Types of Level Measurement: Technologies and How to Choose
The main level measurement technologies — radar, ultrasonic, hydrostatic, capacitance, float, guided wave — how each works, and how they connect to a PLC.
The main types of level measurement are radar (non-contact and guided wave), ultrasonic, hydrostatic/differential pressure, capacitance, float/displacer, and point-level switches including vibrating fork and conductive probes. Each technology measures tank or vessel level by a different physical principle, which determines which medium, range, and process conditions it can handle — and how it connects to a PLC.
Choosing the wrong technology costs money twice: once on purchase and again during troubleshooting when readings drift or fail. This guide covers how each technology works, where it excels, where it fails, and how every device type wires into a PLC analog or discrete input.
How Level Measurement Works: Continuous vs Point
Level measurement divides into two fundamental categories before any technology choice is made.
Continuous level measurement produces a live analog signal that represents the actual level at every moment — for example, 2.4 m in a 5 m tank. The transmitter outputs a 4–20 mA signal (or digital protocol such as HART, PROFIBUS, or IO-Link) that the PLC reads on an analog input module and scales to engineering units.
Point level measurement — also called level detection or level switching — produces only a binary on/off signal. The device reports "level has reached this point" (high alarm) or "level has dropped below this point" (low alarm or pump protection). The output wires to a discrete (digital) input on the PLC.
Most real-world tanks use both: a continuous transmitter for process control and inventory management, and one or two point switches as independent overfill protection or dry-run interlocks.
Radar Level Measurement (Non-Contact)
How It Works
A non-contact radar transmitter mounts at the top of the tank and directs microwave pulses downward toward the liquid surface. The device measures the time-of-flight (ToF) — the round-trip travel time from antenna to surface and back. Because microwaves travel at a known speed, the transit time converts directly to distance, and the PLC subtracts that distance from known tank height to get level.
Modern transmitters use FMCW (Frequency-Modulated Continuous Wave) radar rather than pulsed radar for better accuracy at short distances and in turbulent conditions. FMCW sweeps across a range of frequencies and analyzes the frequency difference between transmitted and reflected signals to determine distance.
Strengths
- No contact with the medium — suitable for corrosive, viscous, or high-temperature liquids
- Unaffected by changes in density, dielectric properties (for most modern transmitters), pressure, or steam above the liquid
- Works through foam in many cases
- Long measurement ranges — typically up to 30 m or more depending on product
Limitations
- Signal can be attenuated or lost in low-dielectric media (liquids with very low relative permittivity)
- Requires proper antenna selection for dusty bulk solids
- Tank geometry (internal agitators, heating coils) can cause false echoes; false echo suppression mapping is required during commissioning
PLC Connection
Non-contact radar transmitters almost universally output 4–20 mA with HART. The 4 mA represents empty (0%), and 20 mA represents full span (100%). The PLC analog input module scales this current range to engineering units (millimetres, metres, or percentage). HART allows a handheld communicator or the PLC's HART-capable module to read diagnostic values and transmitter configuration without breaking the loop.
Guided Wave Radar (GWR) / TDR
How It Works
Guided wave radar — also called Time-Domain Reflectometry (TDR) — uses the same microwave principle as non-contact radar, but guides the signal down a probe (rod, cable, or coaxial) that extends into the liquid. When the microwave pulse reaches the liquid surface (a zone of high dielectric change), a significant portion reflects back up the probe. The transit time from top of probe to reflection point determines level.
Because the signal travels along a physical probe rather than through open air, GWR is far less sensitive to vapors, foam, and turbulence than non-contact radar.
Strengths
- Excellent performance in turbulent, foamy, or vapor-laden environments
- Works in narrow standpipes and bypass chambers
- Suitable for interface measurement (oil over water layer) with the right probe configuration
- Generally more accurate than non-contact radar at short ranges
Limitations
- Probe must be kept clean; coating or buildup degrades accuracy
- Probe length physically limits measurement range
- Not suitable for very viscous media that coat the probe heavily
PLC Connection
Same as non-contact radar: 4–20 mA with HART is standard. Some devices also support PROFIBUS PA or Foundation Fieldbus for plant-wide digital bus integration.
Ultrasonic Level Measurement
How It Works
An ultrasonic transmitter mounts above the liquid and emits high-frequency sound pulses (typically 40–200 kHz). The sound wave travels through air, reflects off the liquid surface, and returns to the sensor. The transmitter measures the round-trip time and calculates distance — the same ToF principle as non-contact radar but using sound instead of microwaves.
Strengths
- Lower cost than radar — a good fit for straightforward water and wastewater applications
- No contact with the medium
- Simple installation and configuration
Limitations
- Temperature sensitivity: the speed of sound in air changes with temperature; on-board temperature compensation helps but is not perfect
- Cannot be used in vacuum or pressurized vessels
- Heavy vapors (steam, solvent-laden atmospheres) absorb or scatter sound, causing signal loss
- Foam on the liquid surface absorbs sound and prevents a return echo
- Blanking distance near the transducer (typically 200–500 mm) where measurement is unreliable
PLC Connection
4–20 mA output, with some units offering a relay output for a built-in high or low level alarm. Wire to a PLC analog input module and scale identically to radar. Many ultrasonic level transmitters also support RS-485 Modbus RTU, allowing digital integration over a single twisted pair.
For water treatment applications that use ultrasonic level sensors on sumps and clarifiers, see the water treatment PLC programming guide for worked examples of analog scaling and PID fill control.
Hydrostatic / Differential Pressure (DP) Level Measurement
How It Works
This is the oldest and most widely applied continuous level technology. It rests on a simple principle: the pressure at the bottom of a liquid column is proportional to the height of liquid above it. Specifically:
P = ρ × g × h
Where P is pressure (Pa), ρ is liquid density (kg/m³), g is gravitational acceleration (9.81 m/s²), and h is liquid height (m).
A submersible pressure transmitter sits at the bottom of the tank or is mounted externally on the lower nozzle. An open tank needs only a gauge pressure transmitter vented to atmosphere. A closed pressurized vessel requires a differential pressure (DP) transmitter: the high-pressure port connects to the bottom nozzle, and the low-pressure port connects to the top of the vessel so that head pressure above the liquid is subtracted automatically.
Strengths
- Well-understood, mature technology with very wide installed base
- Reliable for most liquid media; unaffected by vapor, foam, or surface turbulence
- Works in pressurized and vacuum vessels with correct DP configuration
- Remote diaphragm seals allow separation from aggressive media
Limitations
- Accuracy depends on knowing the liquid density; if density changes (temperature variation, mixture changes), readings drift
- Not suitable for slurries that can block the pressure port — diaphragm seals or flush-face transmitters required
- Requires process penetration (wetted parts)
PLC Connection
4–20 mA with HART is the dominant output. The PLC scales the current to level using the known density and transmitter zero/span calibration. In pump control applications, the PLC typically reads a submersible level transmitter on a 4–20 mA analog input and uses the scaled level value as the process variable for a PID loop or simple on/off hysteresis control to start and stop pumps.
Capacitance Level Measurement
How It Works
A capacitance probe extends into the tank. The probe and the tank wall (or a reference electrode) form the two plates of a capacitor, with the medium acting as the dielectric between them. As level rises, more of the probe is surrounded by the liquid rather than air or vapor. Since the dielectric constant of most liquids is higher than air, the capacitance of the probe-to-wall system increases measurably with level.
RF capacitance transmitters apply a radio-frequency signal to the probe and measure the resulting capacitance continuously, producing a 4–20 mA output proportional to level.
Strengths
- No moving parts
- Works well with granular solids and powders as well as liquids
- Good for high-temperature and high-pressure applications
- Can detect interface between two immiscible liquids if dielectric constants differ sufficiently
Limitations
- Accuracy depends on the dielectric constant of the medium being stable; if the product composition changes (moisture content in a powder, for example), calibration shifts
- Coating of the probe by the medium causes false high readings ("coating error") — some modern designs compensate electronically
- Not ideal for media with very low dielectric constants (dry non-polar hydrocarbons)
PLC Connection
4–20 mA continuous output for level transmitters. Point-level capacitance switches (single-point probes) output a discrete relay or NPN/PNP transistor signal directly to a PLC digital input module. For an overview of how different sensor output types wire to PLC input cards, see types of industrial sensors.
Float and Displacer Level Measurement
How It Works
Float-based devices use the buoyancy of a float that rides directly on the liquid surface. The float connects mechanically or magnetically to a position indicator, transmitter, or switch. As level rises or falls, the float rises or falls with it.
Displacer transmitters work on a different principle: a heavy cylinder (the displacer) is suspended in the liquid on a torque tube. As liquid level rises around the displacer, buoyancy increases, reducing the apparent weight of the displacer. The torque tube detects this weight change and converts it to a level signal.
Magnetic float switches are a common, low-cost point-level device: a magnet inside the float actuates a reed switch inside the stem at a preset level.
Strengths
- Float switches are mechanically simple, low-cost, and extremely reliable for point level in clean liquids
- Displacers are accurate for interface measurement and work well in high-pressure or high-temperature conditions
- No power required for purely mechanical float-and-tape gauge systems
Limitations
- Moving parts are susceptible to fouling in dirty or viscous liquids
- Float switches must be sized for liquid specific gravity
- Displacer transmitters require density calibration; density changes affect accuracy
- Limited to clean or mildly contaminated liquids; not suitable for slurries
PLC Connection
Float switches: discrete (on/off) output — either a dry contact relay or a solid-state NPN/PNP output wires directly to a PLC digital input. A magnetic float switch used as a high-level alarm wires in series with an interlock ladder rung.
Displacer transmitters: 4–20 mA analog output to PLC analog input module.
Vibrating Fork (Point Level Switch)
How It Works
A tuning-fork probe vibrates at its natural resonant frequency (typically around 100–1,000 Hz) driven by a piezoelectric element. When the fork is immersed in liquid, the vibration frequency drops measurably because of the additional mass load of the surrounding liquid. The built-in electronics detect this frequency shift and switch the output state.
Vibrating fork switches are available for both liquids (fork detects damping) and bulk solids/powders (fork detects frequency shift and attenuation).
Strengths
- Highly reliable with no moving parts in contact with the liquid
- Not affected by flow, turbulence, foam, bubbles, or buildup (fork design largely self-cleaning)
- Wide range of process conditions: temperatures to 280 °C and pressures to 100 bar in process-grade versions
- SIL-rated versions available for safety instrumented systems (SIS)
Limitations
- Point level only — not a continuous measurement
- Some highly viscous media can prevent the fork from vibrating freely
PLC Connection
Discrete relay or transistor output to PLC digital input. Typically SPDT relay: use normally-closed (NC) contact for fail-safe high-level detection (relay de-energizes = trip) or normally-open (NO) contact for dry-run protection (relay de-energizes = low level = stop pump).
Conductive and Capacitive Point Switches
Conductive (Conductivity) Probes
Conductive level switches consist of one or more bare rod electrodes extending into the tank at different heights. When the conductive liquid (typically water or water-based solutions) rises to contact a probe, it completes an electrical circuit between the probe and the tank wall (or a ground electrode). This current flow is detected by the controller and switches the relay output.
Strengths: extremely simple, low-cost, no moving parts, very reliable for clean conductive liquids such as water. Limitations: the medium must be electrically conductive; not suitable for oils, most organic solvents, or deionized water. PLC connection: discrete relay or transistor output to PLC digital input.
Capacitive Point Switches
A single-point capacitive probe switches at a specific level rather than measuring continuously. The electronics detect whether the probe tip is surrounded by air (low capacitance) or liquid (higher capacitance) and switch the relay accordingly.
Strengths: works with both conductive and non-conductive media; no minimum conductivity requirement. Limitations: coating of the probe can cause false activation; needs calibration for the specific medium. PLC connection: discrete relay or transistor output to PLC digital input.
Level Technology Selection Table
| Technology | Medium Type | Typical Range | Contact/Non-Contact | Relative Cost | Best For |
|---|---|---|---|---|---|
| Non-contact radar | Liquid, some solids | Up to ~30 m | Non-contact | Medium–High | Chemical, oil & gas, agitated tanks |
| Guided wave radar (TDR) | Liquid, interface | Up to ~6 m | Contact (probe) | Medium | Foamy, turbulent, narrow standpipes |
| Ultrasonic | Liquid | Up to ~10 m | Non-contact | Low–Medium | Water, wastewater, simple tanks |
| Hydrostatic / DP | Liquid | Limited by transmitter span | Contact | Low–Medium | Most liquids, pressurized vessels |
| Capacitance (continuous) | Liquid, powder, solid | Up to ~6 m | Contact | Medium | Powders, granules, corrosive liquids |
| Float / displacer | Liquid, interface | Tank height | Contact | Low–Medium | Clean liquids, interface, simple duty |
| Vibrating fork | Liquid, bulk solid | Point only | Contact | Low–Medium | High/low alarms, SIS interlocks |
| Conductive probe | Conductive liquid | Point only | Contact | Very Low | Water sump level, simple alarms |
| Capacitive point switch | Liquid, solid | Point only | Contact | Low | Non-conductive liquids, powders |
Cost guidance is relative and varies significantly by manufacturer, process rating (pressure, temperature, materials), and approval requirements (ATEX/IECEx for hazardous areas, SIL for safety functions). Always compare installed cost including mounting, wiring, and commissioning — not just purchase price.
Continuous vs Point Level: When to Use Each
Use continuous level measurement when:
- The PLC needs to control level at a setpoint (PID loop, variable-speed pump control)
- Inventory management or custody transfer reporting is required
- The process requires proportional feed control based on current volume
- You need trending data for process optimization
Use point level switches when:
- Independent overfill protection is required (a continuous transmitter should never be the sole overfill protection for safety-critical tanks)
- Simple pump start/stop based on high and low levels is sufficient
- Budget is constrained and the application does not require a continuous reading
- The SIS (Safety Instrumented System) requires a discrete voted logic (e.g., 2-out-of-3 high-level detection)
Use both together for most industrial tanks: the continuous transmitter handles control, and one or two independent point switches handle the high-high alarm/shutdown interlock. This separation ensures that a single instrument failure does not simultaneously destroy control and protection.
How Level Devices Connect to a PLC
Continuous Transmitters: 4–20 mA Analog Input
The vast majority of continuous level transmitters — regardless of technology — output a 4–20 mA current loop signal. This is a two-wire, loop-powered circuit:
- The PLC's analog input module supplies 24 V DC to power the transmitter through the loop
- The transmitter regulates loop current between 4 mA (0% level, "live zero") and 20 mA (100% level)
- The PLC input module converts current to a raw integer (e.g., 0–27,648 on Siemens S7, 4,095 counts at 20 mA on many Allen-Bradley modules) that the program scales to engineering units
Scaling example (Siemens S7-1200):
The NORM_X and SCALE_X instructions (or equivalent) convert the raw analog word to engineering units. If the transmitter span is 0–5,000 mm, SCALE_X maps 4 mA (raw 5,530) to 0 mm and 20 mA (raw 27,648) to 5,000 mm.
The "live zero" principle is important: a 0 mA reading means a broken wire or lost power, not an empty tank. The PLC program should monitor for this condition and raise a fault alarm rather than treating it as a valid low-level reading.
Point-Level Switches: Discrete Digital Input
Point-level switches wire to the PLC's digital input module:
- Relay output switches: typically wired as a dry contact (SPST or SPDT) in series with the 24 V DC supply feeding the PLC digital input
- NPN transistor output (sinking): the switch pulls the PLC input terminal to 0 V (common) when active; compatible with PLC digital inputs configured for NPN
- PNP transistor output (sourcing): the switch sources 24 V to the PLC input terminal when active; compatible with PLC digital inputs configured for PNP
Always check whether the PLC digital input module is designed for NPN (current sinking) or PNP (current sourcing) input devices — mixing types is a common wiring error.
Tank Level Control Logic Basics
A simple two-setpoint hysteresis control for a pump-fed tank requires only the level transmitter's analog input and the pump's digital output:
- Scale the analog input to engineering units (e.g., 0–100% or 0–5,000 mm)
- Define a FILL_START setpoint (e.g., 20%) and a FILL_STOP setpoint (e.g., 80%)
- In the ladder or structured text: if level falls below
FILL_STARTand no fault is active, set pump run bit; if level rises aboveFILL_STOP, clear pump run bit - The independent high-level float switch or vibrating fork wires to a separate digital input and provides a hard interlock that stops the pump and raises an alarm regardless of the continuous transmitter reading
For a full PLC pump control worked example including analog input scaling, hysteresis logic, and fault handling, see the pump control PLC programming guide.
Level transmitters share the same 4–20 mA and HART architecture used by pressure transmitters — if you are familiar with pressure measurement wiring and scaling, the skills transfer directly to level. Similarly, the discrete output wiring of point-level switches follows the same conventions as the broader family of types of industrial sensors covered in the sensors overview.
When level measurement is part of a flow calculation (for open-channel flow via a weir or flume), the level transmitter's output feeds a flow calculation in the PLC; see flow meter types for the alternative direct-flow measurement technologies.
Frequently Asked Questions
What are the types of level measurement?
The main types of level measurement are radar (non-contact), guided wave radar (GWR/TDR), ultrasonic, hydrostatic/differential pressure, capacitance, float and displacer, vibrating fork, and conductive or capacitive point switches. They divide into continuous technologies that produce a live 4–20 mA analog signal and point technologies that produce a discrete on/off switch output.
What is the most accurate level measurement technology?
For liquids, guided wave radar (TDR) and non-contact FMCW radar consistently deliver the highest accuracy — typically ±1–3 mm depending on model and installation. Hydrostatic DP transmitters are also highly accurate when the liquid density is constant and the transmitter is correctly calibrated. Ultrasonic is less accurate because the speed of sound varies with air temperature and vapor density, introducing additional error even with compensation.
What is the difference between continuous and point level measurement?
Continuous level measurement outputs a proportional analog signal (4–20 mA) representing the actual liquid height at all times, enabling PLC control loops and inventory tracking. Point level measurement outputs only a binary on/off signal when the liquid reaches a specific height — used for alarms, interlocks, and simple pump start/stop control. Most industrial tanks use both: a continuous transmitter for control and one or more point switches for independent overfill or dry-run protection.
How does a level sensor connect to a PLC?
Continuous level transmitters connect to the analog input module of the PLC via a 4–20 mA two-wire current loop. The module converts the current to a digital value that the PLC program scales to engineering units using normalization and scaling instructions. Point-level switches connect to the digital input module as discrete on/off signals, wired as relay dry contacts or NPN/PNP transistor outputs depending on the module type. HART-capable transmitters can additionally communicate digital diagnostic and configuration data over the same two-wire loop.
