As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
For most applications, a Coriolis flowmeter is one of the most accurate flowmeter types available, especially when you need direct mass-flow measurement. For water systems, though, an electromagnetic flowmeter, or magmeter, is often the better practical choice. It can measure water flow very accurately with no moving parts and little added pressure loss.
For a rainwater harvesting system, the best choice usually depends on where you are measuring. A Coriolis meter may provide greater laboratory-grade accuracy, but a magmeter often makes more sense for measuring water moving through a full pipe from a tank, pump, or cistern.
What Makes a Flowmeter Accurate?
Flowmeter accuracy is usually stated as a percentage of the measured flow. A smaller percentage means the reading should be closer to the actual flow under the meter's specified conditions.
However, the number printed on the specification sheet is only part of the story.
Real accuracy also depends on:
- Flow rate
- Pipe diameter
- Whether the pipe stays full
- Air bubbles in the water
- Sediment and debris
- Pipe layout
- Straight pipe before and after the meter
- Fluid conductivity
- Temperature
- Calibration
- Installation quality
NIST notes that flowmeter performance can change when the fluid, temperature, pressure, flow rate, or installation conditions differ from the conditions used during calibration.
That means an expensive, highly accurate meter installed badly can give worse readings than a simpler meter installed correctly.
Coriolis Flowmeters Offer Very High Accuracy
A Coriolis flowmeter measures mass flow directly.
Inside the meter are tubes that vibrate while fluid moves through them. The moving fluid changes the way the tubes vibrate. Sensors measure this change and calculate how much mass is flowing through the meter.
High-quality Coriolis meters can achieve very low measurement errors. Technical guidance from Emerson notes that liquid mass-flow accuracy around 0.1% is common in Coriolis instrumentation, with some meters capable of about 0.05% under specified conditions.
Why Coriolis meters can be so accurate
They have several advantages:
- They measure mass directly.
- They do not depend on a spinning turbine or paddle.
- They can provide strong repeatability.
- Changes in fluid density have less effect on the basic mass-flow measurement.
- Many can also measure density and temperature.
Coriolis meters are often used where small measurement errors matter.
That does not automatically make them the best meter for a rain barrel or home cistern.
The meter still has to match the pipe size and flow range. It can also add some resistance to the water flow. For large rainwater pipes or simple irrigation systems, its level of measurement performance may be more than the system needs.
Electromagnetic Flowmeters Are Often Best for Water
An electromagnetic flowmeter, commonly called a magmeter, measures the movement of an electrically conductive liquid through a magnetic field.
Water normally contains enough dissolved material to conduct some electricity, although the meter's minimum conductivity requirement should always be checked.
Magmeters are especially useful for:
- Rainwater
- Irrigation water
- Well water
- Tank transfer systems
- Cistern systems
- Water treatment systems
One major advantage is that there are normally no moving parts in the flow path.
There is also no small turbine or paddle that has to turn as water passes through.
Some modern water magmeters specify maximum measurement errors around 0.5% of the reading, with higher-accuracy configurations around 0.2% under their stated operating conditions.
Those numbers are examples rather than a guarantee for every electromagnetic meter.
Why magmeters work well in rainwater systems
A magmeter can be a strong choice when water is moving through a completely full pipe.
For example, imagine a cistern feeding an irrigation system through a pump. A correctly sized magmeter installed in the discharge pipe can measure both instantaneous flow and total water use without putting mechanical parts in the water.
This can be useful for tracking:
- Gallons used by irrigation
- Water transferred between tanks
- Pump output
- Household non-potable water use
- Cistern consumption
For many homeowners who want accurate water measurement rather than laboratory-grade mass measurement, this is a good balance.
Coriolis vs. Magnetic Flowmeter Accuracy
The important difference is what the meters measure.
A Coriolis meter primarily measures mass flow.
A magnetic meter measures volume flow.
If you simply want to know how many gallons of rainwater went to your garden, volume flow is normally what matters.
If a process requires very precise mass measurement, a Coriolis meter has an advantage.
| Flowmeter type | Best suited for | Main limitation |
|---|---|---|
| Coriolis | Very high-accuracy mass flow | More complex than most home water systems need |
| Electromagnetic | Accurate water flow in full pipes | Liquid must meet conductivity requirements |
| Positive displacement | Low-flow liquid measurement | Moving parts can wear or foul |
| Turbine | Clean water at steady flow | Debris and changing flow can affect performance |
| Ultrasonic inline | Water without moving parts | Installation and flow profile matter |
| Clamp-on ultrasonic | Measuring without cutting the pipe | Accuracy depends heavily on pipe and installation |
| Paddle wheel | Basic water and irrigation monitoring | Moving sensor and flow profile affect readings |
There is therefore no single meter that is most accurate under every possible condition.
Positive-Displacement Meters Can Be Very Good at Low Flow
A positive-displacement meter divides flowing water into small, known volumes.
Each cycle represents a certain amount of water.
This approach can provide useful accuracy even when flow is relatively low.
Positive-displacement meters are common where total water volume matters more than rapid changes in flow.
Their main drawback in untreated rainwater is the mechanical measuring system.
Roof-collected rainwater can contain:
- Fine sediment
- Organic particles
- Small grit
- Biofilm
Good prefiltration can reduce these problems, but no prefilter removes the need to inspect and maintain equipment.
A positive-displacement meter can therefore be a good choice for clean, low-flow water, but it may require more attention than a meter with no moving parts.
Turbine Flowmeters Need Clean, Steady Water
A turbine meter contains a small rotor. Water flowing through the meter spins the rotor, and the meter converts its rotation into a flow reading.
Turbine meters can be accurate when:
- Water is clean
- Flow stays within the correct operating range
- The pipe stays full
- The meter is installed correctly
- The flow entering the meter is reasonably smooth
They are more sensitive to sediment and mechanical wear than magnetic meters.
For filtered rainwater flowing from a tank to an irrigation system, a turbine meter may work well. It is less attractive where leaves, grit, or other debris may reach the meter.
Ultrasonic Flowmeters Avoid Moving Parts
Ultrasonic meters use sound waves to calculate water velocity.
There are two broad arrangements.
An inline ultrasonic meter becomes part of the plumbing.
A clamp-on ultrasonic meter attaches to the outside of an existing pipe.
Clamp-on meters are useful when cutting into a pipe is difficult or undesirable. But convenience does not automatically mean maximum accuracy.
Results can depend on:
- Pipe material
- Pipe wall thickness
- Pipe diameter
- Sensor spacing
- Sensor alignment
- Air in the water
- Flow profile
- Straight pipe length
A correctly installed inline meter usually provides more predictable conditions than a temporary clamp-on installation.
Accuracy and Repeatability Are Not the Same Thing
Two terms often appear in flowmeter specifications.
Accuracy describes how close the measurement is to the actual flow.
Repeatability describes how consistently the meter gives the same reading when conditions stay the same.
A meter can be very repeatable but still be wrong by a small amount.
For example, a meter could repeatedly report 10.2 gallons when exactly 10 gallons pass through it. The reading is consistent, but it still has a measurement error.
Calibration is what helps determine and correct that difference.
NIST's liquid-flow calibration facilities compare meters with highly controlled reference systems. NIST also includes the meter's own reproducibility when determining calibration uncertainty.
Flow Range Matters as Much as Maximum Accuracy
Do not choose a flowmeter from its headline accuracy specification alone.
First find the flow range your system actually produces.
Suppose a meter performs well from 5 to 100 gallons per minute, but your drip irrigation system normally uses only 1 gallon per minute. Even if the meter has an excellent accuracy specification, it may not perform well that far below its intended range.
Account for a suitable flow meter for irrigation to understand control options for dependable low-pressure operation.
The opposite problem can also happen.
A small meter may restrict flow if a transfer pump regularly pushes much more water than the meter is designed to handle.
Check both:
- Minimum useful flow
- Maximum permitted flow
Also check the pressure rating and connection size.
A 1-inch pipe does not automatically mean every 1-inch flowmeter will perform properly in that system.
The Pipe Usually Needs to Stay Full
This is especially important in rainwater harvesting.
Many inline flowmeters are designed for a full pipe, meaning the entire internal cross-section contains water.
A pump discharge pipe normally stays full while the pump is running.
A rainwater downspout often does not.
Water may run along the bottom or wall of the downspout while the rest contains air. Installing a standard full-pipe flowmeter in that location can produce poor measurements or prevent the meter from working properly at all.
If your goal is measuring how much water arrives directly from the roof, confirm that the chosen measurement method can handle partially filled or open-channel flow.
Do not assume a meter intended for pressurized plumbing will accurately measure water falling through a downspout.
Straight Pipe Can Affect Accuracy
Some flowmeter types need relatively smooth water movement entering the sensor.
Elbows, valves, pumps, tees, and sudden changes in pipe diameter can disturb the flow.
This disturbance is sometimes called a flow profile problem. Instead of moving evenly through the pipe, water can swirl or move faster on one side.
The amount of straight pipe required depends on the specific meter.
Follow the manufacturer's installation requirements rather than relying on one general rule for every flowmeter.
This matters especially with turbine, paddle-wheel, and some ultrasonic meters.
Sediment Changes the Decision for Rainwater
Rainwater systems are not the same as clean laboratory water systems.
Roof runoff may carry:
- Dust
- Pollen
- Grit
- Roof particles
- Leaves
- Insect material
- Organic debris
Screens, first-flush devices, settling, and filters can reduce the amount that reaches downstream equipment.
A first flush is a device or arrangement that diverts some of the early roof runoff before water enters storage. That early runoff may contain more accumulated roof debris.
Meters with moving parts are generally more sensitive to debris than meters with an open measuring tube.
This is one reason electromagnetic meters are attractive for many rainwater applications.
It does not mean a magmeter eliminates the need for prefiltration or system maintenance.
Choose the Meter Based on Where It Goes
The most useful answer changes with the measurement point.
Pump discharge from a cistern
An electromagnetic meter is often a strong choice if the pipe stays full and the water meets the meter's conductivity requirement.
Irrigation supply line
Magnetic, ultrasonic, turbine, or other water meters may work. Flow range and debris tolerance may matter more than achieving the smallest possible measurement error.
Very low-flow water use
A meter specifically designed for low flow may be more suitable than a larger meter with a better headline accuracy figure.
Positive-displacement technology can be worth considering when the water is sufficiently clean.
Tank-to-tank transfer
A magnetic meter can work well where a pump creates steady full-pipe flow.
Roof downspout
A normal full-pipe meter may be a poor fit because the pipe may contain both water and air.
Precision process measurement
A Coriolis meter is usually one of the first types to consider when very accurate mass flow is required.
Flow Rate or Total Volume?
Make sure the meter measures what you actually want to know.
Flow rate tells you how fast water is moving, such as gallons per minute.
A totalizer keeps adding the flow over time and reports the total amount that passed through the meter.
For a rainwater system, the totalizer may be more useful.
For example, knowing that an irrigation zone is flowing at 8 gallons per minute can help diagnose a system.
Knowing that the zone used 400 gallons during the week helps track storage use.
Many electronic flowmeters can provide both, but this should be confirmed before choosing one.
What Is the Most Accurate Choice for Rainwater?
For pure measurement performance, Coriolis meters are among the most accurate commonly used flowmeter technologies.
For most rainwater harvesting systems, however, an electromagnetic flowmeter is usually the more practical high-accuracy option when water is moving through a full pipe.
A good rainwater flowmeter should match:
- The expected minimum and maximum flow.
- The pipe diameter and connections.
- Whether the pipe remains completely full.
- The water's conductivity.
- The amount of sediment and debris.
- The available power supply.
- The required measurement accuracy.
- Whether you need flow rate, total volume, or both.
A well-matched meter installed correctly is more useful than choosing a meter type only because it has the smallest advertised error.
Frequently Asked Questions
Is a Coriolis flowmeter more accurate than a magnetic flowmeter?
Coriolis meters can provide extremely accurate direct mass-flow measurements and are often used where measurement uncertainty needs to be very low. Magnetic flowmeters can also provide excellent accuracy for water and are often more practical for rainwater, irrigation, and tank-transfer systems.
What is the best flowmeter for rainwater?
An electromagnetic flowmeter is often a good choice for rainwater moving through a completely full pipe. It has no moving measuring parts and can tolerate typical water-service conditions well. Check the meter's flow range, pipe requirements, and minimum conductivity before using one.
Can a flowmeter measure water coming directly down a downspout?
Not every flowmeter can. Many inline meters require the pipe to remain completely full of water. A downspout often contains both water and air, so a standard full-pipe meter may give unreliable readings. Use a measurement method designed for the actual flow conditions.
Are clamp-on ultrasonic flowmeters accurate?
They can provide useful measurements when installed correctly, but their accuracy depends strongly on the pipe material, dimensions, sensor position, flow conditions, and installation. They are useful when you do not want to cut into existing plumbing.
Does pipe size affect flowmeter accuracy?
Yes. The meter must be correctly sized for both the pipe and expected flow range. An oversized meter may perform poorly at low flows, while an undersized meter can restrict flow or operate beyond its intended range.
Does dirty rainwater damage flowmeters?
Sediment and debris can affect some meters, especially those with turbines, paddles, or other moving parts. Screens, settling, and suitable prefiltration can reduce the problem. The meter should still be inspected and maintained according to its instructions.
Does a highly accurate flowmeter need calibration?
Calibration is important when measurement accuracy matters. Calibration compares the meter with a known reference and helps identify measurement error. The required calibration interval depends on the meter, its use, and the level of measurement confidence needed.
