Continuous Liquid Level Measurement Technologies Used in Industry

Magnetic level indicator coupled with
guide wave radar level transmitter
Courtesy Vega

Although continuous level measurement technologies have the ability to quantify applications for bulk solids, slurries, and granular materials, liquid level technologies stand out as being exceptionally crucial to the foundation of process control. Called “transmitters,” these continuous liquid level measurement devices employ technologies ranging from hydrostatics to magnetostriction, providing uninterrupted signals that indicate the level of liquid in a vessel, tank, or other container.

Hydrostatic devices focus on the equilibrium of dynamic and static liquids. There are three main types of hydrostatic transmitters: 1) displacer, 2) bubbler, and 3) differential pressure.

The displacer transmitters utilize a float placed within the liquid container. With its buoyancy characterized to the liquid and the application, the float, a connecting stem, and a range spring or similar counterbalance represents the liquid level in terms of the movement of the displacer (float). The displacement, or movement, of the assembly is converted into an electric signal for use by the monitoring and control system.

Bubbler transmitters are used for processing vessels that operate at atmospheric pressure. This method introduces a purge gas or an inert gas, e.g. air or dry nitrogen, into a tube extending into the liquid vessel. Precise measurement of the pressure exerted on the gas in the dip tube by the liquid in the tank is used to determine the height of the liquid.

Differential pressure (DP) transmitters rely directly on, in a basic explanation, the pressure difference between the bottom and top of the container. Precise pressure measurement is used to determine the height of the liquid in the tank. One of the most advantageous aspects of DP transmitters is that they can be used in pressurized containers, whereas displacer and bubbler transmitters cannot.

Other examples of level transmitter technologies––which are not hydrostatic devices––are magnetostrictive, capacitance, ultrasonic, laser, and radar.

In magnetostrictive level transmitters the measuring device, a float, has a series of magnets that create a magnetic field around a wire enclosed in a tube. Electrical pulses sent down the wire by the transmitter head product a torsional wave related to the position of the float, which moves with changes in liquid surface level. The transit time of the torsion wave back to the sensing head is measured and the depth of the liquid, as indicated by the float position, can be determined.

Capacitance transmitters are best applied to liquids that have high dielectric constants. Essentially, changes in the capacitance of the sensor / tank / liquid assembly will vary proportionately with the liquid level. The change in capacitance is measured and converted to an appropriate electrical signal.

Ultrasonic level transmitters emit ultrasonic energy from the top of the vessel toward the liquid. The emissions are reflected by the liquid surface and them time required for the signal to return to the source is used to determine the distance to the liquid surface.

Laser level transmitters operate similarly to an ultrasonic level transmitter. However, instead of using ultrasound signals, they use pulses of light.

Radar level transmitters involve microwaves emitting downward from the top of the container to the liquid’s surface and back again; the measurement is the entire time-frame. One variable radar level measurement echoes capacitance measurements: they both involve dielectric contact of liquid.

The precise measurement of transmit time for a wave or pulse of energy is employed in several of the technologies, the measurement of pressure in others. Each technology has a set of attributes making it an advantageous selection for a particular range of applications. Share your liquid level measurement challenges with an application expert, combining you process knowledge with their product application expertise to develop effective solutions.

Level Measurement - Simple Accurate Bubbler Method

Instrument layout for level
measurement

Measuring liquid level in a tank or vessel can be accomplished in a number of ways, all of which require some arrangement of instrumentation to either infer the liquid level from the measurement of a related physical property, or directly deliver the liquid level visually using a scaled gauge arrangement. One indirect method of level measurement is often referred to as the bubbler method, so named because it employs a purging gas that continually vents from the bottom of a tube extending into a tank of liquid. Through a simple apparatus, the level of a liquid can be inferred by the amount a back pressure exerted upon the gas flowing through the tube.

Probably the greatest advantage of this method of liquid level measurement is that the liquid does not contact the sensing instrumentation. The only portion of the apparatus in contact with the liquid is a tube immersed into the tank. Basically, a purge gas flows through the immersion tube and may bubble out the immersed end of the tube, which is open to allow the contained liquid to exert a hydrostatic pressure on the purge gas. The back pressure on the gas that is exerted by the liquid contained within the tank will vary directly with the depth of the liquid. The back pressure can be correlated to a liquid level. The accuracy of the measurement is related to the proper regulation of the purge gas (explained in the application note below) and the measurement capability of the pressure transmitter. Once the depth of the liquid is derived, further calculations, employing tank shape, dimensions, and the liquid density can provide an indication of the volume and mass of the liquid. Here is an illustration of the setup, provided courtesy of Yokogawa, a recognized leader in flow and pressure measurement with a global presence. Included below, an application note from Yokogawa on applying rotameters to the setup, as well as data sheets for the instruments employed in the illustrated apparatus.

Share your level measurement challenges of all types with the application specialists at Classic Controls. Combining your process expertise with their product application knowledge will yield the best process measurement and control solutions.

Bubbler Method Liquid Level Measurement from Classic Controls, Inc.

Differential Pressure Transmitter From Yokogawa from Classic Controls, Inc.

Rotameter Variable Area Flowmeters from Classic Controls, Inc.