Continuous Liquid Level Measurement Technologies Used in Industry

Process measurement connectivity with wireless technology
is suitable for almost any application 

Although continuous level measurement technologies have the ability to quantify applications for bulk solids, slurries, and granular materials, this article will focus on level measurement technologies applied to liquid level measurement utilized in process control. A distinction should be made between continuous level measurement and point level measurement. Point level measurement acts like a switch, changing state when a single level condition is achieved. Called “transmitters,” 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.

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

In magnetostrictive level transmitters the measuring device, a float, has a magnet that creates a magnetic field around a wire enclosed in a tube. Electrical pulses sent down the wire by the transmitter head produce 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. The measured time for receipt of a return sign reflecting off the liquid surface enables calculation of the distance from the sensor to the liquid surface.

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.

Analynk Wireless provides connectivity solutions enabling the wireless transmission of measurement data across the room, across the property, across the globe. Share your process measurement and control connectivity challenges with the experts at Analynk for a solution that is simple to implement and cost effective.

Magnetic Level Indicators For Tank Level Measurement

Magnetic Level Indicator
Courtesy Orion

The magnetic level indicator (MLI), also called a magnetically coupled liquid level indicator or a magnetic level gauge, is in wide-spread use throughout process industries around the world. Originally designed as an alternative to sight glass gauges, MLIs are now commonly specified in new construction and plant expansions.

Principle of Operation

Magnetic level indicators use magnetic force couple the position of a float to a scale that indicates actual liquid level in a tank. The float movement and scale can be used to activate a switch or provide continuous level data via a transmitter. Unlike a sight glass, magnetic coupling allows the MLI to indicate liquid levels without direct contact between the externally-mounted visual indicator and the fluid in the vessel.

A magnetic field consists of imaginary lines of flux originating from the north and south poles and completely surround the magnet. This field acts on other objects (magnets or ferromagnetic materials) through these forces. When a magnetic field acts upon another body with sufficient force to influence it, the pair are said to be magnetically coupled to each other. In the case of a magnetic level indicator, the float is magnetically coupled to the indicator scale.

The MLI float, located inside the chamber and specifically engineered to provide proper buoyancy in the targeted fluid, dynamically tracks the surface of the liquid as it rises and falls. The magnet assembly inside the float generates a magnetic field that penetrates through the chamber wall to couple with the visual indicator.

Typical applications include:

• Alkylation units 
• Boiler drums
• Feedwater heaters
• Industrial boilers
• Oil / Water separators
• Process vessels
• Propane vessels
• Storage tanks
• Surge tanks
• Wastewater tanks

Advantages of the MLI

A magnetic level indicator is often used in applications where a sight glass (or glass sight gauge) is unsafe, environmentally risky, or difficult to see.

Typical shortcomings of glass sight gauges include:

• High pressures, extreme temperatures, deteriorating seals, and toxic or corrosive materials may cause a risk of fugitive emission of dangerous substances. 
• Some chemical materials within a process vessel or storage tank can attack the glass, causing discoloration of the sight gauge, thus decreasing level visibility. 
• Liquid/liquid interfaces can be very difficult to read in a sight glass particularly if the liquids are of similar color. Clear liquids can also be difficult to see in a sight glass. 
• Liquids that tend to coat or build-up on surfaces can hinder visibility by forming an opaque film on the glass. 
• To cover a large measuring span, sight glass assemblies typically must be staggered using multiple sections. 

Key reasons for selecting an MLI over a sight glass are:

• Improved safety due to the absence of fragile glass and a substantially reduced number of potential leak points. 
• Greatly increased visibility 
• Reduced maintenance. 
• Easier initial installation and addition of transmitters and switches without interrupting the process 
• Lower long-term cost of ownership and legitimate return-on-investment benefits. 
• Single chamber measurement over 20 ft. (6 m) without staggering chambers.

When using a transmitter in conjunction with a magnetic level indicator, consider using a wireless connection to deliver the process signal from the transmitter to the monitoring and control unit. Analynk has effective solutions that can be easily implemented. A video shows an application example.

Introduction to Level Measurement

In many industrial processes, the measurement of level is critical. Depending on the nature of the material being measured, this can be a simple or complex task. Several different technologies for sensing level are briefly explained here.

Level Gauges or Sightglasses

Sight Glass or Gauge

The simplest form of level measurement for direct measurement of level (almost always visually) in a vessel. A level gauge (sightglass) is usually a clear tube connected to the a vessel at the highest and lowest part of the level range. The fluid level inside the vessel will be at the same hight as the level in the tube.

Floats

Float level indicator

Another very simple approach to level measurement for fluids or solids is the float. The float sits on
top of the material being measured and is visually, magnetically, or electronically located and equated to the level inside the vessel. It is important that the float material be compatible with the process media and that it freely moves on top of the process.

Hydrostatic Pressure

Hydrostatic Level

A very popular way to measure level because of the ease in equating the pressure of a fluid column with the level inside the vessel. In it's simplest form, a pressure sensor (gauge or transmitter) is attached to the bottom of a vessel and measures the pressure of the column. This pressure reading is then interpreted as level.

Bubbler Principle

Bubbler Systems

A variation of the hydrostatic pressure method, bubbler systems measure the pressure of a purge gas being injected into the fluid in a vessel through a dip tube. This approach comes in handy when sensing the level of corrosive fluids. The principle of operation is that the amount of pressure to "push" an inert purge gas through the dip tube will change according to the level in that vessel, and therefore can be correlated to the level.

Displacer Level

Displacer Level

Based upon the laws of buoyancy, a float (either inside its own isolated cage, or hanging in the process directly) is calibrated the the level of the fluid being measured. The displacer is usually a sealed metal tube and hang's in place in the process media. As more of the displacer’s volume becomes submerged, the buoyant force is increased on the making the displacer "lighter".

Echo level measurement

Echo (Ultrasonic, Radar, Laser)

Level measured by bouncing some wave form (sound, light, electromagnetic) off the surface of liquids and measuring their time of flight.

Capacitance level measurement

Capacitance Level

Capacitive level instruments measure the electrical capacitance of a conductive rod inserted vertically into a process vessel. As process level increases, the capacitance between the rod and the vessel walls increases, causing a signal change in the instruments circuitry.

Weight

Level is measured by knowing the empty weight of a vessel and the full weigh of a vessel and calibrating the points between. The shape of the vessel is can also be a factor.

Industrial level control requires deep knowledge and understanding of many process variables, such as media compatibility, interfaces, head pressures, material densities, and mechanical considerations. It's always recommended that an experienced consultant be involved with the selection and implementation of any industrial level device.

Image attribution: courtesy of "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt