PC-05Process measurement · LT
Level Measurement for Liquids & Bulk Solids
Continuous level for inventory and control, point level for alarms and overfill protection. The vessel, the surface of the medium and what fills the space above it decide which principle reads reliably.
01Scope
What's included
Continuous and point level for liquids and bulk solids.
- Radar level transmittersNon-contact radar for liquids and bulk solids, and guided wave radar with rod or cable probes.
- Hydrostatic and DP levelSubmersible and flange-mounted pressure transmitters; DP transmitters with remote seals for closed vessels.
- Ultrasonic transmittersNon-contact level for water, wastewater, open channels and simple silos.
- Float, displacer and magnetic gaugesMechanical level and interface measurement with local indication and transmitter options.
- Point-level switchesVibrating forks, capacitance probes and float switches for high, low and overfill alarms.
02Compare
Measuring principles
Contact or non-contact, continuous or point — compare against the surface conditions and the vessel before accuracy.
| Principle | How it works | Suited for | Considerations |
|---|---|---|---|
| HydrostaticMedia: Liquid | How it worksThe pressure at the bottom of an open vessel is proportional to liquid height multiplied by density. | Suited forOpen tanks, wells, sumps and reservoirs; submersible probes for water and wastewater. | ConsiderationsDensity must be known and stable; closed vessels need the vapour-space pressure compensated (DP). |
| Differential pressureMedia: Liquid | How it worksThe DP between the vessel bottom and top cancels the vapour-space pressure. | Suited forPressurized vessels, boiler drums, hot or corrosive liquids with remote seals, and interface. | ConsiderationsDensity dependent; wet or dry legs and seal capillaries add temperature effects. |
| Non-contact radarMedia: LiquidBulk solids | How it worksMicrowave signals (pulse or FMCW) reflect from the surface; their travel time gives the distance. | Suited forLiquids and bulk solids across wide pressure and temperature ranges; unaffected by density changes. | ConsiderationsFoam, internal obstructions, low-dielectric media and nozzle geometry need care; antenna and frequency selection matter. |
| Guided wave radarMedia: LiquidBulk solids | How it worksA microwave pulse travels along a rod or cable probe and reflects from the surface — and from an interface layer. | Suited forLiquids and interface, turbulent or foaming surfaces, small or cluttered vessels; cable probes for bulk solids. | ConsiderationsA contact method: build-up, probe length and mechanical load (in solids) must be considered. |
| UltrasonicMedia: LiquidBulk solids | How it worksSound pulses reflect from the surface; the echo time gives the distance. | Suited forWater and wastewater, open-channel flow, sumps and simple bulk-solid silos. | ConsiderationsTemperature, vapour, foam, dust and turbulence affect the echo; a blocking distance applies below the sensor. |
| Float and displacerMedia: Liquid | How it worksBuoyancy moves a float, or changes the apparent weight of a displacer, with the liquid level. | Suited forClean liquids, interface, and magnetic level gauges with local indication. | ConsiderationsMoving parts; displacers depend on density; build-up and turbulence affect operation. |
| Point level — vibrating fork and capacitanceMedia: LiquidBulk solids | How it worksA fork's resonant frequency changes when the medium covers it; a capacitance probe detects the change in capacitance. | Suited forHigh and low alarms, overfill and dry-run protection in liquids and bulk solids. | ConsiderationsSwitches, not continuous measurement; heavy build-up and very viscous media need attention, as do dielectric changes for capacitance. |
General engineering guidance. Limits and performance depend on the specific instrument and variant — confirm them against the manufacturer's datasheet for the selected model.
03Specify
What to specify
The parameters that decide the selection. State each with its unit; where a value is not known yet, say so — it becomes part of the engineering review.
Function
Inventory, control, alarm or overfill protection — decides continuous versus point level and whether devices must be independent.
Medium
Liquid or bulk solid, dielectric constant, density and its stability — each principle depends on a different property.
Measuring range
Vessel height and the dead zones at top and bottom; blocking distances and probe lengths follow from it.
Vessel and internals
Open or closed, pressurized, agitated, with heating coils or baffles — internals create false echoes.
Surface conditions
Foam, turbulence, vapour, condensation and dust decide whether non-contact methods see a clean surface.
Process temperature and pressure
Set the materials, seals and the antenna or probe construction.
Process connection
Nozzle size and height, flange rating or thread, and roof access determine what can be mounted.
Accuracy
Inventory and transfer measurements need more than control or alarm points.
Output and communication
4–20 mA / HART or fieldbus for continuous measurement; relay or discrete outputs for point level.
Area classification
Tank farms and solvent storage are often hazardous areas — certification must match the zone.
04Integrate
Outputs & communication
Interfaces typically offered in this category, and what each means for the control system. Availability depends on the specific model.
- 4–20 mAAnalog
- Current loop with a live zero: 4 mA is the bottom of the range, so a broken wire (0 mA) is distinguishable from a zero reading. Two-wire (loop-powered) on most transmitters; four-wire with an active output on line-powered instruments such as analyzers — the I/O channel must match. Accepted by practically every PLC and DCS analog input.
- HARTDigital over analog
- A digital signal (Bell 202 FSK, 1200/2200 Hz) superimposed on the 4–20 mA loop — configuration, diagnostics and secondary variables over the existing two wires.
- Modbus RTUSerial
- Serial client/server (formerly master/slave) protocol, typically over RS-485: several devices share one multi-drop cable and the client polls each for its values — common on analyzers, multivariable meters and packaged equipment.
- PROFIBUS PAFieldbus
- Two-wire, bus-powered process fieldbus at 31.25 kbit/s, coupled to PROFIBUS DP — several instruments share one cable, with intrinsically safe variants for hazardous areas.
- FOUNDATION Fieldbus H1Fieldbus
- Two-wire, bus-powered digital fieldbus at 31.25 kbit/s with function blocks in the field devices — used mainly where the DCS architecture is built around it.
- Relay / discreteDiscrete
- Switched contact or transistor output for alarms, interlocks and pump or valve control — it carries a state, not a value.
05Install
Installation practice
- Keep non-contact devices away from the fill stream, vessel walls and internals, and check nozzle height and diameter against the antenna.
- Stilling wells or bypass chambers calm the surface and give a defined measuring path in turbulent or foaming liquids.
- Overfill protection is commonly a separate, independent point-level device rather than the transmitter used for control.
- For inventory, convert level to volume with the tank's strapping (linearization) table in the transmitter or the control system.
06Products
Level products
Status · no models listed
Specific models are not listed on the site.
Our engineers can recommend instruments against your process data. The selection checklist on this page is the information that decides it — send what you have.
How a review runs
LT01Share the process data
Medium, range, conditions, connection and the system it reports to.
02Engineering review
The measuring principle and the installation are checked against the application.
03Instrument options
Suitable devices are proposed with the data needed to confirm the selection.
Next steps
Where to go next
- 01SolutionInstrumentationProcess measurement and final control elements — selected for the process conditions, installed, calibrated and loop-checked.
- 02ResourcePressure vs Differential Pressure MeasurementGauge, absolute and differential pressure — what each measures and where each is used.
- 03Related categoryDifferential PressureDP transmitters for flow, level, filter and pressure-drop measurement.
Engineering enquiry
Choosing a level principle for your vessel?
Share the medium, the range, the process conditions and the system it has to connect to. Our engineers can help match the principle and the instrument to the application.