Lle103101：i Can Tell You How To Choose A Suitable Level Sensor.

I read an electronic magazine in which there is a report on level sensors. After reading, I became interested in level sensors and want to find something more about this topic, so I searched some related information and materials about it on net, but it is not specific at all. Based on the above theories for materials of level sensors, I thought of how to choose a suitable level sensor. Today, I come to this forum to discuss my understandings or guesses are appropriate.

Here are my following understandings:

Many industry processes are used in liquid level sensors, which are divided into two categories. The point level measurement sensor is used to indicate a single discrete level level, indicating a preset level condition. Normally, the effect of this type of sensor is the upper limit alarm, which indicates an overfill condition, or indicates a lower limit alarm condition. Continuous level sensor is more complex, the entire system can be liquid level monitoring. They can measure the level of a certain level, rather than a point of the liquid level, it will produce directly with the container level associated with the analog output. To establish a level measurement system, this output signal is transmitted to the process control loop and the visual indicator.

Floating switch

In such a level of liquid level sensor, the magnetic float will move with the liquid surface, and will promote the rod in the sealed "reed switch." This simple and low maintenance rate design is not only quick to install, minimizes shock, vibration and pressure, but also for a wide range of media. The reed switch can be single-pole single throw (SPST), or it can be a single pole double throw (SPDT).


Non - contact ultrasonic sensor

This type of sensor includes an analog signal processor, a microprocessor, multiple binary-to-decimal (BCD) range switches, and an output driver circuit. The pulse and a gate signal are emitted from the microprocessor and transmitted via the analog signal processor to the sensor, causing the sensor to send the ultrasonic beam to the liquid surface. The sensor then detects the echoes reflected by the liquid level, sends them back to the microprocessor, and generates digital information that represents the distance between the sensor and the level of the liquid level. By constantly updating the received signal, the microprocessor calculates the average value, thereby finally achieving the liquid level measurement.

When using a continuous sensor, the microprocessor converts the average to a 4 to 20 mA analog signal in linear relationship with the liquid level. When the level return does not return to the sensor within 8 seconds, the system output signal will drop below 4mA, indicating that the liquid level is too low or the pipeline is empty. When using a point sensor, the microprocessor compares the average with the BCD switch setting and indicates that the output relay is powered on to indicate that the level is too high or the level is too low. If the signal is lost for more than 8 seconds, the relay is de-energized and the original state is restored. In addition, in order to alleviate the surface turbulence phenomenon, the electronic part of the addition of half a second delay.


Contact ultrasonic sensor

These sensors contain low-energy ultrasonic equipment that measures the level of a particular point. The contact ultrasonic sensor consists of a field-mounted sensor and a solid-state integrated amplifier, without any moving parts, and without calibration. Normally, they are provided with terminal blocks for connecting power and external control devices. The ultrasonic signal passes through the half-inch wide gap in the sensor and controls the relay switch when it contains liquid. Horizontal mounted sensor detection plane In the middle of the gap, the vertical mounting sensor detects the plane at the top of the gap. When the level drops below this level, the ultrasonic signal will be weakened and eventually the relay will switch to the previous state.

These sensors are used to automatically operate pumps, solenoid valves and upper and lower limits in containers or pipelines. Two such sensors are required to fill and empty the tank and measure the amount of liquid. Such sensors are suitable for most liquids, which are not affected by the cover, droplet adhesion, foam and steam. But excessive inflation of the liquid and the viscosity is too large and can fill the sensor gap of the liquid, it may lead to failure.

Capacitive level sensor

As with ultrasonic sensors, capacitive sensors can also be used for point level measurement or continuous level measurement. They use the probe to monitor the level of the tank changes, and electronically adjust the output capacitance and resistance values, and then converted into analog signals. The probe and the container wall are equivalent to the two plates of the capacitor, the liquid is equivalent to the dielectric. Since the transmission of the signal is only related to the change in the level, the material buildup on the probe does not have any effect. For non-conductive liquid containers, two probes or an external conductive sheet are required.

The probe can be divided into two kinds of rigid and flexible, usually using PTFE insulated wire. It uses stainless steel as the base metal for the probe, providing a higher sensitivity for non-conductive liquids, granular liquids, or liquid measurements with lower dielectric properties (dielectric constant less than 4). A flexible probe must be used when there is not enough clearance to accommodate a rigid probe or in applications where a long probe length is required. The stability of the rigid probe is higher, especially in the presence of turbulence, because the rocking of the probe can cause fluctuations in the signal fluctuation.

How to choose a suitable level sensor ?


Level measurement sensor type

Non-contact ultrasonic level sensor

With a variety of power and programming options, RS-232 or RS-485 communication, with button calibration and temperature compensation signals, LVU40 series sensors to continuous non-contact level measurement flexibility to a new height. They combine the filtering mechanism to virtually eliminate all pseudo-echoes of peripheral obstructions and provide high-precision measurements over the entire range of 0.3 to 18.3 m (1 to 60 '). With the PLC (programmable controller) or OMEGA CNi series controller used in conjunction, can achieve point level measurement.


Radar level sensor

For applications that require non-contact level measurement, the affordable LVRD500 radar transmitter is a rational choice for ultrasonic sensors. They are well suited for applications where vapor, dust, or foam can cause obstruction to ultrasonic measurements on liquid surfaces. The radar sensor using microwave pulse technology, the target liquid from the top of the antenna to the bottom of the cylinder tracking. This advanced "echo-tagged" signal processing method provides reliable continuous pulse waveforms that are not affected by the environment. RS-232 and RS-485 products are also available.

Dry propeller level switch

LVD-800 series propeller switch for a variety of granular, pellet and powdery material. It uses patented magnetic technology for fault-safe monitoring of tanks, hoppers, silos and silos. A microprocessor with a self-diagnostic function continuously monitors the rotation of the shaft and the motor mounting plate and responds immediately to the presence of the material and to mechanical or electrical faults. The magnetic sensing mechanism avoids the inherent malfunction of the optical system. Use a revolving lid without any bolts. There are two conduit entries, and most models have a status indicator. The high temperature model can withstand temperatures of 399 ° C (750 ° F).


Capacitive measuring probe

The LV3000 / 4000 series probes are capable of high temperature and high pressure and are resistant to a wide range of corrosive items, providing reliable continuous level measurement in harsh environments. They are suitable for liquids, pastes and partially solids (both conductive and nonconductive), which do not contain any moving parts and are easy to install. After rectifying and filtering the input power supply and generating a radio frequency signal and calculating the current change, the electronic circuit will produce a 4 to 20 mA 2-wire output signal proportional to the process level. Convenient zero and range adjustment functions allow the user to set various variables, including media type, container size, pole length and mounting position. If a stainless steel probe is used near the wall of the conductive container, a PTFE insulation model can be selected to prevent short circuits.




Those are just my understandings. I know that my understandings may not be mature or inefficient. Today, I come to this forum to turn to you to discuss this topic. What is your idea ? Do you agree with my ideas ? Any of your ideas would be highly appreciated.

May someone would like to help ?

thanks in advance.

 

top spec is always a matter of context.

if you just want to turn on or off a sump pump, a float with a pair of contacts is pretty much all you need.

or to turn a signal red to stop a train before it gets to water over the tracks.

but sometimes it isn't level but flow rate that you're interested in.

or other factors. again, its always a matter of context.

 

so now do you know how to choose a level sensor ? Any questions about this topic, you can feel free to ask me. Any time you are welcome to ask.

 

Please don't worry; I am so pleased to receive your news. thanks in advance.

 

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