Float, Displacer, and Differential Pressure Level Measurement

Why Level Measurement Matters

Accurate level measurement prevents overfills, protects pumps from dry-run, and keeps inventories and batching on target. From crude oil tanks and polymer reactors to sumps and clarifiers, choosing the right level technology improves safety, quality, and energy efficiency. This guide compares three proven workhorses: Float, Displacer, and Differential Pressure (DP) level instruments.

Overview: What Are These Level Instruments?

• Float Level Instruments – A buoyant float follows the liquid surface. Motion is converted into a level signal via linkage, magnet & reed switches, or a transmitter. Simple, visual, and widely used for local indication and switch control.
• Displacer Level Instruments – A heavy cylindrical displacer is partially immersed; changes in buoyant force (Archimedes’ principle) correspond to level. Force is sensed by a torque tube or spring and converted to an analog signal.
• Differential Pressure (DP) Level – Static pressure at a tap near the tank bottom is proportional to liquid head. Using a single pressure transmitter (vented tanks) or a DP transmitter (pressurized/closed tanks), level = pressure / (ρ·g).

Float Level Measurement: Principles and Applications

How It Works

1. A buoyant float rises and falls with the liquid surface.
2. Motion actuates a local indicator, limit switches, or a transmitter (magnetostrictive/ magnetic coupling).
3. The device outputs level or triggers alarms (high/low) for control.

Practical Example

A utilities tank farm uses magnetic level gauges with floats for local indication and high/low alarms on diesel day tanks—no external power needed for basic switching.

Advantages

• Simple, visible, and intuitive operation
• Excellent for discrete level switching (pump start/stop)
• Works without a control system when used as a gauge
• Suitable for many clean liquids; sanitary designs available

Limitations

• Moving parts → wear in viscous/dirty service
• Foam, turbulence, and buildup can affect movement
• Density changes affect float buoyancy and accuracy

Displacer Level Measurement: Principles and Applications

How It Works

1. A cylindrical displacer hangs in the vessel or chamber.
2. As level rises, buoyant force on the displacer increases (Archimedes’ principle).
3. A torque tube/spring mechanism senses force change, converting it to a level signal (pneumatic or electronic).

Practical Example

In a separator drum with moderate vapor and temperature swings, a displacer transmitter provides stable level control to a level-control valve on the outlet.

Advantages

• Robust and proven in many refinery and chemical services
• Less sensitive to surface foam than floats (measures buoyant force)
• Handles moderate temperature/pressure with appropriate chambers

Limitations

• Accuracy depends on fluid density—recalibration may be needed if density varies
• Requires mechanical linkages; potential for sticking or corrosion
• Not ideal for heavy buildup or severe slurry service without chambers

Differential Pressure (DP) Level Measurement: Principles and Applications

How It Works

1. Hydrostatic pressure at the bottom tap is proportional to liquid head (P = ρ·g·h).
2. For vented tanks: a single pressure transmitter referenced to atmosphere can infer level.
3. For pressurized/closed tanks: a DP transmitter measures bottom pressure minus vapor space pressure.

Practical Example

A closed solvent tank uses a DP transmitter with a bottom capillary and an upper reference leg to remove vapor pressure effects, delivering accurate inventory data to MES.

Advantages

• No moving parts; suitable for high pressure/temperature
• Works on opaque, viscous, or contaminated liquids
• Remote seals and capillaries enable isolation from corrosive or hot media

Limitations

• Depends on density; temperature or composition changes introduce error
• Install complexity (impulse lines, seals, capillaries) and maintenance for plugging
• Not ideal for rapid sloshing without damping/filtering

Comparison Table: Float vs Displacer vs DP

Typical Applications

1) Oil & Gas / Petrochemical

• Displacer: Separator/knock-out drums, reflux accumulators
• DP: Pressurized hydrocarbon tanks with remote seals
• Float: Sump tanks, day tanks with simple alarms

2) Chemical & Specialty Liquids

• DP: Corrosive/viscous media using PTFE-lined remote seals
• Displacer: Moderate vapor applications in external chambers
• Float: Non-critical storage with magnetic gauges

3) Water & Wastewater

• Float: Lift stations—simple pump control with float switches
• DP: Sludge tanks where optics/acoustics struggle

4) Food & Beverage

• Float: Hygienic float gauges for local viewing
• DP: CIP tanks with stable density assumptions

Installation & Reliability Best Practices

• Process Compatibility: Verify wetted materials (floats, displacer cages, DP seals) against chemistry and temperature.
• Mounting Geometry: For floats/displacers, use external chambers to isolate turbulence; for DP, keep impulse lines short and sloped to prevent air/gas pockets.
• Density Management: DP and displacer accuracy depend on ρ. Use temperature compensation or enter corrected density tables where available.
• Maintenance: Schedule inspection for buildup; flush impulse lines; verify torque tube freedom; test alarms with simulated levels.
• Validation: Cross-check with a sight glass, portable gauge, or transfer measurement during commissioning.

Frequently Asked Questions (FAQ)

Do floats or displacers work with foam?

Light foam is often manageable in displacer systems (buoyant force) but troublesome for floats. For persistent foam, consider radar level sensors.

How do density changes impact DP level?

DP assumes known density. Temperature/composition shifts change ρ and cause error. Use density compensation, temperature input, or alternative technologies when ρ varies widely.

Can I use DP on pressurized tanks?

Yes. Use a DP transmitter with high side at the bottom, low side to vapor space (or a remote seal) to cancel vapor pressure effects.

When is a float switch still the best choice?

For simple pump control, overflow protection, or local alarms in benign liquids—float switches are cost-effective and easy to maintain.

Float, Displacer, and DP instruments remain reliable choices for industrial level measurement. Floats excel for simple indication and switching, displacers thrive in many refinery/chemical vessels, and DP offers robust inventory measurement across opaque or harsh fluids. Match the method to your fluid properties, tank conditions, and accuracy needs to achieve safe, stable, and efficient operations.