The Metal Tube Float Flowmeter is a variable area flow measurement instrument composed of a measuring tube, float, indicator, and process connections.
With its compact structure, wide measuring range, and easy installation, it has been widely applied in industrial process control systems for the measurement of liquids, gases, and steam, especially in low flow velocity and small flow rate applications.
The metal tube float flowmeter is available in local indication type and intelligent remote transmission type.
Local type provides pointer indication for instantaneous flow measurement.
Intelligent type features pointer indication combined with an LCD display, showing instantaneous flow and totalized flow, and supports high and low limit alarm outputs, pulse output, standard two-wire 4–20 mA current output, and HART communication protocol.
Flow Meter Description
Metal Tube Flow Meter-Features
Simple and Robust Design – The metal tube rotameter features a compact structure with excellent stability and long-term reliability, ensuring consistent performance in demanding industrial environments.
Independent of Medium Properties – Accurate flow measurement is maintained regardless of the medium’s conductivity, dielectric constant, or other physical and chemical properties.
Versatile Application – Suitable for a wide range of media, including corrosive, toxic, and explosive substances, making it ideal for diverse industrial processes.
Multi-Media Measurement – Capable of measuring interfaces or levels of two different media with varying densities, providing flexible process monitoring.
Advanced Signal and Display Options – Supports two-wire 4–20 mA output and digital LED display (0.56” or 0.8”), enabling local reading and easy integration with automated control systems.
Metal Tube Flowmeter-Applications
With its excellent performance, high reliability, and cost-effective solution, the metal tube float flowmeter is widely used in:
Petroleum and chemical industries
Steel and metallurgical processes
Power generation and energy systems
Light industry and food processing
Pharmaceutical production
Water and wastewater treatment
By selecting different wetted materials, the flowmeter is suitable for non-corrosive, corrosive, and highly corrosive media, meeting the demands of various industrial applications.
Rotary Flow Meter-Working Principle
The metal tube float flowmeter measures flow based on the variable area principle. The float rises or falls inside the tapered tube until the upward hydrodynamic force balances the weight minus buoyancy:
Ff = Cd × 0.5 × ρ × v2 × A
Fg = (ρf – ρ) × V × g
At equilibrium, the flow rate Q is proportional to the float’s position:
Q = K × Aeffective × √[ 2 × (ρf – ρ) × V × g / ρ ]
Key Points:
Rotary Flowmeter-Flexible Installation Structures
According to different process connection requirements, the metal tube float flowmeter is available in multiple measuring tube configurations, including:
Bottom inlet – top outlet
Bottom inlet – side outlet
Side inlet – side outlet
Right inlet – left outlet
Left inlet – right outlet
Top inlet – bottom outlet
Users can select the appropriate structure to meet specific installation and process requirements.
Flow Meter Parameter
| Measuring range | Water:1-150000L/H(20℃) |
| Air: 0.7~3000m3/h (20℃, 0.1013MPa) | |
| Measuring range proportion | Standard: 10:1 |
| Accuracy class | Standard:1.5, 2.5; Special: 1.0 |
| Pressure rating | Standard & sanitary one:DN15~DN50≤4.0MPa; DN80~DN200≤2.5MPa |
| Special one:DN15~DN25≤42MPa;DN50~DN100≤16MPa | |
| Jacket one: 1.6MPa | |
| Pressure loss | 7kPa~70kPa |
| Medium temperature | Standard:-70℃~+180℃; PTFE:-50℃~+100℃(Change frequently is not allowed) |
| Environment temperature | Remote type: -40℃~+85℃ (LCD display: -35℃~+70℃) |
| Local needle indicator: -40℃~+100℃ | |
| Connection type | Flange; Sanitary tri-clamp |
| Jacket connection | DN15/PN1.6MPa or 1/2” ANSI 150LB RF or φ12mm tube |
| Flange Standard | Standard: GB/T 9119-2010, HG20592, ASME/ANSI B 16.5, DIN2501, SH3406 |
| Food type: SMS, DIN 11851, Tri-clamp | |
| Wiring connection | M20×1.5; 1/2NPT female thread |
| Power supply | Standard: 24VDC two-line 4~20mA(12VDC~32VDC) |
| AC type:85~260VAC | |
| Battery type:3.6V lithium battery(2-3 years lifetime) | |
| Loading resistance feature | Two-line: max loading resistance 500Ω(24VDC) |
| Multi-line: max loading resistance 500Ω | |
| Warning signal output | Reed pipe warning switch output, upper limit and lower limit flow warning(contact capacity 250V 0.05A or 24VDC 0.2A) |
| LCD display | Instant flow display scope:0~99999 |
| Total flow display scope: 0~99999999 | |
| Protection grade | IP65, IP67 |
| Explosion-proof grade | Exia II CT6Ga Exd II BT6Gb |
| Installation Height: | Standard height: 250mm for DN15-DN200; High pressure type: 300mm for size > DN80 |
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By combining selection expertise with application understanding, Instrava helps customers reduce risk, improve efficiency, and achieve dependable results in complex industrial environments.
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What is the measurement principle of a metal tube float flowmeter and what physical parameters does it depend on?
A metal tube float flowmeter operates on the variable area flow measurement principle, where the flow rate is determined by the equilibrium position of a float inside a vertically mounted, tapered metal measuring tube.
The float reaches a stable position when the following forces are balanced:
Upward force generated by fluid flow
Gravity acting on the float
Buoyancy force of the medium
Drag force related to fluid density and viscosity
Therefore, the measurement result directly depends on:
Medium density
Medium viscosity
Flow regime (laminar or transitional)
Float geometry and mass
This is why metal tube float flowmeters are factory-calibrated for a specific medium and operating condition, and recalibration or correction is required if the process conditions change.
Under what process conditions is a metal tube float flowmeter technically suitable or unsuitable?
Metal tube float flowmeters are technically suitable for stable, continuous flow measurement under harsh process conditions, but they are not universal instruments.
High pressure systems: up to 25–40 MPa
High temperature processes: up to 300–450°C
Corrosive fluids: with proper material or lining
Steam, gas, and liquid applications
Low to medium flow rates with stable flow
Strongly pulsating flow
Highly unstable density or viscosity
Applications requiring very high accuracy (< ±0.5%)
Rapidly changing process conditions
Severe vibration without proper damping
From an engineering standpoint, correct application matching is more critical than the flowmeter model itself.
How accurate is a metal tube float flowmeter and what factors influence its measurement accuracy?
The typical accuracy of a metal tube float flowmeter is ±1.0% to ±1.5% of full scale (FS) under rated conditions.
Medium Density Deviation
Any deviation from the calibrated density will cause proportional flow error.
Viscosity Change
Increased viscosity increases drag force, causing the float to rise higher than expected.
Installation Conditions
Must be installed vertically
Straight pipe lengths upstream improve stability
Temperature & Pressure Variation
Extreme changes may alter medium properties and affect readings.
In practical industrial applications, metal tube float flowmeters are used for process indication, monitoring, and control, not custody transfer.
How should material selection be handled for different process media?
Material selection directly determines safety, service life, and measurement stability.
| Process Medium | Recommended Tube Material | Float Material |
|---|---|---|
| Water / Steam | SS304 / SS316 | SS316 |
| Corrosive acids | SS316L + PTFE lining | PTFE / Hastelloy |
| Alkalis | SS316L | Alloy / PTFE |
| Hydrocarbons | SS316 | SS316 |
| High-temperature steam | Alloy steel | Alloy steel |
Incorrect material selection may lead to:
Float sticking
Chamber corrosion
Measurement drift
Safety risks
From a procurement perspective, material compatibility confirmation is mandatory before ordering.
How are output signals generated and how reliable are they for control systems?
Modern metal tube float flowmeters use magnetic coupling to transmit float position to external devices without process penetration.
4–20 mA two-wire transmitter
Local mechanical pointer
Digital LED display
Limit switches (SPST / SPDT)
No direct contact with the medium
Immune to vibration and temperature fluctuations
Long-term stability for PLC and DCS integration
Typical applications include:
Flow interlocks
Low / high flow alarms
Simple closed-loop control
Metal tube flowmeters are preferred when signal stability and mechanical robustness are more important than ultra-high accuracy.
What are the most common selection mistakes made during procurement?
Based on field feedback, the most frequent procurement errors include:
Selecting flow range too close to maximum operating flow
→ Causes float saturation and poor resolution
Ignoring medium density and viscosity
→ Results in systematic measurement error
Using standard materials for corrosive media
→ Leads to premature failure
Assuming metal tube flowmeters are maintenance-free
→ Periodic inspection is still required in dirty media
Using them in highly fluctuating flow systems
→ Leads to unstable readings
A properly selected metal tube float flowmeter should operate reliably for 10–15 years with minimal intervention.
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