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Level is usually measured in one of two ways. From below, a pressure measurement infers the height of the liquid column from its weight. From above, a signal travels to the surface and back, and its time of flight gives the distance. Point-level switches sit alongside both, detecting whether the liquid has reached one particular height. Each method is sensitive to something different — density, vapour, foam, dielectric constant, build-up — and the tank decides which sensitivity matters.
Pressure-based level: the hydrostatic calculation
A liquid column of height h exerts a pressure at its base of ΔP = ρ·g·h, where ρ is the liquid density and g the gravitational acceleration (9.81 m/s²). Rearranged, level is pressure divided by density and gravity: h = ΔP / (ρ·g). The transmitter measures pressure; density turns it into level. On an open tank, atmospheric pressure acts on the surface and on the transmitter reference alike and cancels, so a gauge transmitter at the base is enough.
Step 1: Define the span
Product density 1,100 kg/m³, level range 0–6.0 m above the lower tap. At full level, ΔP = 1,100 × 9.81 × 6.0 = 64,746 Pa, so the transmitter is ranged 0–647 mbar.
Step 2: Read a value
A signal of 14.0 mA is (14.0 − 4) ÷ 16 = 62.5 % of span, which is 0.625 × 6.0 = 3.75 m of liquid.
Step 3: Change the density
The tank now holds a lighter batch at 1,050 kg/m³, but the configuration is unchanged. A true level of 4.00 m produces 1,050 × 9.81 × 4.00 = 41,202 Pa, which the transmitter reports as 41,202 ÷ (1,100 × 9.81) = 3.82 m.
Step 4: The consequence
The reading is 0.18 m low — an error of about 4.5 % of reading, equal to the relative change in density. Where density varies with product, blend or temperature, compensate with a density measurement or choose a method that measures distance instead.
Mounting height matters too. A transmitter installed below the lower tap sees the extra head of liquid in its impulse line, which is removed by ranging it with a suppressed zero. Sediment settling on a bottom-mounted sensor adds an error of its own.
Closed tanks: dry leg, wet leg and seals
In a closed or pressurised tank, a transmitter at the base sees the liquid head plus the pressure in the vapour space. A DP transmitter cancels the vapour pressure by connecting its low side to the top of the tank, leaving only the liquid head. How that low-side connection is made is the main design decision.
- Dry leg
- The low-side impulse line stays free of liquid. Suitable when the vapour does not condense. Condensate collecting in the leg adds a false head that makes the level read low, so a drain pot and a regular drain routine are needed.
- Wet leg
- The low-side line is deliberately filled with a reference liquid to a fixed height, for vapours that condense. At zero level the transmitter sees a negative differential — an elevated zero — and is ranged accordingly. The leg must stay full and at a known density.
- Diaphragm seals
- Seals on both taps with filled capillaries replace impulse lines for corrosive, viscous, crystallising or hygienic liquids. The low-side capillary behaves like a permanent wet leg, and capillary temperature effects belong in the error budget.
- Electronic DP
- Two pressure sensors with the difference calculated electronically avoid long capillaries on tall vessels, at the cost of resolution when the vapour pressure is large compared with the liquid head.
Engineering note
Worked example — wet-leg elevation
Taps 3.0 m apart, with the wet leg filled with water at 1,000 kg/m³. At zero level the low side carries the full leg: ΔP = −1,000 × 9.81 × 3.0 = −29.4 kPa, or −294 mbar. With the tank full of a liquid of the same density, the two heads balance and ΔP = 0. The transmitter is ranged −294 to 0 mbar: the output rises from 4 mA to 20 mA as the level rises, even though the differential is negative across the range and reaches zero only at full level.
Caution
Boiler drums need density compensation
In a steam drum, the densities of both the water and the steam change with pressure, so a DP drum-level measurement set up for operating pressure reads incorrectly at other pressures, including during start-up. Pressure-compensated level calculation is common practice for drum level.
Radar, guided wave radar and ultrasonic
Time-of-flight instruments mount at the top of the tank and measure the distance to the surface; level is the reference height minus that distance. They do not depend on density, which is their main advantage over pressure-based methods.
- Non-contact radar
- Microwaves reflected from the liquid surface. Their propagation is barely affected by temperature, pressure or most vapour spaces — high-pressure steam is a notable exception that needs compensation. Echo strength depends on the liquid’s dielectric constant, so low-dielectric liquids such as many hydrocarbons return weaker signals; heavy foam, agitation and tank internals are the usual sources of trouble. Higher-frequency devices give a narrower beam for a given antenna size, which helps in tanks with internals or small nozzles.
- Guided wave radar
- The pulse travels along a rod, cable or coaxial probe that concentrates its energy, which makes it robust in small or turbulent vessels and with low-dielectric liquids. It can also measure the interface between two liquids when the upper layer has the lower dielectric constant. The probe is in contact with the process, so coating, build-up, mechanical load and probe length need attention.
- Ultrasonic
- A sound pulse through the air above the liquid — economical and well suited to open tanks, sumps and channels. The speed of sound in air changes by about 0.17 % per °C, so instruments compensate for temperature, but gradients in the air column, vapours, foam, dust and condensation still weaken or distort the echo. Ultrasonic sensors do not work in vacuum and cannot measure within a blocking distance close to the transducer.
Point level for alarms and protection
Point-level switches answer a simpler question — is the liquid at this height or not — and are the basis of high, high-high and low alarms, pump dry-run protection and overfill protection.
- Vibrating forks change frequency when covered. They are largely independent of the liquid’s properties and suit most liquids; designs exist for some bulk solids.
- Float switches are simple and mechanically robust, but moving parts can stick where the liquid coats or carries solids.
- Conductive probes are economical, but work with conductive liquids only.
- Capacitance and optical switches suit particular media and are selected for the product and its tendency to coat.
For overfill protection, keep the high-level switch independent of the continuous level transmitter, so that one failure cannot disable both the measurement and the alarm. The degree of independence and the proof-testing interval follow from the plant’s risk assessment.
Selection table
| Method | Measures | Strong where | Watch for |
|---|---|---|---|
| Hydrostatic (gauge) | Pressure at the base | Open tanks, sumps and wells; simple and economical | Density changes; sediment at the sensor; unsuitable alone for closed tanks |
| Differential pressure | Liquid head, vessel pressure cancelled | Closed and pressurised tanks; high temperature and pressure | Density; condensate in dry legs; wet-leg level; seal temperature effects |
| Non-contact radar | Distance to the surface | Most liquids; vapour, temperature and pressure; no wetted parts | Low-dielectric liquids; heavy foam; internals and nozzle design |
| Guided wave radar | Distance along a probe | Small or turbulent vessels; low-dielectric liquids; interfaces | Coating and build-up on the probe; mechanical load; probe length |
| Ultrasonic | Distance through air | Open tanks, sumps and channels; economical | Vapour, foam, dust, temperature gradients; vacuum; blocking distance |
| Point-level switch | Presence at one height | High, low and overfill alarms; dry-run protection | Principle matched to the medium; build-up; proof testing |
Engineering questions
- How do you calculate tank level from pressure?
- Divide the hydrostatic pressure by the liquid density and gravitational acceleration: h = ΔP / (ρ·g). For example, 64,746 Pa under a liquid of 1,100 kg/m³ corresponds to 64,746 ÷ (1,100 × 9.81) = 6.0 m. The result is only as good as the density value, so compensate when density varies.
- Should I use radar or ultrasonic level measurement on a tank?
- Ultrasonic is economical and works well on open tanks, sumps and channels with clear air above the liquid. Radar is largely unaffected by temperature, pressure and most vapours, so it is usually preferred for closed process tanks, vapour-laden spaces and wider temperature ranges.
- What is the difference between a wet leg and a dry leg?
- Both connect the low side of a DP level transmitter to the top of a closed tank. A dry leg is kept free of liquid and suits non-condensing vapours. A wet leg is deliberately filled with a reference liquid, suits condensing vapours, and shifts the calibration to an elevated, negative zero.
- Can a continuous level transmitter provide overfill protection?
- Overfill protection is normally provided by a separate high-level switch or independent transmitter, so a single failure cannot disable both the measurement and the alarm. The required independence and proof testing follow from the plant’s risk assessment.