A flow meter is a device used to measure the volume or mass of a gas or liquid moving through a pipe, tube or open channel. Flow meters are referred to by many names, such as flow gauge, flow indicator, liquid meter, flow rate sensor or flow transmitter, depending on the particular industry. Whatever the name, they all measure flow.
Open channels, like rivers or streams, may be measured with flow meters. More frequently, the most utility from a flow meter and the greatest variety of flow meters focus on measuring gases and liquids in a pipe. Improving the precision, accuracy, and resolution of fluid measurement are the greatest benefits of the best flow meters.
What does a flow meter measure?
A flow meter measures flow rate: how much fluid passes a point in a given amount of time. Volumetric flow meters report volume per unit of time, such as liters per minute or gallons per minute. Mass flow meters report mass per unit of time, such as kilograms per hour. Most meters also total the flow, giving the volume or mass delivered since the last reset.
Flow meter units of measurement
| Quantity | Common units |
|---|---|
| Volumetric flow rate, liquids | cc/min (mL/min), L/min, gpm, m³/h |
| Volumetric flow rate, gases | SCFM, SLPM, Nm³/h, stated at standard or normal conditions |
| Mass flow rate | g/min, kg/h, lb/h |
| Velocity | ft/s, m/s |
| Totalized flow | liters, gallons, cubic meters, kilograms |
Gas volume changes with pressure and temperature, so gas flow is usually stated at standard conditions or measured as mass. Liquids are close to incompressible, so liquid meters report volume directly. Max meters, for example, are specified in cc/min and liters per minute with the gpm equivalent alongside.
How does a flow meter work?
A flow meter works by placing a sensing element in the path of the fluid and converting what the fluid does to that element into a flow signal. Positive displacement meters trap and count fixed volumes of fluid. Velocity meters measure how fast the fluid moves and multiply by the pipe’s cross-section. Mass meters measure the mass of the fluid directly, through its inertia or its heat capacity.
The signal leaves the meter in one of a few forms. Mechanical meters drive a register or pointer. Electronic meters produce a pulse train, where each pulse represents a fixed volume set by the meter’s K-factor (pulses per liter or pulses per cc), or an analog signal such as 4-20 mA that scales with the flow rate. A flow transmitter is the electronics mounted on or near the meter that converts the raw sensor signal into one of these standard outputs and sends it to a display, data logger, PLC or control system. On a positive displacement meter the transmitter can also report direction of flow, so the same meter measures forward and reverse flow.
What are flow meters used for?
Flow meters are used wherever the amount of fluid matters: billing and custody transfer of fuel, water and gas; dosing chemicals into a process; monitoring production lines; testing engines, valves and injectors; dispensing adhesives, coatings and resins; and measuring fuel consumption. The accuracy required sets the meter type, from 5% for irrigation to 0.1% of reading for a test stand.
A few examples from Max customers: measuring the fuel burned by a single injector or a single-cylinder engine during gasoline and diesel fuel measurement, metering odorant and chemical injection in oil and gas, verifying hydraulic and servo-valve performance in aerospace test cells, and controlling adhesive bead volume on a production line. In every case the meter is there because the process cannot be proven, billed or controlled without a trustworthy number for how much fluid moved.
Why do I need a precision flow meter?
You might not! Precision flow meters are used to provide accurate monitoring and/or flow control. Some industrial applications require precise calculation of quantity, such as precision servo-valve development for the aerospace industry. On the other hand, an application to measure water flow to a vineyard may only require a measurement accuracy of 5% to 10%.
Not all flow meters measure and provide you with data in the same way. Because all sensors, including flow meters, have uncertainty or error in the data provided, the industry uses several terms to describe how close to the “true” flow the meter can report. Terms you will see include accuracy of reading, accuracy of full scale, repeatability, and resolution.
One significant difference in how flow meter data is presented is the difference between accuracy of reading and accuracy of full scale. A meter rated 0.5% of full scale is allowed the same absolute error at every flow rate, so at 10% of its range the error is 5% of what it is reading. A meter rated 0.5% of reading holds 0.5% across the whole range. Read more on accuracy of reading vs. accuracy of full scale.
What makes a meter a Max Precision Flow Meter?
Max Precision Flow Meters are positive displacement piston, gear and helical meters built for low, intermittent and bi-directional flows. They hold 0.2% accuracy of reading over a 200:1 turndown, respond in 1.6 milliseconds, deliver K-factors up to 12,000 pulses per cc, measure flow rates down to 0.005 cc/min, and ship with NIST traceable calibration across their full flow range. Read what makes a meter a Max Precision Flow Meter.
What are the various types of flow meters?
Flow meters fall into three families based on what they actually measure: volume, velocity or mass. Every meter on the market belongs to one of them.
Positive displacement flow meters
Positive displacement flow meters, also known as volumetric flow meters or PD flow meters, are unique as they are the only meter to directly measure the actual volume. All other types infer the flow rate by making some other type of measurement and equating it to the flow rate. With PD meters, the output signal is directly related to the volume passing through the meter.
The family includes bi-rotor types (spur gear, oval gear, helical gear, rotary vane), nutating disc, reciprocating piston, and oscillating or rotary piston meters. Because each measurement is a trapped volume, PD meters are not affected by the fluid’s viscosity, density or flow profile, need no straight pipe runs, and measure very low and intermittent flows that other technologies miss. Accuracy runs from 0.1% to 1.0% of reading with turndown from 10:1 to 200:1 or more. They need relatively clean fluid and are not economical for large pipe diameters. Read the full guide to positive displacement flow meters.
Mass flow meters
The output signal is directly related to the mass passing through the meter. Thermal and Coriolis flow meters fall into this category. Read the full guide to mass flow meters.
Coriolis flow meters
Coriolis flow meters vibrate a tube, or a pair of tubes, that the fluid passes through. Mass moving through the vibrating tube twists it, and the phase shift between the inlet and outlet ends of the tube is proportional to the mass flow rate. The same measurement gives the fluid’s density. Coriolis meters are accurate to 0.1% to 0.5% of reading with turndown up to 100:1, have no obstruction in the flow path, and are unaffected by pressure, temperature, viscosity or density. They are the most expensive industrial flow meters, mostly priced between two and twelve thousand dollars, and they do not respond fast enough for injection or rapidly cycling flows.
Thermal mass flow meters
Thermal mass flow meters place a heated sensor in the flow stream and measure how much heat the fluid carries away. Because heat transfer depends on the number of molecules passing, not their volume, the result is mass flow. They are used mainly for gases, are accurate to 1% to 2% of reading, and have no moving parts and a nearly unobstructed flow path. Gas sensors cost from a few hundred dollars up to a thousand; liquid versions cost thousands.
Velocity flow meters
The output signal is directly related to the velocity of the fluid passing through the meter. Flow rate is calculated as velocity multiplied by the cross-sectional area of the pipe, so velocity meters need a full pipe and, for most types, a settled flow profile with straight pipe upstream and downstream. Read the full guide to velocity flow meters.
Electromagnetic flow meters
Magnetic flow meters operate on Faraday’s law of electromagnetic induction: a conductor moving through a magnetic field produces a voltage. The liquid is the conductor, coils outside the flow tube create the field, and electrodes in the pipe wall pick up a voltage proportional to velocity. They only work on electrically conductive liquids such as water, wastewater, slurries and many chemicals, and cannot measure deionized water or hydrocarbons. There are no moving parts and almost no pressure drop. Accuracy is 0.3% to 1% of reading with flow ranges up to 100:1.
Ultrasonic flow meters
Ultrasonic flow meters send high-frequency sound through the fluid. Transit-time meters measure the difference in travel time of a pulse sent with the flow and against it; Doppler meters measure the frequency shift of sound reflected off particles or bubbles in the fluid. Because sound passes through pipe walls, transducers can clamp on the outside of the pipe with no process connection. Accuracy is about 1% of rate. Slurries with excess solids or entrained gas can block the signal.
Turbine, propeller and paddle wheel flow meters
A turbine flow meter is a rotor that spins in the pipe as fluid moves through it. A magnetic pick-up senses each blade, producing a pulse train proportional to flow. Turbine meters suit clean liquids with viscosities up to about 50 centistokes and gases, reach 0.5% to 1.5% of reading, and can turn down 100:1 when calibrated for a single fluid at constant conditions. They need several diameters of straight pipe on each side. Paddle wheel meters put a small rotor at the pipe wall, touching only part of the flow; they are inexpensive, typically 1% to 3% of full scale, and used for water and water-like fluids.
Vortex shedding flow meters
A vortex meter puts a bluff body across the flow. Fluid sheds eddies alternately from each side of it, and the shedding frequency is proportional to velocity. Vortex meters are used for steam, gas and clean liquids, are accurate to 0.5% to 1.0% of reading, tolerate changes in process conditions well and have low wear. They perform poorly on slurries, high-viscosity liquids and very low flows. Most cost $1,000 to $3,000.
Target flow meters
A target meter measures the force of the fluid on a disk suspended in the stream. Simple versions are hinged plates that act as flow indicators; instrumented versions use a strain gauge to read the force. They handle dirty and corrosive liquids, cost little, and offer 10:1 ranges with accuracies of 0.5% to 1.0% of full scale.
Variable area flow meters and rotameters
A variable area meter is a vertical tapered tube with a float inside. Flow lifts the float until the annular gap around it lets the fluid pass at equilibrium, and the float’s height indicates the flow rate on a graduated scale. Rotameters are simple, cheap, and have low pressure drop, with turndown up to 10:1 and accuracy of 1% to 5% of full scale. Magnetic floats add alarm and signal outputs. Prices run from $50 to $1,000.
Differential pressure flow meters
Orifice plates, flow nozzles, venturi tubes and pitot tubes all infer flow from the pressure drop across a restriction or the pressure of the moving fluid against a probe. They are simple, inexpensive and available in almost any material and pipe size, which is why they remain common in large lines, gas, steam and air ducts. Their weakness is range: an orifice plate turns down less than 5:1 and loses accuracy at low flow, a venturi or averaging pitot tube reaches about 10:1, and a worn orifice edge reduces accuracy further.
Flow meter comparison chart
| Meter type | Measures | Typical accuracy | Turndown | Works well for | Avoid for |
|---|---|---|---|---|---|
| Positive displacement | Volume, directly | 0.1% to 1.0% of reading | 10:1 to 200:1 or more | Low flows, viscous fluids, intermittent and bi-directional flow, custody transfer, test stands | Dirty fluids, large pipe diameters |
| Coriolis | Mass and density | 0.1% to 0.5% of reading | Up to 100:1 | Custody transfer, batching, mass-based dosing, gases and liquids | Rapidly cycling or injection flows, small budgets |
| Thermal mass | Mass, mostly gases | 1% to 2% of reading | About 100:1 | Gas flow, compressed air, low pressure drop gas lines | Most liquids, wet or dirty gas |
| Electromagnetic | Velocity | 0.3% to 1% of reading | Up to 100:1 | Water, wastewater, slurries, conductive chemicals | Hydrocarbons, deionized water, gases |
| Ultrasonic | Velocity | About 1% of rate | Wide | Large pipes, clamp-on retrofits, clean liquids | Slurries with solids, entrained gas |
| Turbine | Velocity | 0.5% to 1.5% of reading | Up to 100:1 on one fluid | Clean low-viscosity liquids and gases, fuels | Viscous or dirty fluids, pulsating flow |
| Paddle wheel | Velocity | 1% to 3% of full scale | About 10:1 | Water and water-like fluids on a budget | Viscous fluids, high accuracy needs |
| Vortex | Velocity | 0.5% to 1.0% of reading | About 20:1 | Steam, gas, clean liquids | Slurries, high viscosity, very low flow |
| Variable area (rotameter) | Velocity, by float position | 1% to 5% of full scale | Up to 10:1 | Local indication, purge flows, low cost | Remote data, high accuracy |
| Differential pressure (orifice, nozzle, venturi, pitot) | Velocity, by pressure drop | 1% to 2% of full scale | Under 5:1 orifice, about 10:1 venturi | Large lines, gas, steam, air ducts | Low flow rates, wide operating ranges |
Flow meter, flow gauge, flow sensor or flow transmitter?
“Flowmeter” and “flow meter” mean the same thing, and both spellings are in common use. A flow gauge or flow indicator usually means a simple local readout, often a variable area meter, with no signal output. A flow sensor is the measuring element itself. A flow transmitter is the electronics that convert the sensor’s signal into a standard output such as pulses or 4-20 mA.
In practice the terms overlap. A Max piston meter is a flow sensor, ships with a transmitter, and is sold as a flow meter. When comparing products, look at the specification rather than the name: what the device measures, how it reports it, and its accuracy of reading over its flow range.
How do you read a flow meter?
On a mechanical or variable area meter, read the flow rate where the float or pointer sits against the graduated scale. On an electronic meter, the display or transmitter shows the current rate and a running total. Pulse-output meters report flow as a pulse count; divide the count by the meter’s K-factor, in pulses per liter or per cc, to get the volume that passed.
When reading any meter, keep its accuracy specification in mind. A reading of 500 cc/min from a meter rated 0.2% of reading is within 1 cc/min of the true flow. The same reading from a 1,000 cc/min meter rated 1% of full scale could be off by 10 cc/min, and at 50 cc/min that same 10 cc/min error is 20% of the reading.
What type of flow meter is best?
There are no “universal” flow meters which are suitable for all applications. Selecting the proper technology for your application requires writing a flow specification which covers the use of the meter. There are usually trade-offs with each meter type, so knowing the critical specifications will be important.
Things you must know:
- What is the gas or liquid being measured?
- Minimum and maximum flow rates.
- What are the accuracy requirements?
- The fluid temperature and viscosity.
- Fluid compatibility with the materials of construction.
- The maximum pressure at the location.
- What pressure drop is allowable?
- Is the meter mounted in a hazardous location where explosive gases may be present?
- Is the fluid flow continuous or intermittent?
- What type of output signal or readout do you need?
Use your collected information to eliminate the technologies that do not apply. Turbines don’t work for viscous fluids, Coriolis meters don’t respond fast enough for injection flow, and positive displacement meters aren’t economical for large pipe diameters like a 12 inch line. You will then have a comparison of the remaining technologies. Accurate meters are priced based on their capabilities. It is better to locate the type of meter which fits your application before trading features for cost savings. Closely evaluate your extreme conditions, such as low flow rates, high pressure or temperature, or the need to measure over a wide operating range. If these conditions are critical to quality for you, do not use lower priced alternatives applied outside of their capabilities.
Max Precision Flow Meters
Max Machinery has designed and manufactured positive displacement flow meters in Healdsburg, California since 1967. Three technologies cover flows from 0.005 cc/min to 500 liters per minute.
- Piston flow meters: 0.005 cc/min to 35 L/min, 0.2% of reading over a 200:1 range, 1.6 millisecond response. The choice for fuel consumption testing, injector and valve testing, chemical injection and laboratory work. See the P002 piston flow meter for 0.5 to 2,000 cc/min at up to 7,250 psi, or the P215 piston flow meter for 25 cc/min to 35 L/min.
- Gear flow meters: 0.5 cc/min to 240 L/min, 0.3% of reading over a 100:1 range, fluids from 1 to 100,000 centistokes. Built for hydraulics, adhesive dispensing, lubricants, fuels and chemicals. The G004 gear flow meter covers 0.5 cc/min to 4 L/min and the G015 gear flow meter covers 10 cc/min to 15 L/min, both rated to 6,000 psi.
- Helical flow meters: 100 cc/min to 500 L/min, 0.2% of reading, viscosities to 1,000,000 centistokes. Made for heavy fuels, coatings, polymers and resins.
Not sure which meter fits your application? Call +1 (707) 433-2662 to speak with an application engineer, or request a quote.
Frequently asked questions
A flowmeter, also written flow meter, is an instrument that measures the rate at which a liquid or gas moves through a pipe or channel, in volume or mass per unit of time. Both spellings refer to the same device.
Flow meters are used to bill and transfer fluids, dose chemicals, monitor and control processes, test engines, injectors and valves, dispense adhesives and coatings, and measure fuel consumption. Any process that depends on knowing how much fluid moved uses one.
A flow meter measures flow rate, in units such as liters per minute, gallons per minute or kilograms per hour, and usually totalizes the volume or mass that has passed. Some meters also report fluid density, temperature or direction of flow.
On a piping and instrumentation diagram a flow meter is drawn as a circle with an ISA tag: FE for the flow element, FI for a flow indicator, FT for a flow transmitter, or FQ for a totalizer, with the loop number below. Orifice plates are also shown as a pair of short lines across the pipe.
A flow sensor is the element that responds to the fluid; a flow meter is the complete instrument, including the sensor and the electronics that turn its signal into a rate, a total or an output. Suppliers use the two words interchangeably for the same products.
A mass flow meter measures the mass of fluid passing through it rather than its volume, so the reading does not change with temperature or pressure. Coriolis and thermal meters are the two mass flow technologies.