Liquid Level Measurement Techniques and How They Work

liquid-level

The relevance of level measurement in several industries such as the Food and Beverage Industry, Petrochemical Plants, and Water Reservoirs on measuring tanks, canals, wet wells, ponds and weirs cannot be overstated. Incorrect measurements can result to distressing troubles where high levels can cause overflow and create safety problems, whereas low levels can cause damaged equipment. Hence, when working in an industry setting, ensuring accurate level measurements can save firms with time and money, and can optimize the performance of systems and processes.
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Liquid Level Measuring Technologies

Following the importance of accurate level measurement, there are now a myriad of unique methods developed for doing the process. However, with these many options, selecting what works though for a particular application can be a daunting task.

With an array of competent brands offering level measurement technologies, all the systems you’ll see in the market will seem reliable. But not all gauges and sensors are equal with each having their own particular advantages and disadvantages.

Every level measurement system rely on certain characteristics. As for liquid materials, it will be density, capacitance, pressure, and temperature. It is then important to be familiar with the fundamental mechanics of the different liquid level measurement techs you are considering to check if it is the appropriate method for your particular application.

Liquid Level Measurement Sensors and Gauges

Sensors and gauges are the most common devices used in level measurement these days. They come with important functions in different industries and the fact that they deliver utmost accuracy, compared to manual measurement techniques, they can ensure that all processes and operations in firms will run without a glitch.

All liquid level sensors and gauges function to monitor and control liquid levels in reactors, tanks and other vessels essential in industrial process applications. These devices also provide measures in maintaining the levels where time-to-time readings are being transmitted to a monitor like those that are used in King-gage systems.

The sectors where liquid level measurement systems are utilized are so varied. To better address to each industry’s unique needs, several types were developed and they can be: Hydrostatic pressure, Radar, Ultrasonic, Capacitance, Magnetic, and the Sight Gauge among the most commonly used devices.

Hydrostatic Pressure
hydrostatic-pressure
Among the level devices, the hydrostatic transmitters are the most cost-effective and loop-powered that can measure the liquid level accurately and consistently. They come in a compact size, which is ideal for fitting into pipes with smaller sizes. And because of these device’s simple installation, proven reliability, and high robustness, more and more industries are opting to integrate their monitoring and control systems with the hydrostatic pressure.

Some of the major advantages to using the hydrostatic pressure level include:

  • Sensors are in direct contact with the liquid, hence delivering more accurate reading.
  • Long proven history of reliable measuring principle.
  • Alternative sensor technologies and designs available to serve almost any application. Variations include Bubbler, Submersible pressure, and
  • Independent system where measurement can’t be influenced by existing installed equipment and vessel geometry and other medium characteristics like dielectric coefficient, conductivity and viscosity.
  • Robust measuring processes unaffected by external factors such as buildup, contaminants, dust, foam, vapor, etc., resulting to highly precise data.

Limitations:

  • Unsuitable for bulk material
  • Accurate measurement requires either media with constant density or continuous density measurement of the medium

BUBBLERS FOR WATER LEVEL MEASUREMENT

Bubblers measure water levels based on the amount of pressure it takes to push an air bubble out of an orifice line into the liquid body. This pressure called “line pressure” requires changes with the elevation of the water.  While the elevation rises or falls, so does the line pressure needed to discharge bubbles. Consequently, the line pressure value (in psi) will be converted into the desired units of measurement to represent water level from the point of discharge to the water’s surface.

Simply, the pressure measured at the bottom of a tank of liquid is proportional to the level of liquid in the tank according to the relationship: Height of liquid = Pressure at bottom of tank / density of liquid.

The major benefits of the continuous bubble are isolation, response, reliability, and accuracy. Any rise or fall of water level (even to the slightest), the Bubbler can accurately sense this change, and within 30 seconds adjusts the tank pressure to keep the line pressure continually following the current water level. Its accuracy ranges from 0.2% to 0.075%. Ask any experienced operator, and they will probably give two thumbs up to this versatile device.

SUBMERSIBLE TRANSMITTER FOR WATER LEVEL MEASUREMENT

Submersible pressure transmitters are notable for their accuracy of 0.5 % and long-term stability which all contribute to the operational reliability of the transmitter. It measures pressure at the bottom of a tank or any vessel to determine liquid level. However, if you choose to use this sensor, you should opt for the one that’s robust enough to deal with the turbulence and debris that is often present at the bottom of containers.

A vent tube is typically attached to submersible transmitters to eliminate the impacts of ambient or atmospheric pressure outside of the container on the sensor reading. This increases the accuracy of the reading.

Although it is a popular choice for continuous measurement in lift stations, the above mentioned factors make more people choose to use the bubbler for its simpler operation than the submersible. Also, submersible pressure transducers can be exposed to a very nasty environment- at the bottom of deep tanks or wells. Because of this, the idea of pulling that sensor for maintenance of any type is far from appetizing.

But this is not the only nasty thing about submersible sensors. When a pump is involved, the liquid can be placed under some pretty serious turbulence. This puts a risk on the transmitter getting damaged. It is for these reasons that most turn to bubbler transmitters when choosing to use hydrostatic pressure transmitters.

Radar Fill Level Transmitters
radar-fill-level
Radar fill, as it name suggests, use radio wave emissions in measuring levels. Devices are usually mounted at the top of a tank filled with a liquid. The radar fill transmitter will send a signal into the liquid and receives a reflection of the signal in return. The system will then asses the current fill level depending on the time taken by the transmitted signal to return. For instance, if the time required to receive the signal is longer, then the distance between the sensor and the liquid is farther. This goes down to the conclusion that the liquid level in the container is low.

The major advantages in using radar fill are:

  • The analyzation of data of the tech is accurate
  • Minimal contact with the substance being measured means lesser maintenance required
  • Easy installation and accessibility

Radar level transmitters are often considered more accurate than other sensors but there are some drawbacks to using them too which include:

  • Installation hassles depending on the tank design
  • Accuracy is compromised when there is foam build-up from liquid substances
  • Blocking distance
  • Turbulent surfaces may confuse readings

Ultrasonic Fill Level Transmitters
ultrasonic-fill-level
Ultrasonic fill level transmitter operates similarly to radar fill level transmitter. It is also typically mounted at the top of a container but instead of radar waves, it sends out an ultrasonic pulse. This pulse travels until it hits the surface of the liquid, and gets reflected. The sensor computes the fill level depending on the time between the sent and the received signal. As the speed of sound can vary based on the gas mixtures present, it is important to note what gases are present in the tank while using the ultrasonic fill transmitters.

There are not much advantages to using the ultrasonic fill level transmitter, but some notable functions of this technology include:

  • It can measure level without making any physical contact with the substance being measured.
  • The precision of the reading remains unchanged even after alterations in the chemical composition.

Ultrasonic level sensors may be inappropriate for certain materials which hamper the quality of a sensor’s received echo. Instances like the following may require the use of other types of sensors:

  • Tank materials may absorb the ultrasonic sound waves rather than reflecting them
  • Very high tank depths may result to inaccurate readings

Capacitance Level Transmitters
capacitance-level
Capacitance level transmitters work on the principle of variation of electrical capacitance of a capacitor formed by the sensor, dielectric material, and vessel wall. The capacitors come with two conductive plates, which are separated from each other by a dielectric. Reading is done by measuring the change in capacitance which is directly proportional to the liquid level. However, it will be necessary to know the dielectric constant – significant factor in level measurement application – of the liquid to use this type of transmitter.

Capacitance level transmitters offer the following advantages:

  • Mechanically simple in structure with no moving parts
  • Minimal maintenance is required
  • Resistant to turbulence

And the notable disadvantage to this technology:

  • Not suitable for low dielectric material

Magnetic Level Transmitters
magnetic-level
Magnetic level transmitters use a small, highly magnetic object, which is suspended in a buoyant float, inside a narrow auxiliary column to restrict the lateral movement of the float. This float will rest on the liquid surface, and a magnetic device will track the movement of the float. This will help the user to read the current fill level. Such method is useful for continuous measurement, since the float will sink or rise depending on the liquid level in the tank. However, it will only work if the tank, auxiliary column, and float are made of non-magnetic materials.

Here are some of the advantages of magnetic level transmitters:

  • Can be easily retrofitted into current level chambers
  • Easy process takeoffs with less connections
  • It continues to operate even in an event of electrical power failure
  • Gauge, transmitter, and switches use the same hardware/float
  • Transmitter and switches can be calibrated while the system is still online
  • Ease of mounting

And the drawbacks:

  • Water must contain a certain amount of Microsiemens (uS), giving it the power to conduct heat and/or electricity. Some magmeters can work down to 2-3 uS/cm, while other require 10 uS/cm or more.
  • Accuracy may be affected by air space in the pipe

Sight Glasses
sight-glass
Every technology has an ancestor and when it comes to level measurement, the sight glass or glass gauge is the simplest and oldest. A manual approach to measurement, sight glasses have always had a number of limitations.

Liquid level is measured by sight glass technique where the sight glass level mirrors the level inside the tank. Reader will then manually collect the data. Remote reading is also possible where the operator in the control room monitors the level with the help of CCTV system. The OCD camera is focused at the sight glass and viewed in the control room through a monitor.

This can result to less accurate readings. The material used for its transparency can also suffer catastrophic failure from ensuing environmental insult, hazardous conditions for personnel, fire, and explosion. Following these, sight glasses are rapidly replaced by more advanced technologies.

Advantages of sight glass technique:

  • Manual and direct reading is possible
  • Special designs are available for use up to 3000’c and 650 kg/cm^2.
  • Glassless designs are available in numerous materials for corrosion resistance

Disadvantages of using the sight glass:

  • Reading is done where the tank is located which is not always convenient
  • Since sight glasses are located outside the tanks, liquid in the sight glass may freeze in the cold, thus causing reading errors
  • Heavy, viscous liquids that fall out of solution and clog the tube cannot be measured by sight glass
  • Overlapping gauges might be required long level spans
  • Accuracy and readability depends on cleanliness of the sight glass and fluid, and the measurement reading skills of the reader

 

So What’s the Takeaway?

The trend in the measurement technology determines which rules the market. Most industries prefer simplistic systems but accurate, which brings us to the option of using the hydrostatic pressure sensors.

Refined digital electronics connected to the system for reading are making level sensors more user-friendly, more reliable, easier to set up, and less expensive. This improved communication interfaces feed level measurement data into a company’s existing control and/or information system.

Today’s process applications use an increasing variety of materials so a good sensor should be able to withstand harsh environments like acids and oils. It should also be able to combat external factors like extremes of temperature pressure. The different sensors are now being created with new materials to help strengthen their make and fulfill their tasks and specialized requirements.

Indeed, not all sensors are created equal. Choosing the best among these types boils down to your preference and your application process’ requirements and conditions.

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