Continuous level transmitters measure liquid, slurry, or solid surface level continuously throughout the vessel height and output a 4–20 mA signal proportional to level — enabling control loops, inventory tracking, and alarm functions. Free-space radar (FMCW or pulse): microwave signal emitted by antenna reflects from the liquid surface and returns — time-of-flight is proportional to distance (level). Guided wave radar (GWR): microwave pulse travels down a probe rod or cable, reflects from the liquid surface on the probe, returns — immune to vapour, foam, and dust unlike free-space radar. Hydrostatic: submerged pressure transmitter at tank base measures fluid head pressure (ρgh) — simple, accurate, unaffected by foam or vapour. Capacitance: insertion probe forms a capacitor with tank wall — capacitance changes proportionally to liquid level as fluid dielectric replaces air.
FMCW radar: transmitter emits a frequency-modulated continuous wave (chirp) from 26 GHz to 80 GHz; the reflected signal has a frequency difference (beat frequency) relative to the emitted signal that is proportional to the distance to the liquid surface — measured with centimetre accuracy. 80 GHz transmitters have a very narrow beam (3–5°) enabling use in small tanks and avoiding nozzle reflections. GWR: time-domain reflectometry (TDR) — a microwave pulse on the probe wire reflects when it encounters the dielectric discontinuity at the liquid surface; reflection time is directly converted to level. Hydrostatic: differential pressure cell with one port submerged at tank bottom, one port vented to atmosphere — output proportional to ρgh (liquid head).
Challenges
Radar Signal Reflection Loss from Low-Dielectric Constant Fluids (<2)
Hydrocarbons and solvents with ε <2 produce very low reflection amplitude — reliable level measurement requires beam focusing.
Hydrostatic Transmitter Density Variation Causing Level Calculation Error
Temperature or composition changes alter fluid density — hydrostatic pressure remains constant but calculated level shifts.
Ultrasonic Dead Band (Transmitter Cannot Measure Within 150–600 mm of Transducer)
Ultrasonic transducer cannot distinguish transmit burst from reflected signal close to sensor — dead band limits top-of-range measurement.
Capacitance Probe Fouling by Coating Material Building Up on Probe Surface
Product coating on the probe changes apparent capacitance — level reading drifts upward (false high level).
GWR Probe Drag and Damage in Agitated Vessel with Rotating Impeller
GWR probe hangs in the vessel — rotating impeller or agitator contacts probe; damage and measurement loss.
Solutions
- 80 GHz Focused Radar for Low-Dielectric Hydrocarbons (ε <2): 80 GHz focused beam achieves reliable reflection from ε = 1.5 — use highest-frequency radar for low-dielectric service.
- Density Compensation in Hydrostatic System (Density Transmitter + Ratio Correction): Second DP cell measures density of known liquid column — PLC automatically corrects level calculation for density variation.
- Ultrasonic with Extended Horn Antenna (Reduces Dead Band to 100 mm): Extended horn reduces dead band — allows level measurement to within 100 mm of transducer face.
- Coaxial or Rod GWR Probe (Self-Cleaning Profile) for Coating Service: Coaxial probe resists coating build-up; robust signal reflection remains despite thin coating film on outer conductor.
- Side-Mounted GWR or External Radar (Bypass Bridle) in Agitated Vessels: External bypass pipe (bridle) side-mounted to vessel — level in bridle equals vessel level; probe in still bridle; impeller cannot reach.
Applications
- Chemical Tanks: Radar or GWR on chemical storage tanks — non-contact, no process wetted parts except GWR probe.
- Pharmaceutical Vessels: Flush diaphragm hydrostatic transmitter on bioreactor and mixing vessel — no intrusion; CIP/SIP compatible.
- Food & Beverage: Sanitary radar or GWR on dairy tanks, brewing vessels, CIP vessels — Tri-Clamp; Ra <0.8 µm wetted.
- Water & Wastewater: Non-contact ultrasonic or radar on open channels, sumps, and storage — no submersible parts required.
- Silos and Hoppers (Solids): 80 GHz radar on powder and bulk solid silos — high beam focus avoids wall reflection; dust immune.
PLC System Size Reference
Continuous Level Transmitters — Technology Comparison & Range Reference | ||||||||||
Technology | Measurement Range [m] | Accuracy | Process Temp [°C] | Max Pressure [bar] | Min Dielectric Constant | Foam / Vapour Immune | Contact with Process | Beam Angle / Profile | Output Signal | Best Application |
Free-space radar (26 GHz FMCW) | 0.3–30 m | ±2–5 mm | −40 to +200 | 40 | ε >1.5 (low dielectric care) | Good (vapour); moderate (foam) | No (antenna only) | 10–15° wide beam | 4–20 mA + HART/FF | Large tanks, open liquids, non-agitated service |
Free-space radar (80 GHz FMCW) | 0.05–30 m | ±1–2 mm | −40 to +200 | 40 | ε >1.4 | Excellent | No | 3–5° narrow beam | 4–20 mA + HART/FF | Small tanks, low-dielectric, nozzle bypass, solids |
Guided wave radar (rod probe) | 0.1–6 m | ±2–5 mm | −40 to +200 | 400 | ε >1.4 | Excellent (GWR) | Yes (rod) | On probe (guided) | 4–20 mA + HART/FF | Agitated vessels (with bridle); interface level; small tanks |
Guided wave radar (cable probe) | 0.5–30 m | ±2–5 mm | −40 to +300 | 420 | ε >1.4 | Excellent | Yes (cable) | On cable (guided) | 4–20 mA + HART/FF | Deep tanks; heavy liquid; interface level measurement |
Ultrasonic (non-contact) | 0.1–15 m | ±2–5 mm | −40 to +80 | 1 (non-press.) | Any (non-contact) | Poor (heavy vapour) | No | 6–15° beam | 4–20 mA + HART | Open channels, sumps, non-pressurised tanks, water |
Hydrostatic (submersible) | 0.1–250 m (water depth) | ±0.1% FS | −5 to +60 | 25 (DP cell 250 bar) | N/A (pressure) | Excellent | Yes (diaphragm) | Omnidirectional | 4–20 mA + HART | Deep wells, open tanks, pits, rivers — simple robust |
Hydrostatic (DP cell external) | 0.1–20 m | ±0.1% FS | −40 to +150 | 700 | N/A | Excellent | Diaphragm only | Omnidirectional | 4–20 mA + HART/FF | Closed pressure vessels, reactors, fermenters |
Capacitance (insertion probe) | 0.1–20 m | ±0.5% FS | −40 to +250 | 250 | ε >1.5 (2× air) | Moderate | Yes (full probe) | Full probe length | 4–20 mA + HART | Conductive and non-conductive liquids; coating-resistant types |
Magnetostrictive (float + wire) | 0.1–7 m | ±0.1 mm | −40 to +100 | 100 | N/A (float) | Moderate | Float (wetted) | Float position only | 4–20 mA + SSI + HART | Interface level; fuel; clean liquid — highest accuracy |