A comparison of fluid level measurement technologies and how they work

The demands of sophisticated automated processing systems, the need for ever-tighter process control and an increasingly stringent regulatory environment drive process engineers to seek more precise and reliable level measurement systems. This article looks at how these demands are being met by advances in level measurement technologies offering improved accuracy and reliability.

Level measurement technology in transition

Probably the most well-known industrial level measuring device is the sight glass, which for a long time served as the method by which operators assessed process conditions.

As a manual approach to measurement, however, sight glasses have always presented a number of limitations.

As a manual approach to measurement, however, sight glasses have always presented a number of limitations. The material used for transparency can suffer catastrophic failure, with ensuing environmental impact, hazardous conditions for personnel and or fire and explosion. Their seals are prone to leaks whilst build-up, if present, can obscure the visible level. These, plus other drawbacks, have seen the steady replacement of conventional sight glasses with more advanced technologies.

Established level sensing technologies

In explaining the various techniques available for measuring level, this article will assume the density of the vapour in the headspace to be negligible compared with that of the process fluid. We will also assume there is only one, uniform process fluid in the tank, although some of the technologies detailed can be used for multilevel applications where two or more immiscible fluids share a vessel.

Floats

Floats work on the simple principle of placing a buoyant object with a specific gravity intermediate between those of the process fluid and the headspace vapour into the tank, then attaching a mechanical device to read out its position. The float sinks to the bottom of the headspace vapour and floats on top of the process fluid. While the float itself is a basic solution to the problem of locating a liquid's surface, reading a float’s position is still problematic. Early float systems used mechanical components such as cables, tapes, pulleys and gears to communicate level. Magnet-equipped floats are popular today.

Early float level transmitters provided a simulated analogue or discrete level measurement using a network of resistors and multiple reed switches, meaning that the transmitter's output changes in discrete steps. Unlike continuous level measuring devices, they cannot discriminate level values between steps.

Hydrostatic devices

Displacers, bubblers, and differential pressure transmitters are all hydrostatic measurement devices.

Displacers work on Archimedes' principle. A column of solid material (the displacer) is suspended in the vessel. The displacer's density should always be greater than that of the process fluid so that it will sink in the process fluid and it must extend from the lowest level required to at least the highest level to be measured. As the process fluid level rises, the column displaces a volume of fluid equal to the column's cross-sectional area multiplied by the process fluid level on the displacer. A buoyant force equal to this displaced volume multiplied by the process fluid density pushes upward on the displacer, reducing the force needed to support it against the pull of gravity. The transducer, which is linked to the transmitter, monitors and relates this change in force to level.

A bubbler-type level sensor is used in vessels that operate under atmospheric pressure. A dip tube with its open end near the vessel bottom carries a purge gas, typically air, into the tank. As the gas flows down to the dip tube's outlet, the pressure in the tube rises until it overcomes the hydrostatic pressure produced by the liquid level at the outlet. That pressure equals the process fluid's density multiplied by its depth from the end of the dip tube to the surface and is monitored by a pressure transducer connected to the tube.

A differential pressure (DP) level sensor measures the difference between total pressure at the bottom of the tank and the static or head pressure in the vessel. As with the bubbler, the hydrostatic pressure difference equals the process fluid density multiplied by the height of fluid in the vessel.

A vent at the top keeps the headspace pressure equal to atmospheric pressure. In contrast to bubblers, DP sensors can be used in pressurised vessels. All that is required is to connect the reference port to a port in the vessel above the maximum fill level. Liquid purges or bubblers may still be required, depending on the process's physical conditions and / or the transmitter's location relative to the process connections.

Load cells

A load cell or strain gauge device is essentially a mechanical support member or bracket equipped with one or more sensors that detect small distortions in the support member. As the force on the load cell changes, the bracket flexes slightly, causing output signal changes.

To measure level, the load cell must be incorporated into the vessel's support structure. As process fluid fills the vessel, the force on the load cell increases. Knowing the vessel's geometry and the fluid’s specific gravity, it is straightforward to convert the load cell's known output into the fluid level.

Although advantageous in many applications because of their non-contact nature, load cells are expensive and the vessel support structure and connecting piping must be designed around the load cell’s requirements of a floating substructure. The total weight of the vessel, piping, and connecting structure supported by the vessel will be weighed by the load cell system in addition to the desired net or product weight. This total weight often creates a very poor turndown to the net weight, meaning that the net weight is a very small percentage of the total weight.

Load cell weighing system requirements must therefore be a paramount consideration throughout initial vessel support and piping design in order to avoid performance being degraded.

Magnetic level gauges

Similar to float devices, magnetic level gauges communicate the liquid surface location magnetically. The float, carrying a set of strong permanent magnets, rides in an auxiliary column attached to the vessel by means of two process connections. This column confines the float laterally so that it is always close to the chamber's side wall. As the float moves with the fluid level, a magnetised shuttle or bar graph indication moves with it, showing the position of the float and providing the level indication. The system can work only if the auxiliary column and chamber walls are made of non-magnetic material.

Many manufacturers provide float designs optimised for the specific gravity of the fluid being measured as well as a large selection of float materials, enabling the gauges to handle high temperatures, high pressures, and corrosive fluids. Where build-up is anticipated, oversized float chambers and high-buoyancy floats are also available.

Where required, chambers, flanges, and process connections can be made from engineered plastics such as Kynar or exotic alloys such as Hastelloy C-Z76. Special chamber configurations can also be devised to handle extreme applications.

Numerous metals and alloys such as titanium, Incoloy and Monel are available for varying combinations of high temperature, high pressure, low specific gravity, and corrosive fluid applications. Today's magnetic level gauges can also be outfitted with magneto-restrictive and guided-wave radar transmitters to allow the gauge's local indication to be converted into 4-20 mA outputs that can be sent to a controller or control system.

Capacitance transmitters

These devices operate on the basis that process fluids generally have dielectric constants (Er), significantly different from that of air, which is very close to 1.0.

This technology requires a change in capacitance that varies with the liquid level, created by either an insulated rod attached to the transmitter and the process fluid, or an uninsulated rod attached to the transmitter and either the vessel wall or a reference probe. As the fluid level rises and fills more of the space between the plates, the overall capacitance rises proportionately. An electronic circuit called a capacitance bridge measures the overall capacitance and provides a continuous level measurement.

The trend today is to replace mechanical and pressure-based measurement tools with systems that measure the distance to the fluid surface by a timing measurement

New technologies

The trend today is to replace mechanical and pressure-based measurement tools with systems that measure the distance to the fluid surface by a timing measurement, commonly termed as ‘time of flight’. Put simply, this technique operates by measuring the distance between the liquid level and a reference point at a sensor or transmitter near the top of the vessel. A pulse wave generated at the reference point travels through either the vapour space or a conductor, reflects off the liquid surface, and returns to a pickup at the reference point. An electronic timing circuit measures the total travel time. By dividing the travel time by twice the wave's speed, the distance to the surface of the liquid can be derived.

Magnetostrictive, ultrasonic, laser and guided-wave radar transmitters are among the most versatile technologies available. Such systems use the sharp change of a given physical parameter at the process fluid surface, such as density, dielectric constant and sonic or light reflection, to identify the level. These technologies make use of the latest electronic techniques and incorporate embedded microprocessor-based digital computers for control, analysis, and communication functions.

Magnetostrictive level transmitters

The advantages of using a magnet containing a float to determine liquid level have already been established. Instead of mechanical links, magnetostrictive transmitters use the speed of a torsional wave along a wire to find the float and report its position. In a magnetostrictive system, the float carries a series of permanent magnets. A sensor wire is connected to a piezoceramic sensor at the transmitter and a tension fixture is attached to the opposite end of the sensor tube. The tube either runs through a hole in the centre of the float or is adjacent to the float outside of a non-magnetic float chamber.

To locate the float, the transmitter sends a short current pulse down the sensor wire, creating a magnetic field along its entire length and simultaneously activating a timing circuit. The field interacts immediately with the field generated by the magnets in the float. The overall effect is that during the brief time the current flows, a torsional force is produced in the wire, much like an ultrasonic vibration or wave. This force travels back to the piezoceramic sensor at a characteristic speed. When the sensor detects the torsional wave, it produces an electrical signal that notifies the timing circuit that the wave has arrived and stops the timing circuit. The time interval between the start of the current pulse and the wave's arrival is then measured. Using this information, the float's location can be very precisely determined and presented as a level signal by the transmitter.

Key advantages of this technology are that the signal speed is known and constant with process variables such as temperature and pressure, and the signal is not affected by foam, beam divergence, or false echoes. Another benefit is that the only moving part is the float that rides up and down with the fluid's surface.

Ultrasonic level transmitters

Ultrasonic level sensors measure the distance between the transducer and the surface using the time required for an ultrasound pulse to travel from a transducer to the liquid surface and back. These sensors use frequencies in the tens of kilohertz range, with transit times of 6 ms/m. The speed of sound depends on the mixture of gases in the headspace and their temperature. While the sensor temperature is compensated for – assuming that the sensor is at the same temperature as the air in the headspace – this technology is limited to atmospheric pressure measurements in air or nitrogen.

Laser level transmitters

Designed for bulk solids, slurries, and opaque liquids such as dirty sumps, milk, and liquid styrene, lasers use the speed of light to find the level. A laser transmitter at the top of a vessel fires a short pulse of light down to the process liquid surface, which reflects it back to the detector. A timing circuit measures the elapsed time and calculates the distance. The key is that lasers have virtually no beam spread and no false echoes, and can be directed through spaces as small as two square inches. Lasers are very precise, even in vapour and foam. They are ideal for use in vessels with numerous obstructions and can measure distances up to 1,500 ft.

For high temperature or high pressure applications, such as in reactor vessels, lasers must be used in conjunction with specialised sight windows to isolate the transmitter from the process. These glass windows must pass the laser beam with minimal diffusion and attenuation and must contain the process conditions.

Radar level transmitters

Through-air radar systems beam microwaves downward from either a horn or a rod antenna at the top of a vessel. The signal reflects off the liquid surface back to the antenna, and a timing circuit calculates the distance to the liquid level by measuring the round-trip time.

A drawback of through-air radar systems is that the radar waves suffer from the same beam divergence that afflicts ultrasonic transmitters. Internal piping, deposits on the antenna, and multiple reflections from tank build-up and obstructions can all cause erroneous readings. To overcome these problems, complex algorithms using fuzzy logic must be incorporated into the transmitter. Transmitter set-up can be tedious and any changes in the process environment, such as build-up, can be problematic.

These problems can be overcome by using guided wave radar (GWR) systems, whereby  a rigid probe or flexible cable antenna system guides the microwave down from the top of the tank to the liquid level and back to the transmitter. As with through-air radar, a change from a lower to a higher dielectric constant  causes the reflection.

Guided wave radar exhibits many of the advantages and few of the liabilities of ultrasound, laser, and open air radar systems

As the guide provides a more focused energy path, guided wave radar is 20 times more efficient than through-air radar. Different antenna configurations allow measurement down to dielectric constants of 1.4 and lower. Moreover, these systems can be installed vertically, or, in some cases horizontally, with the guide being bent up to 90° or angled to provide a clear measurement signal.

GWR exhibits many of the advantages and few of the liabilities of ultrasound, laser, and open air radar systems. Radar's wave speed is largely unaffected by vapour space gas composition, temperature, or pressure. It works in a vacuum with no re calibration needed, and can measure through most foam layers. Confining the wave to follow a probe or cable eliminates beam-spread problems and false echoes from tank walls and structures.

Summary

Refined digital electronics are making level sensors and other measurement devices increasingly accurate, reliable, easy to use and less costly to purchase and own than ever. Improved communication interfaces are also offering expanded opportunities for relaying measurement data into control and/or information systems.

With the increasing variety of materials and alloys capable of use in harsh environments, level instruments are also able to be used in virtually any application.

For more information, email enquiries.mp.uk@gb.abb.com or call 0870 600 6122 ref. ‘Level technology’.