Continuous Liquid Level Measurement Technologies Used in Industry

Pressure measurements can be utilized to determine liquid level
Courtesy Azbil

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 many facets of the process control industrial sphere. Called “transmitters,” these continuous liquid level measurement devices employ technologies ranging from hydrostatic 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
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.

Guided wave radar liquid level transmitter
joined with magnetic liquid level gauge
Courtesy Jerguson

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.

Liquid Level Measurement With Differential Pressure

Flange mount differential pressure transmitter
Courtesy Azbil North America

Industrial process operations frequently involve the use of liquid storage tanks for raw materials, in-process liquids, or finished product. Regardless of the tank usage, the data point indicating the liquid level in the tank is useful for effective process operation and safety.

Deriving tank liquid level can be accomplished utilizing a number of technologies, classed as direct or inferred. Direct level readings indicate level by interacting with the interface of the liquid surface and the unfilled area of the tank. A simple example of this would be a float, the position of which is read on a scale or transmitted to a controller. Inferential measurements are derived by directly measuring a condition which is affected in a predictable manner by the tank liquid level.

Differential pressure is an inferential methodology for determining liquid level. A differential pressure transmitter measures the total pressure of the fluid column and subtracts the pressure exerted on the surface of the liquid, delivering a signal indicating the net pressure exerted by the liquid column in the vessel. This value can be used to calculate the height of the liquid column, weight of the fluid, percentage of tank fill, or other useful values. The differential pressure method of liquid level measurement has some key features:

• Cost effective
• Limited moving parts in a complete installation, generally consisting of isolation valves for transmitter servicing
• Best applied to clean liquids with known uniform density
• Provides good accuracy over a wide operating range
• Easily mounted or retrofitted to vessel surfaces
• Low maintenance requirements
• Specifically designed seals can be utilized to provide hygienic vessel connections, or device protection from high temperature or corrosive process media

Like any measurement technology, the differential pressure method of liquid level measurement provides the best results when properly installed and maintained. There are numerous installation configurations and options to fulfill a wide variety of applications. A product application specialist can help you specify the best arrangement for your application.

Azbil Industrial Pressure Transmitters from CTi Controltech