Guided Wave Radar Level Measurement Instrumentation
Guided wave radar (GWR) liquid level measurement sensors have been on the market since the mid to late 1990s, though it is still considered by many as a new technology. GWR can be used in a wide range of applications and media including solids and liquids. Guided wave radar transmitters are very commonly used in liquid interface level measurement.
Radar Level Transmitter Operating Principle
Radar level transmitters measure the distance from the transmitter to the fluid surface by measuring the time of flight of a high frequency electromagnetic radio wave, typically in the microwave frequency range - GHz. The distance from the transmitter to fluid surface is subtracted from the tank depth to give the liquid level.
Difference between Guided Wave Radar and Radar Level Measurement
Guided wave radar level measuring instruments use a probe to guide the electromagnetic waves to and from the process liquid, as shown in the diagram below:
Advantages of GWR Level Transmitters over Pulse Radar Level Transmitters
Non-contact radar devices experience more signal loss than guided-wave radar devices, due to dispersion of the electromagnetic waves. Radar waveguides combat this signal loss by channeling the radio energy along a straight-line path.
Guided Wave Radar Waveguide Types
There are various types of wave guide available commercially, including single metal rods, parallel pairs of metal rods - also known as twin element probes, and coaxial metal rod and tube structure. For all types of probe it is recommended that PTFE probes are used for liquid level measurement of viscous fluids.
Single Element Rod Probes
The single element rod probe is the least efficient and exhibits the greatest energy losses. However, single rod probes are more tolerant of process fouling than two-rod or coaxial probes, where viscous liquid or solid matter may cling to the wave guide. Clinging liquid or solids can cause electromagnetic wave reflections that fool the transmitter into thinkinging it is seeing a reflection from a liquid level or liquid interface surface.
Coaxial probes are the most efficient waveguides and suffer the least energy loss. For this reason guided wave radar transmitters with coaxial probes are used in the more difficult low dielectric hydrocarbon applications.
Twin Elelment Waveguide Probes
Twin elelment waveguide probes are less efficient than coaxial probes, but more efficient than single element probes. These are good general purpose probes that are typically used in longer range applications, or where flexible probes are required.
Guided Wave Radar and Liquid Dielectric Permittivity
With all radar level instruments, including GWR transmitters, the necessary condition for electromagnetic wave reflection is a sudden change in dielectric permittivity. When an electromagnetic wave encounters a sudden change in dielectric permittivity, some of that wave’s energy will be reflected resulting in another wave traveling in the opposite direction, while the remainder of the wave’s energy continues forward to propagate into the new material. The strength of the reflected signal depends on how greatly the two materials’dielectric permittivity differs, i.e. their relative permattivity, or as it is sometimes called - dielectric constant. A strong reflected wave is required for good operation of the guided-wave radar measuring sensor, therefore radar level instruments function best when there is a large difference in relative permittivity between the two substances at the interface.
With air as the gas medium above the liquid we have realative permativities of approximately 2 for gasoline, 42 for glycerin, and 80 for water.
Compensating for Changes in Dielectric Constant
Changes in dielectric constant can alter the accuracy of radar level measurement. Factors that can alter the dielectric constant of gases include pressure and temperature, therefore the accuracy of a radar level instrument will vary as the pressure and/or temperature of the gas above the liquid vary. Often, the variation is insufficient to cause concern.
In high accuracy applications it is common to compensate for the error by either
- Pressure and Temperature compensation The compensation calculation can be performed within the plant control system, or some manufacturers offer this capability within their guided wave radar transmitter.
- Use of a Reference Signal In this case the radar level instrument is furnished with a reference probe of fixed length who's entire length is always above the liquid level (i.e. it only senses gas).
Guided Wave Radar Transmitters - Typical Features
Guided wave radar accuracy: expect to achieve 2.5mm or 0.1% of distance for measurements up to 5 metres
Transmitter mounting: Head mounted and remote mounted options are widely available
Explosion protection: sensors and transmitters are widely available for use in hazardous areas, either as explosion proof, or intrinsically safe (IS)
Transmitter power supply: 2 wire, loop powered universally available
Output signal: 4-20mA superimposed with a digital HART®, FOUNDATION™ Fieldbus, or Modbus® widely available
Operating temperature and pressure ranges: can cope with wide temperature range (-170°C to 450°C), and from full vacuum to 400 bar
Measurement range: guided wave radar is a contact level measurement technique, that requires the radar waveguide length to be at least as long as the required measurement range (span). Using flexible cable probes as the waveguide allows level measurements up to 50m.
Guided Wave Radar Installation
Guided wave radar level measurement requires a relatively flat fluid surface. If the surface is turbulent then the use of a Stilling Well or chamber should be considered.
For low dielectric liquids consider using a level chamber - a metal chamber acts as a shield and amplifier to the radar signal, giving more reliable reflections.
Position the insertion probe such that it is subject to a minimum of lateral force.
For long probes, the lower end of the probe should be be fixed to the bottom of the vessel.
Ensure adequate headroom, i.e. clearance above the vessel is required for insertion and removal of the probe.
Nozzles used for guided wave radar instruemnts should be a minimum of 4 inch - note that mounting flange sizes vary by GWR supplier, so be sure to check.
The following pages on Control and Instrumentation.com give more detail on various other liquid level measurement methods:
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For those who want to delve further into the techniques employed in level measurement and the differing types of level instrumentation, then the following books from Amazon will be of interest: