Level Gauges

What a level gauge does

A level gauge measures the degree of filling in a container or sieve, either continuously
(gives an analogue value 0-100 %) or as point level (high/low alarm).
The choice of technology depends on the medium (liquid/solid), dielectric constant,
temperature, pressure, vapours, foam, etc. The five demanded are described below
methods, their working principle, strengths, limitations and typical
uses.

Guided Wave Radar (GWR)

• Principle: A short microwave pulse is sent down a probe (rod or wire).
  The pulse is reflected at the interface between two media; the time difference gives the level
  (Time-Domain-Reflectometry).
• Advantages:
  • Accurate ±2-5 mm, independent of pressure, temperature and vapour.
  • Works well at low ε_r-values and can measure interfaces (e.g. oil/water).
  • Requires no air-free zone above the level - can be mounted in narrow towers or standpipes.
• Restrictions:
  • The probe is in contact with the process → risk of coating, abrasion or tensile forces when moving bulk materials.
  • Maximum length about 40 m; requires variant with weight for powder.
• Typical applications: Separators, narrow columns, LNG tanks, sludge in sewage plants.

Level switches (point level switches)

• Objective: Provides only 'on/off' signal for overfill protection, dry running alarm, pump control.
• Common technology: Vibrating fork, float, capacitive, rotating paddle, magnetostrictive, optical, RF admittance.
• Strengths: Cheap, simple, often SIL rated for safety circuits.
• Restrictions: No continuous output signal; each instrument covers only one fixed alarm level.

Radar (free-radiating, 24-80 GHz)

• Principle: Antenna transmits microwave pulses or FMCW sweeps; the time/frequency shift of the reflection indicates the distance to the surface. No contact with the product.
• Advantages:
  • Withstands high pressures (>160 bar) and temperatures (>400 °C).
  • Barely affected by dust, vacuum or aggressive chemicals.
  • Newer 80 GHz sensors have narrow beam → small dead zone and can be mounted close to wall.
• Restrictions:
  • Weak economy at very low ε_r (<1.7) or thick foam.
  • Requires clear view - internal struts or stirrers may produce false echoes (managed with echo cancellation).
• Typical applications: Crude oil tanks, asphalt, cement silos, biogas reactors.

Ultrasound

• Principle: The transducer transmits a sound pulse (20-65 kHz) towards the surface and measures the echo time; the speed of sound in air is assumed or compensated.
• Advantages:
  • Completely non-contact and cost-effective for water, sewers and open channels.
  • Easy installation - no pressure feedthroughs.
• Restrictions:
  • Sensitive to temperature gradients, fog, heavy foam or strong vapours that attenuate sound.
  • Limited range (usually ≤15 m) and needs free surface above the liquid.
• Typical applications: Pumping stations, separators, grain silos (with dust protection).

Continuous capacitive level measurement

• Principle: The probe and the tank wall form a condenser. As the height increases, the
  the dielectric constant in the gap; the capacitance - and thus the output signal - changes proportionally.
• Advantages:
  • Simple, robust, no moving parts; works in pressurised vessels.
  • Low power consumption and good for low levels in small vessels.
  • Can be designed for both conductive and non-conductive liquids and powders.
• Restrictions:
  • Requires calibration against the medium ε_r; changes (temperature, recipe) can drive the measurement.
  • Coatings on the probe give errors; insulated probes or RF admittance reduce the problem.
• Typical applications: Glue silos, plastic granules, acids/bases where radar is problematic due to low dielectricity.