Point level detection of liquids

Typical systems for mid-point level detection in liquids include magnetic and mechanical floats,pressure transducers,conductive sensing or electrostatic (capacitive or inductive) detectors as well as electromagnetic (e.g.magnetostrictive) measurement signals flying to the surface of the fluid time),ultrasonic,radar or optical sensors.

Magnetic and mechanical float 

The principle behind magnetic,mechanical,cable and other float level sensors usually involves opening or closing a mechanical switch by direct contact with the switch or magnetic operation of a reed.In other cases,such as magnetostrictive sensors,the floating principle can be used for continuous monitoring.For magnetically actuated float sensors,switching occurs when a permanent magnet sealed within the float is raised or lowered to the actuation level.With mechanically actuated floats,switching occurs due to the movement of the float relative to a micro (micro) switch.For magnetic and mechanical float level sensors,chemical compatibility,temperature, specific gravity (density), buoyancy, and viscosity can affect stem and float selection.For example,larger floats can be used for liquids with a specific gravity as low as 0.5 while still maintaining buoyancy.The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity-these changes directly affect buoyancy.The float sensor can be designed so that the shield protects the float itself from turbulence and wave motion.The float sensor is suitable for a wide variety of liquids, including corrosive liquids.However,when used in organic solvents,it is necessary to verify that these liquids are chemically compatible with the materials used to build the sensor.Float-style sensors should not be used with high-viscosity (thick) liquids,sludge or liquids adhering to the valve stem or float, or materials containing contaminants such as metal filings; other sensing technologies are better suited for these applications.A special application for float sensors is the determination of the interface level in oil-water separation systems. Two floats may be used,each sized to match the specific gravity of the oil on the one hand and the specific gravity of the water on the other hand.Another special application for rod float switches is the installation of temperature or pressure sensors to create multi-parameter sensors.Magnetic float switches are popular for their simplicity, reliability and low cost.A variation of magnetic induction is the "Hall Effect" sensor,which utilizes the magnetic induction indicated by a mechanical gauge.In a typical application, a magnetically sensitive "Hall effect sensor" is attached to a mechanical tank level gauge with a magnetized indicator needle to detect the indicated position of the gauge needle.A magnetic sensor converts the pointer position into an electrical signal, allowing other (usually remote) indications or signals.

Pneumatic

Pneumatic level sensors are used where hazardous conditions exist, where power access is unavailable or limited,or in applications involving heavy sludge or slurries.Since the compression of the diaphragm by the air column is used to actuate the switch, no process fluid contacts the moving parts of the sensor.These sensors are suitable for high viscosity liquids, such as grease, as well as water-based and corrosive liquids.This has the added benefit of being a relatively low-cost technique for point-level monitoring.A variation of this technique is the "bubbler",which compresses air into a tube leading to the bottom of the tank until the pressure increase ceases, as the air pressure becomes high enough to expel air bubbles from the bottom of the tube, overcoming the pressure there.The measurement of steady air pressure indicates the pressure at the bottom of the tank and, therefore,the mass of fluid above the tank.

Conductive

Conductive liquid level sensors are very suitable for point detection of various conductive liquids such as water, especially for highly corrosive liquids such as caustic soda hydrochloric acid, nitric acid, ferric chloride and similar liquids.For those conductive liquids that are corrosive, the electrodes of the sensor need to be made of titanium,Hastelloy B or C, or 316 stainless steel with spacers, separators or the bracket is insulated.Depending on their design,multiple electrodes of different lengths can be used with one holder. Since corrosive liquids become more aggressive with increasing temperature and pressure,these extreme conditions need to be considered when specifying these sensors.Conductive level sensors use a low-voltage,current-limited power supply applied across separate electrodes.The power supply is matched to the conductivity of the liquid, with higher voltage versions designed to operate in lower conductivity (higher resistance) media.Power supplies often include control of some aspect, such as high and low or alternate pump control.Conductive liquid contacting the longest probe (common) and the shorter probe (return) completes the conductive circuit.Conductive sensors are very safe because they use low voltage and low current.Due to the inherently small current and voltage used, the technology is also "intrinsically safe" to meet international standards for hazardous locations for reasons of personal safety. Conductivity probes have the added advantage of being solid state devices and are very simple to install and use.In some fluids and applications maintenance can be an issue.The probe must continue to conduct electricity.If buildup insulates the probe from the media, it will stop functioning properly.A simple check of the probe requires connecting an ohmmeter between the suspect probe and the ground reference.Typically, in most water and waste wells, the well itself and its ladders, pumps, and other metal fixtures provide the ground return.However, in chemical storage tanks and other ungrounded wells, the installer must provide a ground return, usually a ground rod

State dependent frequency monitor

The microprocessor controlled frequency state change detection method uses low amplitude signals generated on multiple sensor probes of different lengths.Each probe is frequency separated from all other probes in the array and independently changes state when exposed to water.Changes in the state of the frequency on each probe are monitored by a microprocessor which can perform a variety of water level control functions.The advantage of condition-dependent frequency monitoring lies in the long-term stability of the sensing probe.Due to electrolysis in sewage, the signal strength is insufficient to cause fouling, degradation or deterioration of the sensor.Sensor cleaning requirements are minimized or eliminated.The use of multiple sensing rods of different lengths allows users to intuitively set the control switches for different water levels.Microprocessors in condition-dependent frequency monitors can drive valves and/or large pumps with very low power consumption.Multiple switch controls can be built into a small package while using a microprocessor to provide complex,application-specific functions.The low power consumption of the controls is consistent in both large and small field applications.This versatile technology is used in applications with a wide range of liquid qualities.