An inline compressed air flow meter installs directly into pipes from 1/8″ to 2″ and measures SCFM, Nm³/h, or m³/h — typically to ±1% accuracy for leak detection, energy audits, and cost allocation. For larger pipe diameters, an insertion-type gas flow meter can be selected. Sino-Inst primarily supplies thermal, vortex, metal rotor, and differential pressure compressed air flow meters.
Sino-Inst offers compresses air flow meters with and without local display. And 4-20mA and 0-10v outputs and for hazardous area applications are available.
An inline compressed air flow meter is a type of inline flow meter that can be installed in conjunction with a compressed air pipeline. The dimensions of an inline compressed air flow meter must match the pipeline dimensions. During installation, the compressed air pipe is typically cut off before the Inline Compressed Air Flow Meter is installed.
Technically speaking, Inline Compressed Air Flow Meters mainly fall into four categories: thermal, vortex, metal rotor, and differential pressure. Here’s how they work:
The working principle of a thermal mass flow meter is based on the principle of heat diffusion, that is, the heat exchange relationship between a fluid and a heat source.
Specifically, a thermal mass flow meter contains two main sensors: a velocity sensor (usually a heater) and a temperature sensor. These two sensors are placed in the gas being measured. The velocity sensor is heated, while the temperature sensor measures the temperature of the gas. As the gas flow rate increases, the heat carried away also increases, causing the temperature sensor’s temperature to drop.
By measuring the linear relationship between the temperature change before and after the temperature sensor reading and the gas mass flow rate through the pipe, the gas mass flow rate can be calculated.
The working principle of the vortex flowmeter is based on the Karman vortex principle, which was discovered and deeply studied by the American-Hungarian scientist von Karman in 1911.
When a fluid (such as gas or liquid) flows through a non-streamlined object, pairs of vortices with opposite rotation directions will appear alternately on both sides of the object’s wake. These vortices are arranged at a certain frequency to form the so-called Karman vortex street.
The frequency of the vortex generation is proportional to the flow rate of the fluid and inversely proportional to the diameter of the vortex generator. The relationship is Sr=fd/V.
Where Sr is the Strouhal constant, f is the vortex frequency, d is the vortex shedding diameter, and V is the flow velocity. The vortex flowmeter measures the vortex shedding frequency to deduce the fluid velocity or flow rate.
The structure of a rotameter flow meter is a vertical tapered metal tube whose cross-sectional area gradually expands from bottom to top. There is a rotor (or float) made of metal or other materials that can rotate freely. The fluid to be measured enters from the bottom of the metal tube and flows out from the top.
When the fluid flows through the vertical tapered tube from bottom to top, the rotor is acted upon by two forces: one is the vertical upward pushing force, which is equal to the pressure difference generated by the fluid flowing through the annular cross-section between the tapered tube and the rotor.
The other is the vertical downward net gravity, which is equal to the gravity on the rotor minus the buoyancy force of the fluid on the rotor. When the flow increases so that the pressure difference is greater than the net gravity of the rotor, the rotor rises. When the pressure difference is equal to the net gravity of the rotor, the rotor is in equilibrium. That is to stay in a certain position.
A reading is engraved on the outer surface of the metal tube. According to the dwell position of the rotor, the flow rate of the measured fluid is read.
The rotameter is a variable cross-section constant pressure differential flowmeter. The pressure difference acting on the upstream and downstream of the float is a constant value. The annular cross-sectional area between the float and the tapered tube changes with the flow rate. The position of the float in the tapered tube reflects the flow rate.
A differential pressure flow meter measures flow by measuring differential pressure. It is based on the principle that there is a certain relationship between the pressure difference and the flow rate generated when the fluid flows through the throttling device.
Also known as a DP flowmeter, it consists of a flow sensor and a pressure/differential pressure transmitter. It can be configured with a variety of restrictive elements, such as orifice plates, venturi tubes, wedges, V-cones, and Annubars. Differential pressure (DP) flowmeters are suitable for water, gas, steam, oil, and other applications.
In summary, inline compressed air flow meters mainly fall into four categories: thermal mass flow meters, vortex flow meters, metal rotor flow meters, and differential pressure flow meters.
Thermal mass flow meters provide direct SCFM readings without pressure/temperature compensation. Vortex and DP-based meters provide volumetric flow and require T/P compensation for mass flow. Rotary meters are the cheapest option but only for local visual indication.
| Type | Working Principle | Accuracy | Turndown | Pipe Size | Pressure Loss | Straight Pipe (U/D) | T/P Compensation | Output | Best For Compressed Air |
| Thermal Mass | Heat dissipation from heated sensor measures mass flow directly | ±1–1.5% of reading | 100:01:00 | DN15–DN300 | Very low (<0.5 kPa) | 15D / 5D | Not required (direct mass) | 4–20 mA, pulse, Modbus, HART | Best overall — direct SCFM, low leak detection, energy audits |
| Vortex | Karman vortex shedding frequency ∝ velocity | ±1% of reading | 30:01:00 | DN15–DN300 | Medium (5–15 kPa) | 20D / 5D | Required (external T/P) or built-in (SI-3305) | 4–20 mA, pulse, HART | Medium–high flow, steam-capable, wide temperature |
| Metal Rotameter | Float rises in tapered tube; flow ∝ float position | ±1.6–2.5% of FS | 10:01 | DN15–DN200 | Low–medium | 5D / 2D | Required (scale is fixed-condition) | Local dial (+ optional 4–20 mA/HART) | Cheapest option, local visual indication only, small branch lines |
| Orifice Plate (DP) | Pressure drop across orifice ∝ (flow)² | ±1–2% of FS | 3:1–4:1 | DN50–DN2000 | High (10–50 kPa) | 10D / 5D | Required | 4–20 mA (via DP transmitter) | Legacy systems, very large pipes, established instrumentation |
| Averaging Pitot (Annubar/Verabar) | Multi-port pitot averages velocity across pipe | ±1–1.5% of reading | 10:01 | DN50–DN9000 | Very low (<1 kPa) | 7D / 3D | Required | 4–20 mA (via DP transmitter) | Very large air mains where low pressure loss matters |
| V-Cone (DP) | Cone-shaped restriction creates stable DP, self-conditioning | ±0.5–1% of reading | 10:01 | DN15–DN1200 | Medium (3–10 kPa) | Only 0–3D / 0–1D | Required | 4–20 mA (via DP transmitter) | Tight spaces with little straight pipe, dirty/wet air |
The choice between an insertion flow meter and an inline flow meter is primarily based on two factors: 1. Pipe diameter. 2. Accuracy requirements.
First, for small pipe diameters, DN2~DN200, inline flow meters are suitable. Installation is simpler on these pipes. For pipe diameters larger than DN200, insertion flow meters are a better choice due to installation difficulty and cost considerations.
Secondly, the required measurement accuracy must be considered. Inline flow meters, installed inside the pipeline, achieve maximum measurement accuracy. Insertion flow meters typically have lower accuracy than inline flow meters.
To measure compressed air flow, install an inline flow meter sized to your pipe and flow range, choose the correct flow unit (SCFM for cost/energy reporting, ACFM for physical pipe velocity, Nm³/h for SI reporting), and add pressure and temperature compensation if the meter doesn’t read mass flow directly.
Thermal mass meters output SCFM without compensation; vortex, DP, and rotameter meters all need a pressure and temperature input to convert volumetric flow into mass flow.
Check our SCFM (Standard CFM) vs. ACFM (Actual CFM) and Calculator.
Mass flow measures how much air (in kg/h or lb/min) moves through the pipe. Volumetric flow measures how much space (in m³/h or CFM) that air occupies. For compressed air, mass flow is what you pay for — every kilogram of air cost electricity to compress.
Mass flow is the only reading that tells you the truth about compressor performance and leak rates.
MFC vs MFM — what’s the difference?
For leak detection, energy audits, and compressor monitoring, you want an MFM (most compressed air applications). For blending gases or dosing air into a reactor at a fixed rate, you want an MFC. Our thermal mass meters are MFMs — they report flow but don’t control it. If you need closed-loop flow control, you can check out MFC.
Whether you need P&T compensation depends on which meter technology you chose:
| Meter Type | T/P Compensation | Why |
|---|---|---|
| Thermal Mass | ❌ Not needed | Measures mass flow directly from heat transfer |
| Vortex | ✅ External required | Vortex frequency ∝ velocity, not mass |
| Precession Vortex | ✅ Built-in | Integrated P and T sensors, auto-compensates |
| Metal Rotameter | ✅ Required (fixed scale) | Scale is calibrated to one pressure/temperature |
| Orifice Plate / Annubar / V-Cone | ✅ External required | DP is proportional to density × velocity² |
Based on our previous introduction, choosing a suitable Inline Air Flow Meter mainly requires you to check and confirm the following:
Four applications usually pay back the meter within 12 months: leak detection, cost allocation between departments, compressor efficiency tracking, and metering N₂/O₂ on shared piping.
US plants typically lose 20–30% of compressed air to leaks. Log flow on a weekend-night baseline — whatever SCFM the meter still reads is 100% leak loss. 45 SCFM leaking = roughly $14,000/year at $0.10/kWh. SI-3501 thermal mass is the right choice: 100:1 turndown catches both low overnight leak flow and full-production peak.
Compressed air costs about $0.25–$0.35 per 1,000 SCF at US industrial rates. One SI-3501 or SI-3502 per branch line gives finance defensible numbers to split the compressor bill by department. A 250 SCFM department running 4,000 hr/yr = ~$18,000/year chargeback instead of plant overhead.
Healthy rotary screws deliver 4.0–4.5 SCFM per HP. When it drops to 3.2, air end or inlet valve wear is costing you money. Install an SI-3301 vortex on each compressor discharge, log SCFM÷kW monthly — when the ratio slips 15% below baseline, schedule service before catastrophic failure.
Same meters work on N₂, O₂, and CO₂ if the gas factor is set at the factory. Thermal mass (SI-3501) needs a gas-specific calibration curve loaded during manufacture — you cannot switch gases in the field. Vortex (SI-3301) is gas-agnostic (same K-factor for air/N₂/O₂) but still needs P&T for mass output. For O₂ service we ship an oil-free, degreased version. See the nitrogen flow meter guide for N₂ generator sizing.
The price of Compressed air flow meters are decided by flollowing factors:
These factors are more or less related to each other. Example – the cost of flow meters increases with accuracy and lifetime quality.
Zhang Wei, possesses 20 years of experience as an automation instrumentation engineer, specializing in the research, design, installation, commissioning, and maintenance of automation instruments.
Face to various instrument communication protocols (such as Modbus, Profibus, etc.), with solid hardware circuit design and software programming skills (proficient in C language and PLC programming). Has extensive project experience; projects he has led and participated in have all achieved outstanding results, improving product accuracy, reducing costs, and increasing production efficiency.
Possesses excellent communication and coordination skills and a strong team spirit, enabling him to quickly respond to customer needs and provide high-quality automation instrumentation solutions.
