude sensing for unique performance. It is able to function on vibrating vessels or turbulent liquid surfaces as short fork sections provide increased vibration immunity.
Tank Level Measurement
There are many methods for measuring tank levels, so identifying which technology or instrument to use can be difficult. Some of the common types of level measurement methods include RF capacitance, conductance, hydrostatic tank gauging, radar and ultrasonic.
- Radio Frequency capacitance uses the electrical characteristics of a capacitor to measure the level. The amount of capacitance is determined by the spacing and area between two conductors and the electrical characteristic of the insulating medium between them.
- Conductance is based on the electrical conductivity of the liquid, used for point level detection.
- The radar or microwave method uses electromagnetic waves for continuous level measurement. A sensor beams down a microwave from the top of the vessel. The time it takes for the beam to reach the bottom is used to calculate the level.
- Ultrasonic and sonic methods uses sound waves to determine the level of a liquid. A transducer sends waves downwards onto the surface of the material, then the echoes return to the trasducer which calculates the distance and converts it into a measurement.
Automatic Tank Gauge (ATG)
Hydrostatic tank gauging with an automatic tank gauge (ATG) is one of the oldest and most common methods of measuring liquids. The density of liquid varies with temperature, so the pressure exerted by a column (or head) of liquid in the tank is measured and expressed in relation to the actual temperature of the measured liquid. Through hydrostatic tank gauging transfers in tank farms can be monitored, and accurate information on tank level, mass, density and volume can be provided.
Semrad Pty Ltd provides level transmitters that measure hydrostatic pressure using an air purge connection to the tank. A downpipe or ‘bubbler’ of open ended pipe that extends downward into the tank measures the differential pressure.