Continuous level measurement of liquids

Senin, 11 Januari 2016

Continuous level measurement of liquids

Magnetostrictive

Magnetostrictive level sensors are similar to float type sensors in that a permanent magnet sealed inside a float travels up and down a stem in which a magnetostrictive wire is sealed. Ideal for high-accuracy, continuous level measurement of a wide variety of liquids in storage and shipping containers, these sensors require the proper choice of float based on the specific gravity of the liquid. When choosing float and stem materials for magnetostrictive level sensors, the same guidelines described for magnetic and mechanical float level sensors apply.

Magnetostrictive level and position devices charge the magnetostrictive wire with electric current, when the field intersects the floats' magnetic field a mechanical twist or pulse is generated, this travels back down the wire at the speed of sound, like ultrasound or radar the distance is measured by time of flight from pulse to return pulse registry. the time of flight corresponds to the distance from the sensor detecting the return pulse.

Because of the accuracy possible with the magnetostrictive technique, it is popular for “custody-transfer” applications. It can be permitted by an agency of weights and measures for conducting commercial transactions. It is also frequently applied on magnetic sight gages. In this variation, the magnet is installed in a float that travels inside a gage glass or tube. The magnet operates on the sensor which is mounted externally on the gage. Boilers and other high temperature or pressure applications take advantage of this performance quality

Resistive chain

Resistive chain level sensors are similar to magnetic float level sensors in that a permanent magnet sealed inside a float moves up and down a stem in which closely spaced switches and resistors are sealed. When the switches are closed, the resistance is summed and converted to current or voltage signals that are proportional to the level of the liquid.

The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy. These sensors work well for liquid level measurements in marine, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications. With the proper choice of two floats, resistive chain level sensors can also be used to monitor for the presence of an interface between two immiscible liquids whose specific gravities are more than 0.6, but differ by as little as 0.1 unit.

Magnetoresistive

Magneto Resistive Level Sensor

Magnetoresistance float level sensors are similar to float level sensors however a permanent magnet pair is sealed inside the float arm pivot. As the float moves up the motion and location are transmitted as the angular position of the magnetic field. This detection system is highly accurate down to 0.02 Degrees of motion. The field compass location provides a physical location of the float position. The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy of the float. The electronic monitoring system does not come in contact with the fluid and is considered Intrinsic safety: or explosion proof. These sensors work well for liquid level measurements in marine, vehicle, aviation, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications.

Due to the presence of a microprocessor and low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal.

Hydrostatic pressure

Hydrostatic pressure level sensors are submersible or externally mounted pressure sensors suitable for measuring the level of corrosive liquids in deep tanks or water in reservoirs. For these sensors, using chemically compatible materials is important to assure proper performance. Sensors are commercially available from 10mbar to 1000bar.

Since these sensors sense increasing pressure with depth and because the specific gravities of liquids are different, the sensor must be properly calibrated for each application. In addition, large variations in temperature cause changes in specific gravity that should be accounted for when the pressure is converted to level. These sensors can be designed to keep the diaphragm free of contamination or build-up, thus ensuring proper operation and accurate hydrostatic pressure level measurements.

For use in open air applications, where the sensor cannot be mounted to the bottom of the tank or pipe thereof, a special version of the hydrostatic pressure level sensor can be suspended from a cable into the tank to the bottom point that is to be measured. The sensor must be specially designed to seal the electronics from the liquid environment. In tanks with a small head pressure (less than 100 INWC), it is very important to vent the back of the sensor gauge to atmospheric pressure. Otherwise, normal changes in barometric pressure will introduce large error in the sensor output signal. In addition, most sensors need to be compensated for temperature changes in the fluid..

Air bubbler

An air bubbler system uses a tube with an opening below the surface of the liquid level. A fixed flow of air is passed through the tube. Pressure in the tube is proportional to the depth (and density) of the liquid over the outlet of the tube.

Air bubbler systems contain no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. Since the point of measurement has no electrical components, the technique is a good choice for classified “Hazardous Areas”. The control portion of the system can be located safely away, with the pneumatic plumbing isolating the hazardous from the safe area.

Air bubbler systems are a good choice for open tanks at atmospheric pressure and can be built so that high-pressure air is routed through a bypass valve to dislodge solids that may clog the bubble tube. The technique is inherently “self-cleaning”. It is highly recommended for liquid level measurement applications where ultrasonic, float or microwave techniques have proved undependable.

Gamma ray

A nuclear level gauge or gamma ray gauge measures level by the attenuation of gamma rays passing through a process vessel. The technique is used to regulate the level of molten steel in a continuous casting process of steelmaking. The water-cooled mold is arranged with a source of radiation, such as cobalt-60 or caesium-137, on one side and a sensitive detector such as a scintillation counter on the other. As the level of molten steel rises in the mold, less of the gamma radiation is detected by the sensor. The technique allows non-contact measurement where the heat of the molten metal makes contact techniques and even many non-contact techniques impractical.