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How to select level sensor and level switch

- Jun 29, 2018 -

We describe how to select the liquid level sensor (level switch) from the aspects of the level sensor's specifications, electrical specifications, and solutions.


Level sensor specifications

Level sensor specifications are used to measure liquids in containers.


The sensor of the range of maximum measuring distance. Most sensors are available in two or three different ranges.

The height of the tank drops from the distance of the mounted transducer surface to the bottom of the tank.

Raise the height of the transducer surface from the top of the tank

The maximum liquid level required to fill the distance from the bottom of the tank.

The dead zone faces the smallest distance that the sensor's sensor can measure.

 


Level sensor process specification

Allow sensor overrange - The overrange specification limits the selected overrange factor to the specified range multiplied by the range of the matched product. This makes the scope of the returned match more extensive than the range of products needed and filtered out.

For example: The user has specified a sensor ranging from 0-100. Without overrange specification, the range 0-100, 0-1000, -10,000 to +10,000 sensors return matches. The 50% over-range specification will filter out any sensor with a full-scale range greater than 150.


Process operating conditions - There are two types of process conditions to consider:

Maximum working pressure - Maximum material, rated working pressure of the device.

Temperature range of the material - The maximum temperature of the material device is rated.

Mounting - Installation is defined as side mounting, top mounting, bottom mounting. The mounting sensor relies on the characteristics of the container, including the contact or non-contact measurement of the sensor.


Electrical Specifications

The communication interface has two basic communication interfaces:

Serial-to-serial communication means to send data bits in a sequence of one line (one after the other). Further data can be sent with the advantage of serial communication, the cable connection is simple and uses less wires. Serial communications protocols such as the Universal Serial Bus (USB), RS232, or RS485 serial interface are used.

In parallel parallel communication devices, the number of data bits is transmitted at the same time. More information can be sent quickly and more reliably, but processing takes longer. The parallel interface uses a parallel communication protocol such as the General Purpose Interface Bus (GPIB).

 

Instrumentation - The liquid level sensor may have an instrument option in the user interface

Non-electrical visual or audio output Non-electrical visual indicators, such as a simple data, or buzz. Non-electric visual effects have cheap and easiest to use and install, but they are subject to user reading level errors.

Use an analog gauge needle or an LED indicator to determine the level of fluid in the system.

Digital display device with specific readings for numbers or applications. These displays are easy to read and can show significant numbers.

These include cathode ray tubes and flat panel display video display terminals (VDTs).

 

Level sensor selection output options include:

Analog voltage signal

Analog current signal

Frequency/modulation frequency

Alarm or visual indicator

 


Level sensor application solution

In a variety of applications, primary monitoring is very important. In any tank or container filled or stored liquid that is involved in an industrial process, the level sensor used will be more advantageous. The level sensor is an important part of the process control system and manages the inflow and outflow velocity of the storage vat or reactor.


Level sensors can be used for different levels of sensing and alarms, leak detection, overflow closures, and supervision interfaces between different media. Can be used in manufacturing, food and beverage, chemical, pharmaceutical, marine, medical, fuel/energy management and other industries.


Nuclear (radiation) sensor


Nuclear sensors are a very effective solution, but their cost and special use requirements usually make this technology the last choice. The solution is simple: Place the gamma ray emission source on one side of the container. The sensor is mounted on the other side of the electro-hydraulic level. The solid or liquid fill in the container absorbs gamma rays in a predictable manner, thereby allowing the level to be determined. The accuracy of the measurement depends on the number of sensors, so it is usually used for high and low limit alarms.


The radiation sensor does not have any media requirements and it does not need to penetrate the tank wall, so it is very suitable for high pressure and high temperature environments, high value products, and any other ways of retrofitting existing installations are not available. However, the use of radioactive sources capable of penetrating general stainless steel tanks requires specialized licenses and professionally trained operators, so this method must be carefully considered beforehand.


Electromechanical solutions


These programs have a common element, namely various types of movable parts, which may be buoys on the surface of the liquid, or may be devices that must pass through the measured object.


Buoys - The use of buoys is a simple and reliable level measurement method as long as there is nothing in the liquid that could interfere with the movement of the buoy. Many valves, switches and encoders are activated by buoys, which provide a small range of contact level or continuous readings.


Some of the most professional continuous measurement buoy designs utilize magnetostrictive sensing technology. The buoy is ring-shaped and can be stuck outside the waveguide. The waveguide can be as long as 50 feet (15 meters), so only larger tanks can be used. The buoy contains permanent magnets that interrupt the transmission of one electron pulse to the waveguide. The instrument can measure breakpoints and has fairly good accuracy and repeatability, typically <±.001in. Once installed and set up, no additional calibration requirements are required.


Magnetostrictive technology is of great significance for wireless communication. Pulse response is extremely fast, so the power consumption is minimal.


The waveguide can support two buoys, making magnetostriction one of the only technologies that can continuously measure mixed liquid layers (such as water-capped oil) with one device.


Vibration and Paddlewheel—The two contact level methods are similar: they all need to be inserted into the probe. The vibratory probe inserts a tuning fork-like device into a material that can cause continuous vibration from the piezoelectric crystal. If the probe is not buried in the contents of the tank, it can vibrate freely; if the probe is trapped in it, it cannot normally vibrate, and the mechanism thus recognizes and sends the corresponding signal.


Similarly, the paddle wheel uses a shaft-mounted electromechanical paddle or flag to connect a small motor. If it sinks deep into a solid product, the paddle wheel cannot turn and send a signal. When the contents of the tank are discharged, the paddle wheel can rotate. Both of the above methods are invasive and can easily damage the contents of the tank.


Pressure - As with mass measurements (called tank weights), pressure and differential pressure methods determine the liquid level at the bottom of the tank (or where the meter is located) by measuring the pressure head. If the tank is vented to the atmosphere, a simple pressure gauge is sufficient. However, if the tank is closed and can be pressurized or decompressed, differential pressure readings between the open space at the bottom and top will self-compensate for any internal and atmospheric pressure differences. This method works well, but it requires extra piping.


Electronic probe


When the contact level measurement is sufficient and the contact medium is allowed, the capacitance and conductivity probes provide a simple and reliable solution.


The conductivity probe is a simple, contact-type liquid level reading device that provides conductive liquids. Two or more conductivity probe sets are commonly used to measure high and low liquid levels. If the liquid is non-conductive, another method must be used.


Capacitive probes determine the presence of solid or liquid material by changing the capacitance around the multi-electrode probe. The RF current input electrode can measure the change of capacitance based on the dielectric value of the contact medium. Some designs can determine the dielectric values of media, which means that they can distinguish different fillers. For example, a probe in oil and a probe in water will have different readings. This can help to deal with situations where more than one type of media is present in the tank.


Some capacitive sensors can work outside the non-metallic tank wall to avoid penetrating or contacting the medium while obtaining the contact level signal. This sensor can be mounted against the tank wall or it can be wound around a non-metallic pipe. If it is a metal can, the sensor can be placed in a transparent glass or a well made of plastic tubing. Capacitive sensors are particularly suitable for most solid media such as chlorine-containing detergents, which cover the inner tank wall and change the properties of the plastic. This will cause some capacitive sensors to get the wrong signal, but more advanced devices will not be affected.


The thermal probe needs to be inserted into the tank for use. They use a small electric heating element to heat a small amount of space around them and measure their temperature rise. If there is no liquid around the probe, the increase in temperature is relatively large. However, if there is liquid around the probe, heat will gradually dissipate and the temperature will be much lower.


The beam sensor can sense the obstacle between the beam emitter and the receiver. The presence of a solid or liquid substance reflects or disperses the light beam, thereby indicating the presence of the substance. If the medium absorbs the light beam or is penetrated by light, this technique cannot display correct information.


Weighing


One of the simplest and most reliable methods for determining the contents of a tank is weighing. This method is the only way to give an actual mass reading without relying on the size inside the tank. The loading unit placed under the tank can determine the content by subtracting the weight of the empty machine. This work is suitable for any type of content. This method can be very precise without interference from pipes or other connections. Different sizes of containers may have different limits in practice, but this obvious method should not be overlooked.


If this method proves to be infeasible, then any other method will make things more complicated, and whether it is feasible depends on the comprehensive consideration of various restrictions and requirements.


The biggest deciding factor in selecting the level sensing technology is the material stored in the tank, that is, the object to be measured. For the purposes of this discussion, we define the liquid as a substance that can form a stable level and flow in and out through the pipeline. Solids may have a certain degree of fluidity, but they do not necessarily form a stable surface or spill out of the tank. Liquids that are viscous or contain heavier solids may behave more like solids.


The following is an outline of the level measurement technique and a comparison with the recommended application method in the column. Note: Continuous measurements can be used to implement contact level functions, while multi-contact level measurements can provide valid continuous data to some degree, although not exactly. Of course, neither the application column nor the following technical introduction can cover the entire status quo. Some practical methods are not mentioned in one of the introductions, and there may be exceptions in any of the regular instructions.


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