Everything you need to know about point level detection
Level measurement, also known as level detection, generally comes in two flavors, continuous and point. In continuous level detection, the sensor constantly measures the level of liquid or solid in the tank. But today’s article focuses on the second type, point level detection.
Here, the operator sets a level, and the sensor simply detects the presence or absence of product at that level. You can easily integrate this binary output into a control system for immediate starting or stopping of the process.
Point level detection works in liquids, pastes, powders, or bulk solids in silos. You can run into problems with changing properties, turbulence, foam, vibration, or buildup, but a vast range of sizes, mountings, and materials make detection easy.
Now let’s discuss the types of point level detection you can find.
Vibrating or tuning fork
This sensor detects the frequency changes in a tuning fork. Piezo-electric crystals oscillate the prongs at their natural frequency. If product covers the prongs, then the frequency drops. The device detects this change and sends a signal to the control system. The system then analyzes the change and sends a command back to the device to switch on or off.
This simple method offers reliable and cost-effective data. Its easy installation and low maintenance makes it a good choice for many liquids and powders as well as solid bulk materials. However, the sensor can misbehave if a viscous product causes buildup.
This principle focuses on capacitor changes. The probe and the wall act as the two plates of a capacitor, with the probe insulation as the dielectric. If the product intrudes between the probe and the wall, the capacity increases, triggering a signal for switching. For this type of sensor, the product must have a dielectric constant different from air.
This sensor works in liquids, solids, and mixed materials, but keep in mind its limitations. You’ll get great mileage from it in small tanks with fast-flowing product; its quick response will prevent overfill. However, it needs regular calibration to compensate for different dielectric constants.
This well-known method works on the Archimedean principle of buoyancy, where a buoyant force on an object equals the weight of the fluid displaced. Here, your float must weigh less than the minimum expected specific gravity of your product. The float sits on a movable arm coupled to a micro-switch. The position of the switch depends on the mounting type and tank construction. Depending on the needs of your process, you can get the float set in different positions, like the side, bottom, or top.
This switch doesn’t need field calibration, but you do have to watch for corrosion, which might eat a hole in the float.
Thermal dispersion switches
This detection principle uses convective heat transfer theory. The switch has two resistance temperature detectors (RTDs), one for reference and the other to measure the product temperature. This device also uses a Wheatstone bridge. If a change in the bridge’s balance occurs, then the process will switch.
This method works well in high-temperature and high-pressure applications, but it responds more slowly than other methods. It also requires calibration with the product.
The principle is based on the moment of resistance change of a rotating paddle in air or a medium. The paddle point level switch is the first choice for the bulk solids in Silos for Overfill, Empty and requirement alarm. It is ideal for flowing bulk solids up to a grain size of 50mm.
During normal operation (in Air medium) a synchronous motor slowly rotates the paddle with the frequency of 1Hz. When this paddle rotation broke by the surrounding material, the motor stops rotating and triggers the microswitch to change the state and indicate the alarm or Control action. As the level moves down, the microswitch restarts the motor. Typical application areas are point level detection in bulk solids, e.g. cereals, sugar, plastics, sand, fillers, wood dusts, washing powders, chalk,dry plaster, cement and granulates.
Conductivity point switches
Conductivity point level detection is based on the difference in electrical conductivities between air and the liquid to be measured. The probe is used as one electrode and electrically conducting tank is used as another electrode. If the tank is made of non-conducting material like plastic or concrete, then additional electrode shall be taken into consideration. Conductivity level switch detects the level resistance when their electrode is covered by the medium and an alarm is transmitted.
This type of sensor has many advantages like no moving parts, easy installation and maintenance free. But for the switch to operate correctly, the process fluid must have a minimum conductivity so that current can flow easily through it.
You’ll want to use point level detection when you only need to know about the presence or absence of a product. Because these methods prevent overfill and protect pumps from running dry, you can install them as backups for your continuous level measurement.
If you’d like to know more about one or several of these devices, then check out our device directory, where you’ll find examples of each to compare.