Why level matters
Knowing how much is in a tank or silo is basic to running a plant. Operators need to know when to refill, when to stop a pump, and when a vessel is close to overflowing or running dry. An unnoticed overfill can spill product and create a safety or environmental problem, and a pump running with an empty supply can be damaged. Level readings are also used for inventory and to control flow.
Level instruments are usually described in two groups. Point level devices tell you whether the material has reached a specific height, and are used for high or low alarms. Continuous level devices give a reading across the whole range. Most plants use both, and a separate, independent high-level alarm is a common safeguard against overfilling.
Simple methods
Sight glasses and gauge glasses show the liquid level directly through a transparent tube or window. They are easy to understand but only suit clean liquids at modest pressure, and they need to be checked in person.
Floats ride on the surface of a liquid and move with it. A float can be tied to a mechanical pointer, a magnetic indicator or a switch. They are simple and cost little to run, but can stick if the liquid is sticky, heavy with solids or prone to build-up. A float on a rod or cable is common in smaller tanks.
Displacers hang in the liquid, and the apparent weight of the displacer changes with the depth of immersion. They work in a range of conditions, including some pressurized vessels, though density changes affect the reading.
Pressure-based methods
A column of liquid exerts pressure at the bottom in proportion to its height and density. A pressure or differential pressure transmitter at the bottom of the tank can therefore be used to infer level. This hydrostatic method is widely used and works with a variety of liquids, including ones with foam on top.
The key limit is density. If the liquid’s density changes with temperature or composition, the level reading will drift unless the density is accounted for. In a closed or pressurized tank, a second connection at the top compensates for the vapor pressure. Impulse lines can plug, freeze or leak, and diaphragm seals are often used where the product is thick or corrosive.
Ultrasonic sensors
An ultrasonic sensor mounted at the top sends a sound pulse down and measures how long the echo takes to return. Because the speed of sound depends on temperature, most sensors include temperature compensation. These are non-contact, which is a real advantage with corrosive or dirty liquids and with some solids.
They can struggle where the path is disturbed: heavy foam, thick vapor, dust clouds, condensation on the sensor face, turbulence or obstructions such as ladders or agitators in the beam. Sound needs an air path, so it does not work in a vacuum. For these reasons, ultrasonic devices are usually chosen for relatively simple, open tanks and sumps.
Radar sensors
Radar sensors send out microwave signals and measure the reflection. Non-contact radar is less sensitive to temperature, pressure, vapor and dust than ultrasonic sensing, which is why it is used in many demanding tanks and in many bulk solids silos. Reflection quality depends on the material: liquids with a low dielectric constant return weaker signals, and an uneven solid surface scatters them.
Guided wave radar sends the signal down a probe, such as a cable or rod, which helps in narrow vessels, foam and long ranges, and it can be used with weaker reflectors. The probe is in contact with the product, so build-up and abrasion need to be considered.
Other methods
- Capacitance probes measure how the capacitance between a probe and the tank wall changes with the material, and are common as point switches.
- Vibrating fork or rod switches detect when material touches a vibrating element and changes its frequency, a common high or low alarm for both liquids and powders.
- Rotary paddle switches are a traditional option in silos: a slowly turning paddle stops when covered by material.
- Load cells can weigh the whole vessel and so calculate contents by weight, which is independent of density and of shape. This is especially useful where the material is a bulk solid with an uneven surface.
- Laser and plumb-bob systems are sometimes used for silos, where the surface is a cone rather than a flat plane.
Special problems with bulk solids
Measuring a solid is harder than measuring a liquid. The surface forms a cone when filled and a crater when drawn from the center, so the sensor reading depends on where it points. Dust can foul sensors and block the signal, and material can build up on probes. Different sensors read the same silo differently, and the stored mass can only be calculated from level if the bulk density is known, which itself changes as the material compacts. Many plants therefore use several sensors or combine level with weighing.
Choosing and maintaining a method
Selection depends on the material, the temperature and pressure, the tank shape, the need for contact or non-contact measurement and the accuracy needed. Conditions such as foam, vapor, build-up, agitation and electrical noise all matter. Instruments need regular calibration and checks against a known reference, such as a manual dip or a weighed transfer. Readings from level devices commonly feed control systems such as programmable logic controllers, which can sound alarms and start or stop pumps.
Work on level instruments often involves tank openings, pressure connections or hazardous contents. Isolation, lockout tagout and the correct permits are essential, and the manufacturer’s instructions should always be followed. This article is general information, not engineering advice.