Switches Indicators & Gauges

Hydrostatic Tank Gauging

Fluid pressure is the pressure at some point within a fluid, hydrostatic pressure being the pressure at any given point of a non-moving (static) fluid. Fluid pressure occurs in open conditions, such as the ocean, and closed conditions, such as a tank. Pressure in open conditions is usually static, as the motion of waves and currents create only negligible changes in pressure. Closed bodies of water are either static (when non-moving) or dynamic (when moving, or compressed by air).

After determining the pressure within the fluid, the pressure’s constant of integration is equal to some reference pressure within the system. The properties (such as pressure) of a fluid can be determined from a control volume analysis of a cube of water. Since the stress of all sides of the cube must be normal and equal in magnitude, the pressure gradient can be found to be linearly increasing in a potential gradient. The pressure within a fluid will increase linearly as the product of the fluid’s density and gravity.

Purge Control

Hydrostatic level transmitters measure hydrostatic pressure using an air purge connection to the tank. An integral specialised airflow regulator ensures a linear response throughout the entire operating range. LiquiSeal™ or Purge Control differential pressure air flow regulators are highly refined versions of the basic bubbler air control. ‘Bubblers’ are usually a length of open pipe that extends downwards into the tank.

Semrad’s Purge Control offers continuous level measurement with pneumatic or electronic output, and is suitable for corrosive or aggressive liquids. The electronic transmitter can be applied to a maximum depth of 49m in water, while the pneumatic transmitter has a maximum depth measurement of 91m. Both have a flow rate of one nominal cubic foot per hour, and a temperature range between -18° and +82° C (though the actual process temperature limits may be significantly higher depending on the material used for the downpipe and purge airline).

Magnetic Tracking

Magnetostrictive level transmitters operate through a float containing a magnetic disc moving along a guide tube with a specific magnetostrictive wire in it. Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. This conversion of magnetic energy into kinetic energy is quantifiable by the magnetostrictive coefficient L, which is the fractional change is length as the magnetisation of the material increases from zero to the saturation value.

The NivoTRACK operates by having a pulse generated by the electronics sent along the magnetostrictive wire, which develops a twist once the pulse reaches the float’s magnetic field. Reflected from the torsion point, the pulse creates an acoustic wave that travels back along the wire. The 4 – 20mA output of the transmitter is proportional to the elapsed time between the excitation and detection.

NivoTRACK

NivoTRACK is a magnetostrictive level transmitter distributed by Semrad Pty Ltd. A highly accurate float level transmitter working on the magnetostrictive principle, it is suitable for custody transfer measurement of valuable liquids such as fuels, solvents, alcohol derivatives, and so on. It can also be applied in level measurement for vessels of production technologies, the most suitable being liquids free of solid particles and with low viscosity both in ordinary and hazardous locations.

This level transmitter comes in both stainless steel for ordinary applications, and plastic coated for a wide range of aggressive materials. It comes with HART communication, a rigid or flexible guide tube, plug in field display module, intrusion length up to 10m, and a 1mm accuracy. The units with flexible tubes make it possible for more accurate measurement for higher tanks, as well as offering a more convenient way for delivery and installation.

NivoTRACK can be housed in either powder paint coated aluminium or plastic (PBT fibre-glass reinforced and flame retardant). The transmitter also is designed as a single or double float, the latter for continuous measuring of liquid interfaces. It can measure levels, interface level, level difference, and medium temperature at one point.

Level Transmitters

Level transmitters are electronic devices that send measurements of the liquid or bulksolid levels via an electromagnetic signal to a receiver. The receiver can be either a computer-controlled device or a user. This measurement information could be in the form of point level measurements (measurements taken at a specific level or at certain intervals) or as a continuous measurement (measurements within a specified range).

Transmitters usually consist of a power supply, an oscillator that emits the signal, a modulator to change the signal from one wave form to another, and an amplifier to increase the strength of the signal. Transmitters are contained within all electronic communication devices, as it is the unit used to send information. Transceivers are devices that contain both a transmitter and a receiver.

Level transmitters come in a range of different technologies for different applications, including hydrostatic, ultrasonic, capacitance, magnetic float, optical, conductivity and more.

NivoCAP

NivoCAP is a capacitance level transmitter distributed by Semrad Pty Ltd, designed to measure the level in vessels filled with conductive or non-conductive liquids, powders and granular materials. It is best suited for the measurement of conductive liquids in regularly shaped conductive containers, though with the use of an auxiliary probe it can also measure levels in irregular or non-conductive vessels. The probe must be continuously in contact with the medium for accurate measurement.

With capacitance level measuring, the probe is one ‘plate’ of the capacitor with the metallic wall of the vessel being the other ‘plate’. The dielectric of the capacitor consists of either the material itself if non-conductive, or by insulation on the probe if measuring a conductive medium. Level changes in the material may be correlated with changes in capacitance, which are then transformed in a 4 to 20mA analog signal by the electronics in the probe head.