Walk into any chemical plant tank farm and you will see all three point-level technologies in service — often within 50 meters of each other — because no single technology handles every liquid. A conductive level switch on a caustic soda tank costs $200 and has no moving parts, but it will not measure diesel. A capacitive switch on the diesel tank works fine, but the same sensor on a latex emulsion tank will false-trigger within a day as the emulsion coats the probe. An ultrasonic sensor on the latex tank solves the coating problem by never touching the fluid, but the same sensor on a hot water tank with a steam blanket will lose its echo every time the tank vents. Three technologies, three failure modes, three tank types.
Conductive Level Detection: Simple, Cheap, and Electrically Demanding
A conductive level switch uses two electrodes — or one electrode and the grounded metal tank wall — to complete a circuit through the liquid. When the liquid rises and submerges both electrodes, an AC current (typically 1–10 mA at 50/60 Hz or a few kHz to prevent electrolysis) flows between them, and the electronics detect the drop in resistance. The alarm relay energizes. When the liquid falls below the electrodes, the circuit opens and the relay de-energizes. The entire sensor has exactly one failure mode: the electrodes corrode or become coated with an insulating deposit — scale, grease, or crystallized product — that blocks the current path.
The electrical requirement is the screening question: the liquid must have a conductivity of at least 10 µS/cm. Tap water is 50–800 µS/cm, so conductive sensors work for water, wastewater, acids, bases, and salt solutions. But diesel fuel, deionized water (<0.1 µS/cm), vegetable oils, and most organic solvents are electrically invisible to a conductive probe. For those fluids, you need capacitive or ultrasonic technology. For more sensing options, browse our level sensors category.
Capacitive Level Detection: Any Liquid, But Sensitive to Everything That Sticks
A capacitive level probe forms a capacitor with the tank wall — the probe is one plate, the tank wall or a concentric ground tube is the other, and the material between them is the dielectric. When liquid replaces air around the probe, the dielectric constant changes. Air has a dielectric constant of 1.0. Water has a dielectric constant of approximately 80 — an 80:1 change that is easy to detect. Diesel fuel has a dielectric constant of about 2.1 — a much smaller 2:1 change, but still detectable with a sensitive capacitive circuit.
The Endress+Hauser Liquipoint FTW23 capacitive point level switch is designed for food and beverage applications where hygienic cleaning is mandatory — its flush-mounted PEEK sensor face resists product buildup, and the IO-Link digital output provides continuous sensor health diagnostics including a coating thickness indicator that alerts the operator before a false trip occurs. The FTW23 detects liquids with a dielectric constant as low as 1.5, covering everything from deionized water to cooking oils, with an active build-up compensation that subtracts the capacitance contribution of a coating film from the measurement signal. This is the technology's answer to the coating problem that otherwise limits capacitive sensors to clean, non-fouling liquids.
Ultrasonic Level Detection: Non-Contact, but Blind to What It Cannot Hear
An ultrasonic level sensor fires a 40–200 kHz sound pulse from a transducer mounted above the liquid surface and measures the time of flight until the echo returns. The distance to the liquid surface is half the round-trip time multiplied by the speed of sound in air (approximately 343 m/s at 20°C). Ultrasonic sensors have zero contact with the process fluid — they can measure corrosive acids, abrasive slurries, and sticky adhesives without any wetted parts. This is their single largest advantage over conductive and capacitive technologies.
The weakness is anything that absorbs, scatters, or blocks the acoustic pulse. Foam on the liquid surface attenuates an ultrasonic pulse by 20–40 dB — a 100:1 to 10,000:1 reduction in signal amplitude — because the air bubbles act as millions of tiny acoustic impedance discontinuities. Steam and vapor layers above hot liquids change the speed of sound in the column of air between the transducer and the liquid surface, introducing a temperature-gradient-dependent distance error of 1–3% of range. And turbulence at the liquid surface scatters the echo, producing a noisy, jittery reading that can false-trigger the alarm relay.
Technology Selection Table
| Criterion | Conductive | Capacitive | Ultrasonic |
|---|---|---|---|
| Minimum conductivity | ≥10 µS/cm | None required | None required |
| Coating tolerance | Poor — insulating coating blocks circuit | Fair–Good with compensation | Excellent — no contact |
| Foam tolerance | Fair — foam may conduct | Fair | Poor — foam kills echo |
| Max temperature | 150°C (electrode insulation limit) | 200°C (PEEK/ceramic probe) | 80°C (transducer limit) |
| Relative cost | $150–400 | $300–800 | $400–1,200 |
Can I use a capacitive sensor on a tank with an internal mixer?
Yes, but you must account for the moving metal mass. A mixer blade passing near a capacitive probe changes the capacitance — the blade is a grounded conductor moving through the probe's electric field. The sensor's signal processing must distinguish between a slow capacitance change from liquid level rise (seconds to minutes) and a fast capacitance change from mixer blade passage (milliseconds per revolution). Most capacitive sensors include a response delay filter for this purpose — set the filter time constant to 3–5× the mixer rotation period, and the sensor will ignore the blades while still responding to actual level changes. Mechanically, position the probe at least 150 mm from the mixer blade's swept radius and orient the probe parallel to, not perpendicular to, the blade path to minimize the projected area facing the moving metal.
For more level measurement technologies, browse our level transmitter and level switch catalogs covering continuous and point-level instruments for every tank type.
