Short answer: the choice between non-contact radar and guided wave radar (GWR) follows from conditions at the measuring point, not the technology label. Where product contact is undesirable, where sticky or corrosive product would foul a probe, or where an agitator or a tall tank would load a probe mechanically, non-contact radar is usually evaluated first. For liquid–liquid interface, weak echoes from low-dielectric products, narrow bypass chambers or stilling wells, and small vessels crowded with internals, GWR comes forward. A high-pressure saturated steam phase affects signal propagation for both technologies; in that case look for a solution with a compensation option and a suitable installation, often inside a chamber.
This answer applies to liquid level in closed tanks and process vessels. Foam is a condition where no firm verdict can be given in advance for either technology: foam thickness, stability and product reflectivity decide together. Measuring range, minimum dielectric constant, accuracy and temperature/pressure limits vary by model, antenna and probe type. This article therefore gives no numerical thresholds generalized to a technology; values are confirmed in the manufacturer's data sheet and under project conditions.
One boundary up front: continuous level measurement and independent overfill prevention are not the same function; overfill prevention is handled separately through risk assessment and safety design and is outside the scope of this article.
Conditions that change the decision
Foam
In non-contact radar the signal reaches the surface through free-space propagation. When heavy foam, turbulence and condensation combine with a poorly reflecting product, the measuring range can shrink and the surface echo can become ambiguous. In GWR the energy is guided along the probe, so the echo usually stays stronger; manufacturer documents nevertheless state that surface foam can also affect GWR performance. With thick, unstable foam, compare differential pressure level measurement alongside both radar types.
Vapor and gas phase
Radar measurement relies on a known propagation speed of the signal in the gas phase. In atmospheric tanks this is usually negligible, but high-pressure, high-temperature saturated steam raises the gas-phase dielectric constant, slows propagation and, uncompensated, offsets the level reading. This physical effect is common to non-contact radar and GWR. For GWR, probe options use a reference reflector near the flange to measure and correct for the vapor phase continuously; they are typically applied in a bypass chamber, with a minimum distance between reflector and highest level. For non-contact radar, vapor compensation and use inside a chamber are model-specific and must be confirmed in the manufacturer's documentation. With boiling hydrocarbons, gas bubbles and agitation can also disturb the echo, so a larger chamber or pipe diameter is chosen.
Condensation and buildup
In non-contact radar, condensation and product buildup on the antenna weaken the signal; where heavy buildup is expected, antennas with fully PTFE wetted surfaces, an air purge connection or a flushing ring are considered. In GWR the buildup forms directly on the probe. Single lead probes are relatively tolerant of buildup; with a coaxial probe the product can bridge between the inner rod and the outer tube, so coaxial probes are not recommended for products prone to buildup. Sticky products may require a coated probe and periodic cleaning. If the process is clearly hotter than the connection, crystallization or coating on the upper probe can reduce the signal; ask about this for hot, viscous products.
Turbulence, agitators and fill stream
In agitated tanks a GWR probe must not touch the agitator, and with strong fluid movement it should be anchored or guided to the tank bottom. Otherwise a probe swinging under side forces threatens both measurement and mechanical integrity. Non-contact radar has no probe risk, but agitator blades, heating coils and ladders should be kept out of the beam, and the device should not be mounted in the center of the tank, above the fill stream or on a manway cover.
Nozzle geometry and internals
A long, narrow nozzle is a frequent problem for non-contact radar: nozzle height is limited by diameter and antenna type, and the nozzle must be smooth inside, free of weld beads, rust or deposits; otherwise an upper null zone may be needed to mask the nozzle echo. With GWR, a single lead probe must not touch the nozzle wall; nozzles with reducers and nozzles that protrude into the tank are avoided. Very wide nozzles can weaken the signal, especially with low-dielectric products, and the nozzle height adds to the probe's upper blind zone. A coaxial probe carries the signal inside its own outer tube and is practically unaffected by nozzles and internals.
Stilling wells and bypass chambers
In turbulent, foaming or low-dielectric service, measuring in a stilling well or external chamber is a common solution. For GWR the chamber diameter must suit the probe type; a flexible probe should not be used in small-diameter chambers because it may touch the wall, and large side inlets can disturb the signal. A metal pipe prevents disturbing echoes from objects outside the pipe. Converting existing displacer chambers to GWR is a common retrofit route. Using non-contact radar in a stilling well depends on antenna type, pipe diameter, inner surface and slot or hole pattern, and must be confirmed model by model in the manufacturer's documentation.
Low dielectric constant and interface measurement
Reflection strength increases with the product's dielectric constant. With low-dielectric products such as hydrocarbons the echo weakens for both technologies; in GWR a coaxial probe, or a single lead probe inside a metal chamber or pipe, improves the minimum measurable dielectric limit. For interface measurement GWR uses the energy that is not reflected at the upper surface and produces a second reflection at the surface of the lower liquid. This requires the dielectric constant of the upper product to be known and stable, and clearly lower than that of the lower product; an emulsion layer between the two can spoil the result. Limits such as the minimum dielectric difference and the thinnest detectable upper layer are given per probe type in the manufacturer's data sheet.
Tank height, probe length and mechanical load
For non-contact radar the measuring range depends on product, antenna and process conditions; because the beam widens with distance, clearance to the wall and internals is rechecked in tall tanks. In GWR the probe must extend over the entire range to be measured: rigid probes are limited in length, and tall tanks need a weighted or bottom-anchored flexible probe. Flexible probes have tensile and collapse load limits; flow, agitation and pull-down forces from the product can stress these limits. Blind zones exist at the top and bottom of the probe, and the 4–20 mA range end points should be set outside them.
Installation, maintenance access and existing nozzles
A rigid probe is lowered vertically into the tank and removed the same way; if there is not enough headroom above the nozzle, a segmented or flexible probe is considered. Non-contact radar, with a shorter body, can be fitted to existing small connections, but performance drops if nozzle diameter and height do not suit the antenna type. When reusing a nozzle, measure its diameter, height, flange standard, pressure class and any protrusion into the tank; drawings and site conditions may differ.
Condition, possible effect and review questions
Table 1 – Possible effect of measuring-point conditions on both technologies
Condition | Possible effect on non-contact radar | Possible effect on GWR | Review questions |
Foam | Echo may weaken, measuring range may shrink | Guided signal usually stays stronger; heavy foam can still affect it | How thick and stable is the foam? Should the top of the foam or the liquid surface be measured? |
High-pressure saturated steam | Propagation speed changes; uncompensated reading shows an offset | Same physical effect; reference-reflector compensation options are applied in a chamber | What is the pressure and temperature range? Can a chamber be used? Is there enough distance for the reflector? |
Condensation and buildup | Antenna buildup reduces the signal; cleaning options needed | Buildup forms on the probe; coaxial probe is prone to bridging | Does the product stick or crystallize? What access and frequency for cleaning? |
Agitator and turbulence | Blades and a rough surface produce disturbing echoes | Risk of probe contact and swinging; anchoring needed | What type of agitator and where? Can the probe be anchored to the bottom? |
Long/narrow nozzle and internals | Nozzle height limited by diameter; structures in the beam create echoes | Single lead probe must not touch the nozzle; coaxial probe practically unaffected | Nozzle diameter, height and inner surface? Any reducer? Where are the internals? |
Stilling well or chamber | Depends on model and pipe properties; manufacturer confirmation needed | Strong option with suitable diameter and probe type; metal pipe preferred | Pipe/chamber diameter, material, slot and side inlet layout? |
Low dielectric | Weak echo, shorter measuring range | Minimum limit depends on probe type; coaxial probe or metal chamber helps | Is the product's dielectric constant known, or does it vary? |
Interface measurement | Generally not applied; GWR or another technology is evaluated | If the upper product dielectric is known and stable, total level and interface can be measured together | Dielectric values of both products, thinnest expected upper layer, emulsion? |
Tall tank, long range | Beam widens; wall and internal clearance rechecked | Flexible probe, weight or anchoring; tensile load limits | What is the measuring range and tank height? Is there headroom to lower the probe? |
Non-metallic tank or open basin | Objects outside the tank may cause echoes; radio compliance class may change | Mounting on a metal flange or metal sheet may be required | What is the tank material? Is the tank closed or open? |
Unsuitable conditions and limits
· Weak candidate for non-contact radar: vessels that need interface measurement; points where heavy foam, turbulence and condensation coincide with a very low-dielectric product; tanks whose only available connection is a nozzle that is very tall for its diameter or rough inside.
· Weak candidate for GWR: probe locations close to an agitator where anchoring is not possible; sticky or crystallizing products that would bridge or heavily coat the probe (especially a coaxial probe); heights where there is no headroom to lower the probe or where probe length cannot carry the mechanical load.
· Uncertain for both: thick, unstable foam; an uncompensated high-pressure steam phase; interface with an upper product whose dielectric constant varies. In these cases, also compare alternatives such as differential pressure measurement, keeping in mind that DP level measurement depends on product density.
Tank measuring point data sheet
The form below is meant to be completed separately for each measuring point, for technology selection and quotation review.
Table 2 – Tank measuring point data sheet
Data group | Data to request | Effect on selection |
Measurement purpose | Continuous level, interface, volume; control or inventory; is a separate overfill prevention system existing or planned | Interface need points to GWR; inventory purpose may require a system-level review |
Product | Product name and composition, dielectric constant (range if known), density, viscosity, solids content | Echo strength, probe type, comparison with alternative technologies |
Gas phase | Design and operating pressure/temperature, presence of saturated steam, gas phase composition | Need for vapor compensation and a chamber |
Surface condition | Foam (thickness, persistence), boiling, turbulence, location of the fill stream | Echo stability, mounting position, need for a stilling well |
Buildup | Tendency to condense, stick, crystallize or coat; current cleaning method | Antenna or probe type, cleaning option, maintenance plan |
Tank | Type (vertical, horizontal, spherical), material, open/closed, height, diameter, required measuring range | Measuring range, probe length, radio compliance class |
Internals | Agitator type and position, coils, ladders, inlet pipes, beams (drawing or photo) | Disturbing echoes, probe anchoring |
Nozzle | Diameter, height, flange standard and pressure class, reducer, protrusion into the tank, inner surface condition | Antenna selection, probe-to-nozzle fit, blind zone |
Chamber or pipe | Existing bypass chamber or stilling well, diameter, material, side inlets; for displacer retrofits, displacer length and chamber flange | Probe type and feasibility of conversion |
Access | Headroom above the nozzle, platform, removal options, whether the tank can be taken out of service | Rigid, flexible or segmented probe and maintenance plan |
Electrical and safety | Output and communication (4–20 mA, HART, fieldbus), hazardous area classification, SIL requirement if any | Model and approval selection |
Radio and documents | Country of use, required certificates and material documents | Frequency setting and compliance documents |
A non-contact radar is also a radio transmitter
Non-contact radar level devices use radio spectrum, whereas in GWR the signal is guided along the probe. In Europe there are two separate harmonised ETSI standards for level radars. ETSI EN 302 372 covers tank level probing radars (TLPR) installed at a fixed position on closed metallic or reinforced concrete tanks, or similar structures with comparable attenuation; it lists microwave sealing of flanges and attachments, conductive coating of sight glasses where necessary, closed manholes and downward-pointing antenna mounting as installation requirements. ETSI EN 302 729 addresses level probing radars (LPR) that are not limited to a tank structure and includes installation constraints such as fixed downward antenna orientation and separation distance from radio astronomy sites. The practical consequence: for plastic tanks, open basins or channels, confirm at the quotation stage which radio compliance class the selected device has and that the correct frequency setting will be made for the country of use.
Related product pages
Related pages
· guided wave radar (GWR) level transmitters: Product details on probe types, vapor compensation and interface measurement.
· non-contact radar level transmitters: Antenna options, mounting position and nozzle conditions.
· tank gauging systems: For system-level measurement needs in inventory and multi-tank sites.
· differential pressure level transmitters and diaphragm seal systems: For comparison in foaming or dirty applications where radar-based measurement struggles.