Guided Wave Radar Level Transmitters

What Is a Guided Wave Radar Level Transmitter?

Guided wave radar (GWR) is a contacting level technology that applies the radar principle along a conductive probe. With non-contacting radar the microwave energy spreads freely from the antenna; with guided wave radar the pulse travels along the probe much as it would along a transmission line. The signal therefore does not disperse into the vapor space of the tank, and a strong, stable reflection is obtained even in narrow nozzles, around internal structures, on turbulent surfaces and inside bypass chambers.

The instrument works by time domain reflectometry (TDR): it measures the time between the transmitted pulse and the reflection returning from the product surface, converts this time into a distance and calculates the level. Because the same measuring chain can also evaluate the wave that passes through the upper product and reflects from the surface of a second liquid below it, one transmitter can deliver both the total level and the interface level. Taking level, ullage, volume, level rate and signal strength from a single device reduces the number of process openings on the tank.

With no moving parts and no need for periodic recalibration, GWR transmitters are a common choice for replacing mechanical displacer and float instruments, for interface measurement and for high-pressure chambers containing steam. For tanks where a probe is not wanted, non-contacting radar level transmitters can be considered; the choice between the two technologies should be based on the product's dielectric constant, the tank geometry and the process conditions.

How Guided Wave Radar Works: Time Domain Reflectometry (TDR)

Guided wave radar relies on recording the impedance changes — in other words, the dielectric constant differences — that an electromagnetic pulse meets as it travels along the probe. The measuring chain consists of five basic steps:

1.      Sending the pulse down the probe: The transmitter electronics generate microwave pulses lasting nanoseconds, with a nominal output power of around 300 µW. The pulses are coupled into the probe through the flanged or threaded connection, and the probe guides them downward like a waveguide.

2.      Reflection at the product surface: When the pulse meets a product whose dielectric constant differs from that of the gas phase above it, a portion of the pulse energy travels back up to the transmitter. The higher the dielectric constant, the stronger the reflection: high-dielectric liquids such as water return a pronounced echo, while low-dielectric products such as hydrocarbons produce a weaker one.

3.      Converting time of flight to distance: The time difference between the reference pulse and the returning reflection is measured and converted into a distance. Subtracting this distance from the defined reference height gives the product level; if required, volume is also derived from the tank geometry.

4.      Detecting the interface with the remaining wave: Energy that is not reflected at the top surface keeps traveling through the upper liquid and creates a second reflection at the surface of the liquid below. As the speed of this wave depends entirely on the upper product's dielectric constant, the interface level and the upper layer thickness can be calculated once that value is known.

5.      Signal processing and output: Thresholds defined on the echo curve separate the true surface echo from noise and disturbing reflections. The calculated variables are passed to the control system via 4–20 mA HART, FOUNDATION Fieldbus or RS-485 Modbus; damping is adjustable from 0 to 60 s.

How the Dielectric Constant Affects the Measurement

The main parameter that sets the limits of GWR measurement is the dielectric constant (εr) of the product. Coaxial probes can measure products with a dielectric constant as low as 1.2, whereas the lower limit for single lead probes is 1.4 with the standard seal, falling to 1.25 inside a metallic bypass chamber or still pipe. With flexible single lead probes, the achievable measuring length grows with the dielectric constant: the practical range is about 15 m at a dielectric constant of 1.4 and reaches 50 m for products with a dielectric constant of 6. Software algorithms that evaluate the reflection from the probe end can push the measurable lower limit further down for products with very weak echoes.

Probe Types and Field Propagation

In a coaxial probe the electromagnetic field is confined between the inner rod and the outer tube, so it is almost unaffected by external disturbances and gives the strongest signal in clean, low-dielectric liquids. Around a single lead probe the field spreads into a larger volume; this design tolerates build-up better, but certain distances to the tank wall, the nozzle and internal structures must be kept. For chamber applications the large coaxial probe offers a balanced compromise between interface resolution and tolerance to build-up.

Reference Reflector Compensation in Saturated Steam

High-pressure saturated steam raises the dielectric constant of the gas phase and changes the propagation speed of the pulse, which causes a level error if it is not compensated. Probes designed for steam service carry a reference reflector at a known position close to the flange; it allows the dielectric constant of the vapor phase to be measured continuously and the level value to be corrected dynamically. The method is used in 2, 3 or 4-inch bypass chambers, and when the level approaches the reflector zone the transmitter falls back to static compensation based on the most recent vapor value.

Because the measurement is based directly on the position of the product surface, changes in density or conductivity do not affect the level reading. For interface measurement, on the other hand, a correct result requires the upper product's dielectric constant to be known and stable.

Key Features

·         Multivariable measurement of level and interface: Level, ullage, interface level and distance, upper product thickness, upper and lower volume, level rate, signal strength and internal temperature are all available from one device.

·         A probe family for every application: Choose from coaxial and large coaxial probes, 8 mm and 13 mm rigid single lead probes, segmented rigid probes up to 10 m, and flexible single lead probes with a weight or a bottom anchoring kit.

·         Measurement of low-dielectric products: Measurement is possible down to a dielectric constant of 1.2 with a coaxial probe and 1.4 with a single lead probe (1.25 in a metallic chamber or pipe).

·         Detection of thin upper layers: The signal processing algorithm can distinguish an upper layer as thin as 2.5 cm from the liquid below when a large coaxial probe is used; the figure is 6 cm for a single lead probe and 7 cm for a coaxial probe.

·         Process seals for severe conditions: Standard, medium temperature/medium pressure, high pressure, high temperature/high pressure and cryogenic process seals cover operation from –196 °C to 400 °C and up to 345 bar.

·         Probes with vapor compensation: Steam probes with a reference reflector automatically correct for dielectric changes in the gas phase of chambers containing high-pressure saturated steam, preserving level accuracy.

·         Verification reflector: A reflector on the probe allows the integrity of the transmitter and the upper probe section to be tested without raising the level, and supports continuous monitoring of the high-level condition.

·         Predictive diagnostics: Signal quality indicators that track the relationship between surface echo, noise and threshold detect probe contamination and sudden loss of signal; echo curve and logging tools simplify online troubleshooting.

·         Flexible communication options: Outputs include 4–20 mA HART (revision 7 by default, revision 5 as an option), FOUNDATION Fieldbus and RS-485 Modbus; an external wireless adapter connects the transmitter to a WirelessHART (IEC 62591) network.

·         Modular dual-compartment housing: Electronics and wiring terminals sit in separate compartments; the transmitter head can be removed from the probe without opening the tank and can be rotated in any direction.

·         Probes cut to length on site: Except for the high temperature/high pressure coaxial probe and the PTFE-coated probe, probes can be shortened on site to match the actual tank height.

Technical Specifications

The values below are typical technical data for the guided wave radar level transmitter family covered on this page. Measuring range, temperature and pressure limits and material options vary with the probe type, process seal, flange and O-ring material; final values should be confirmed at project stage.

Maximum measuring ranges are guide values for clean liquids; nozzles, internal structures, foam, particles in the tank atmosphere and build-up on the probe can reduce the range. Remote housing mounting and spacers on the probe can affect accuracy.

Key Advantages

·         Direct measurement unaffected by process changes: No additional compensation or recalibration of the level measurement is needed when density, conductivity, viscosity, pH, temperature or pressure change.

·         Low maintenance: With no moving parts, the maintenance items typical of float and displacer instruments — mechanical wear, sticking and periodic adjustment — disappear.

·         Fewer process openings: Taking level and interface from the same transmitter reduces the number of nozzles on the tank and the number of potential leak points.

·         Straightforward retrofit projects: Flange options that fit existing tank connections and probes that can be cut on site allow old instruments to be replaced without modifying the chamber or nozzle.

·         Suited to difficult atmospheres: Stable measurement is maintained in the presence of vapor, dust, turbulence and foam; steam probes with vapor compensation are available for high-pressure saturated steam.

·         Small tanks and complex geometry: The guided signal produces a stable echo in small tanks, in vessels crowded with internals and inside chambers; the mechanical design of the chamber does not influence the measurement.

·         Suitability for safety functions: A SIL 2 certified configuration and proof tests carried out with the verification reflector make it easier to document safety instrumented functions.

·         Top mounting reduces leak risk: Because the transmitter is installed through the top connection of the tank, no side or bottom connections below the liquid surface are needed.

Application Areas

Guided wave radar is used in process vessels, control loops and safety systems that require liquid level, liquid–liquid interface or solids level. For inventory-oriented measurement in storage tanks, system-level tank gauging solutions should be considered separately.

Process Tanks and Vessels

Used for level control in mixing, intermediate storage and process vessels. In tanks with agitators or strong liquid movement, anchoring the probe to the tank bottom or guiding it is recommended.

Bypass Chambers and Displacer Replacement

For installation in external chambers, a large coaxial or single lead probe is preferred; rigid probes are used up to 6 m, and weighted flexible probes with a centering disc in longer chambers. When replacing displacers, the connection is matched to the chamber flange and the probe length is chosen according to the displacer length.

Liquid–Liquid Interface Measurement

In vessels where oil or an oil-like liquid sits on top of water or a water-like liquid — typically with an upper product dielectric constant below 3 and a lower product dielectric constant above 20 — total level and interface are monitored together.

High-Pressure Saturated Steam Applications

In chambers containing a saturated steam phase, steam probes with a reference reflector are used in 2–4-inch bypass chambers with flanges rated for the pressure and temperature; the dielectric change of the vapor phase is compensated continuously.

Cryogenic and High-Temperature Processes

The cryogenic seal allows use down to –196 °C, and the high temperature/high pressure seal up to 400 °C. Where the temperature limit of the electronics would be exceeded, the housing can be mounted remotely with a 1, 2 or 3 m connection.

Solids Silos

The model dedicated to solids measures level up to 35 m with a 4 mm flexible probe and up to 50 m with a 6 mm flexible probe, unaffected by dust, moisture or material fluctuations. Leaving the probe slightly slack is recommended to avoid high tensile loads.

Safety Instrumented Systems and Overfill Prevention

The configuration certified to IEC 61508 for SIL 2 and supplied with FMEDA data can be used in high-level alarm and overfill prevention functions; the verification reflector helps carry out proof tests without raising the level.

How to Select a GWR Level Transmitter

With guided wave radar, a correct result depends more on the choice of probe and mounting than on the transmitter itself. For a technical assessment we recommend clarifying the following:

Product and Process Properties

·         Dielectric constant of the product (and of the gas phase, if relevant); for products with a very low or variable dielectric constant, a differential pressure measurement should also be compared

·         Viscosity and build-up tendency: single lead probes are recommended up to 8,000 cP and tolerate build-up, large coaxial probes up to 1,500 cP with thin build-up, coaxial probes up to 500 cP with no build-up

·         Presence of foam, emulsion, boiling or turbulence

·         Design and operating temperature and pressure; presence of saturated steam

·         Chemical compatibility of the wetted metal and O-ring materials

Conditions Specific to Interface Measurement

·         The upper product's dielectric constant must be known and must not change

·         The upper product's dielectric constant must be below that of the lower product, and the two values must differ by more than 6

·         Maximum permissible upper-product dielectric constant: 7 for single lead probes, 10 for coaxial probes

·         Expected minimum upper layer thickness and formation of an emulsion layer

Mounting Geometry

·         Tank, or bypass chamber / still pipe? A chamber diameter of 3 or 4 inches (75 or 100 mm) is recommended; at least 4 inches should be preferred for flexible probes

·         Nozzle diameter and height: a 150 mm diameter is recommended for single lead probes, and nozzles of 250 mm or more should be avoided in low-dielectric applications

·         Clearance to the tank wall and internals: at least 100 mm to a smooth metal wall, larger distances to interfering objects like pipes or beams and to plastic or concrete walls

·         Spacing between several single lead probes in the same tank (1.6 m in oil, 1.0 m in water)

·         Need to mount on a metal flange or metal sheet in non-metallic tanks

Measuring Range and Blind Zones

·         Required measuring range and total probe length (the probe must extend over the whole zone in which level is to be read)

·         Blind zones at the top and bottom ends of the probe; the 4–20 mA range points should be set outside these zones

·         Nozzle height adds to the upper blind zone, and a metallic centering disc increases the lower blind zone

System Integration and Safety

·         Output type: 4–20 mA HART, FOUNDATION Fieldbus or RS-485 Modbus

·         SIL requirement (SIL 2 certification is offered only with the 4–20 mA HART output and is not combined with angled probe mounting)

·         Hazardous area classification and required type of protection

·         Project documentation such as hydrostatic testing, material traceability certificates, NACE material recommendation and welding procedure documents

Inside chambers and in low-dielectric applications, choosing a metal pipe prevents disturbing reflections from objects outside the pipe. For uncertain applications a preliminary assessment based on echo curve analysis is recommended.

Approvals, Safety Certifications and Standards

The main certifications, approvals and standards offered for the guided wave radar transmitter family are summarized below. The scope of approval depends on the ordered configuration.

Hazardous-area markings and certificate numbers are configuration-specific, so they are assessed at project stage by the TLY Enerji engineering team together with the relevant product certificates.

Guided Wave Radar Solutions from TLY Enerji

At TLY Enerji, a guided wave radar project starts at the measuring point rather than with the transmitter. We collect the dielectric constant of the product and of any upper liquid, viscosity and build-up tendency, the presence of steam, the tank or chamber geometry and the nozzle dimensions, and then define the probe type, probe length, centering disc and process seal together. In displacer replacement projects we design the conversion around the existing chamber flange and displacer length, so that no pipework or chamber changes are needed on site.

Our engineering team supports customers with the assessment of hazardous-area and SIL requirements, project documentation, PLC/DCS integration via HART, FOUNDATION Fieldbus or Modbus, echo curve analysis and threshold setting during commissioning, and technical support throughout operation. This makes the measurement verifiable and traceable from first commissioning, even in sensitive applications such as interface and steam service.

Frequently Asked Questions

How does a guided wave radar level transmitter work?

The transmitter sends low-power microwave pulses lasting nanoseconds down a probe immersed in the product. When a pulse reaches a surface whose dielectric constant differs, some of the energy returns as an echo. The time between transmission and reflection is measured, converted into a distance and used to calculate the level. Energy that is not reflected at the surface travels on to the liquid below, so the same device can also measure the interface level. This method is called time domain reflectometry (TDR).

What is the difference between guided wave radar and non-contacting radar?

With guided wave radar the signal travels along a probe, which gives a strong echo in difficult conditions such as narrow chambers, internal structures, low-dielectric products and interface measurement. With non-contacting radar the signal spreads freely from the antenna and no part touches the product, which is an advantage for sticky or abrasive products and in tanks where a probe cannot be installed. The choice should be based on the product, the tank geometry and whether interface measurement is needed.

Can guided wave radar measure low dielectric products?

Yes. Coaxial and large coaxial probes can measure down to a dielectric constant of 1.2, and single lead probes down to 1.4 with the standard seal; inside a metallic chamber or still pipe this limit falls to 1.25. With a flexible probe, the achievable measuring length becomes shorter as the dielectric constant decreases. For low-dielectric applications, using a metallic chamber and avoiding large-diameter nozzles is recommended.

What conditions are needed for interface measurement with GWR?

The upper product's dielectric constant must be known, must stay stable and must be lower than that of the lower product, and the two values must differ by more than 6. For the upper product, the dielectric constant may not exceed 7 with single lead probes or 10 with coaxial probes. A typical application is an oil-like liquid with a dielectric constant below 3 on top of a water-like liquid. If an emulsion layer forms between the two phases, the measurement should be assessed separately.

Can a GWR replace a displacer level transmitter?

Yes. Converting existing displacer chambers to guided wave radar is a common retrofit. A standard or special flange matching the chamber flange is selected, and the probe length is set according to the displacer length. The large coaxial probe is the first option to consider in chambers; rigid single lead probes are used up to 6 m, and weighted flexible probes with a centering disc in longer chambers. Moving parts and the need for periodic calibration are eliminated.

How do foam, vapor and probe build-up affect the measurement?

The guided signal tolerates vapor, dust, turbulence and foam, although heavy surface foam and particles in the atmosphere can affect performance. Steam probes with a reference reflector are used in high-pressure saturated steam. For products that tend to form deposits, a single lead probe should be preferred, and a PTFE-coated probe should be considered for sticky products. Diagnostic functions track build-up and help determine when cleaning is due.

Which probe type should be selected?

In clean, low-dielectric liquids the coaxial probe gives the strongest signal but is not suitable where build-up occurs. For chambers and interface applications the large coaxial probe is the preferred option. Single lead probes are chosen for viscous or build-up-prone products, long measuring ranges and tanks with internals; rigid probes are used up to 6 m and flexible probes for longer ranges. The segmented rigid probe makes rigid measurement up to 10 m easier to transport.

Can the probe be shortened on site?

All probes except the high temperature/high pressure coaxial probe and the PTFE-coated probe can be cut on site. Standard and medium temperature/pressure coaxial probes have a restriction: probes longer than 1.25 m can be shortened by at most 0.6 m, while shorter ones can be cut to no less than 0.4 m. For flexible single lead probes the minimum remaining length is 1.0 m. After cutting, the probe length must be updated in the transmitter configuration.

Can guided wave radar be used in safety instrumented systems?

Yes. The version with 4–20 mA HART output can be supplied certified to IEC 61508 for SIL 2 applications, together with FMEDA data. The verification reflector option makes it possible to proof test the transmitter and the upper part of the probe without raising the level and to monitor the high-level condition continuously. The SIL certified configuration must not be used with angled probe mounting, and the safety function design should be assessed for each project.

What is a blind zone and how does it affect the measurement?

A blind zone is the section at the upper and lower ends of the probe where measurement may not be possible or the error may exceed ±30 mm. Accuracy is reduced for measurements close to these zones, so the end points of the 4–20 mA range should be defined outside them. When a single lead probe sits inside a nozzle, the height of the nozzle is added to the upper blind zone; when a metallic centering disc is used, the lower blind zone is 20 cm.