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Tutorial on level measuring techniques for liquid and fluid

There are so many different technologies for measuring fluid/liquid level, selecting the most appropriate one can be extremely difficult and time-consuming. Based on Dwyer Instruments’ extensive knowledge and expertise on Level measuring technologies, we have put together this tutorial to assist in your specification decisions.

The four major level measuring technologies for fluid & liquid applications are; Ultrasonic, Capacitance, Submersible and Radar. This tutorial will give an overview of these technologies, including the advantages and disadvantages in an application context.

Ultrasonic Level Sensors

How Do Ultrasonic Level Sensors Work?

Ultrasonic level sensors

Ultrasonic level sensors work by emitting high frequency acoustic signals that are reflected back and detected by the unit. The transit time of the signal, from the sensor to the target and back again, is correlated to level.

Advantages of Ultrasonic Level Sensors

  • Provides a non-contact measurement – eliminates compatibility concerns
  • Great for measuring level of viscous fluid
  • Full measuring range is programmablFeatures a high F.S. accuracy  

Disadvantages of Ultrasonic Level Sensors

  • Not suitable to high turbulence applications or applications with steam or foam – these applications would prevent the sound wave being properly reflected to the sensor.
  • Not suitable for highly pressurised tanks – Ultrasonics can be affected by the changing coefficient of sound due to moisture, temperature or pressure.

So as long as it is understood that due to the characteristics of sound waves, when there is potential to change the sound wave, there could be inaccuracies and lost signal. Correction factors can be applied to improve the accuracy of measurement. Dwyer offer their Series UTC and UTS with programming features to account for any disrupting/changing factors.

Have a look at this product tutorial on the Series ULB Ultrasonic Level Transmitters:

Capacitance Level Transmitters

How Do Capacitance Level Transmitters Work?

Capacitance units utlise low radio frequency to measure the conductivity of current in a closed circuit that varies proportionally to the level of the application. Capacitance is a function of the dialectic constant of the fluid, the surface area of the capacitor, the probe and the separation distance – it is important all constants remain the same, with only the level of the media changing.

Advantages of Capacitance Level Transmitters

  • Vertical mounting making them easier to install
  • FEP coating increases chemical compatibility
  • Units work well in liquid with solids

Disadvantages of Capacitance Level Transmitters

  • In applications with non-metallic tanks or tanks with irregular shaped walls a ‘probe ground reference’ must be used and purchased beforehand.
  • The dielectric constant of the measuring media must be greater than 3mF and must be used for measuring heights greater than 6 for water.
  • As capacitance is a product of surface area, it is recommended that the probe should be greater than 24” in length.

Dwyer’s CRF2 Series is available with a rigid or flexible probe depending on the application, it is vertically mounted, has FEP coating and can be fitted with a customized probe length.

Submersible Level Transmitter

How do Submersible Level Transmitters work?

Submersible level sensors

Based on the principle that fluids expert a pressure that is a function of height.

Submersible level sensors work  by measuring that hydrostatic pressure formed by the water column directly located above the unit and output a linear 4-20mA output proportional to level.

Advantages of Submersible level Transmitters

  • Suited to applications that contain vapour, foam or any form of agitation

Disadvantages of Submersible Level Transmitters

  • Limited to applications with low viscosity liquids with a specific measuring range
  • Can only be used in non-pressurised tank applications

It is important to note, the vent tube of the unit must be properly vented to atmosphere, clear of any obstructions and free of any moisture that damage electrical components. Dwyer’s units are supplied with a hydrophobic, Teflon filter to avoid any moisture build up.

Guided Wave Radar Level Sensors

How do Radar Level Sensors work?

This technology utilizes Time Domain Reflectometry (TDR) – fast-paced impulses are transmitted along the probe to the surface of the fluid, when the impulses hit the fluid a portion of the signal is reflected back up the probe.

Advantages of Radar Level Sensors

  • Top vertical mounting – easy to install
  • Can be used with liquids with solids, or with coating liquids.
  • Unaffected by emulsification, dust, foam or vapours.
  • Dwyer’s series features a programmable output range, which is unaffected by dielectric or specific gravity changes of the liquid

Disadvantages of Radar Level Sensors

  • Probe length must be the entire length of the sensing range
  • Usually a more expensive level sensing/measurement option
  • If application with bypass chambers or stilling wells, careful attention must be taken when choosing probe type.

Dwyer Instruments are experts in level measuring technologies and their vast range of products cover every application. To avoid costly reinstallation costs, always consult the experts before you specify your level measurement instrument – there could be a more suited product out there.

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