THE WATER PHYSICS LAB
See what the meter is sensing.
Move the controls. Follow the water, the moving parts and the signal. These simplified experiments explain principles; they do not select or certify a product.
01 / HOW IT WORKS
Flow, velocity and pipe area
A flow meter must turn its sensing signal into a volume per unit time. In a full round pipe, volume flow equals cross-sectional area multiplied by mean water velocity. Keep the volume flow fixed and reduce the bore: the same water must travel faster.
Q = A × v̄; A = πD² / 4
What this model assumes
This example holds flow at 3.6 m³/h. Diameter is actual bore, not nominal DN. It assumes steady, incompressible, full-pipe flow with no branch or leakage. A sensor samples a real velocity profile, so its calibration and installation conditions matter.
What this means when buying
Do not buy on pipe DN alone. Check low-flow measurement, maximum continuous flow and pressure loss. Reducing bore can increase velocity and hydraulic losses; this illustration does not calculate those losses.
Explore the buying guidance →02 / HOW IT WORKS
Water pushes. The rotor turns.
Single-jet and multijet meters direct water at an impeller. Its motion drives a register or a sensor. Woltman and propeller arrangements use an axial rotor. Counting movement gives volume; movement per unit time gives flow. Drag the control to turn the illustrated rotor.
V = revolutions / calibrated revolutions per litre
What this model assumes
The rotor angle is a teaching illustration, not a construction drawing. The chosen 2 rev/L calibration is fictional and constant only for this example. Real calibration depends on the exact measuring element, flow range and fluid.
What this means when buying
Friction and wear can affect low-flow response. Particles can obstruct moving parts. Compare the exact minimum flow, strainer requirements and orientation; jet and axial-rotor meters are not interchangeable.
Explore the buying guidance →03 / HOW IT WORKS
Count known portions of water.
Positive-displacement meters repeatedly fill and empty a measuring chamber. The mechanism counts these cycles. Nutating-disc and rotary-piston designs do this with different moving geometries; the central diagram is an abstract chamber, not either exact mechanism.
V = N × Vcycle
What this model assumes
Each completed cycle represents an illustrative 0.25 L. Twelve cycles therefore represent 3 L. Clearances, leakage through the mechanism and calibration are omitted from this ideal model.
What this means when buying
Defined measuring volumes can support useful low-flow performance, but dirt, wear, viscosity and pressure loss still matter. Confirm the chamber design, fluid compatibility and measured flow range.
Explore the buying guidance →04 / HOW IT WORKS
Moving water becomes an electrical signal.
Conductive water moving across a magnetic field produces an electrode voltage. With magnetic flux density B and electrode spacing D fixed, the ideal signal scales with velocity. Drag through zero: the voltage changes sign when flow reverses.
U ≈ B × D × v
What this model assumes
The crosses show a magnetic field into the page, perpendicular to flow and the electrode pair. This example uses B = 0.1 T and D = 0.1 m. Real instruments use excitation and signal processing; this is not an output or accuracy specification.
What this means when buying
Verify minimum conductivity, full-pipe conditions, grounding, electrode and liner compatibility. Low-conductivity RO/DM water needs explicit checking. A lack of moving parts does not remove installation requirements.
Explore the buying guidance →05 / HOW IT WORKS
Listen in both directions.
Transit-time ultrasound crosses the water in two directions. Sound travelling with the flow arrives sooner than sound travelling against it. At zero flow the times are equal. The small timing difference reveals the velocity component along the acoustic path.
tAB = L / (c + v cos θ); tBA = L / (c − v cos θ)
What this model assumes
Illustrative path L = 0.30 m, sound speed c = 1,480 m/s and angle θ = 45° to flow. Sound speed varies with water conditions. Dot separation is deliberately magnified; pipe geometry and refraction are omitted.
What this means when buying
Inline and clamp-on installations both need a supported acoustic path. For clamp-on, pipe dimensions, wall material, lining, coupling and liquid condition affect the measurement. Bubbles and poor signal quality can prevent a valid reading.
Explore the buying guidance →06 / HOW IT WORKS
Listen to moving reflectors.
Doppler measurement uses sound reflected from moving particles or bubbles. Their movement changes the received frequency. The instrument relates that shift to velocity. This is different from comparing two travel times through clean water.
|Δf| ≈ 2 f₀ v |cos θ| / c
What this model assumes
The simplified backscatter example assumes reflectors move with the representative liquid velocity. The chosen frequency and angle are illustrative. More solids do not automatically mean a better measurement.
What this means when buying
A sufficiently reflective, suitable fluid is necessary. Very clean water can lack useful scatterers; excessive aeration or solids can also interfere. Confirm the fluid and application with the manufacturer.
Explore the buying guidance →07 / HOW IT WORKS
Measure water movement and heat transfer.
A thermal-energy system combines flow with a paired supply/return temperature measurement. Water can move without transferring net heat: when the temperature difference is zero, this ideal sensible-heat calculation gives zero power. Accumulating power over time gives energy.
P ≈ ρ × cp × Q × ΔT; E = ∫ P dt
What this model assumes
This example fixes water flow at 10 m³/h and uses 1.163 kW per (m³/h × K), with approximate water properties. Glycol needs different properties. A real energy calculator uses the fluid model, installation side and applicable metrological requirements.
What this means when buying
A water totalizer alone is not an energy meter. Specify the compatible flow sensor, matched temperature pair, calculator and required approval scope. Small temperature differences make sensor-pair errors more significant.
Explore the buying guidance →08 / HOW IT WORKS
Sometimes water level reveals flow.
An open channel has a free surface rather than a full measuring tube. With a suitable calibrated weir, the head above the crest can be related to discharge. Doubling head does not merely double flow: this simplified relationship gives about 2.83 times the reference flow.
For an idealised rectangular weir: Q ∝ h³ᐟ²
What this model assumes
Only a normalised relationship is shown, using 100 mm as the reference head. No absolute discharge is calculated. Real weir equations depend on crest geometry, approach flow, contraction, ventilation and submergence. Flumes use their own rating relationships.
What this means when buying
Do not use a conventional full-pipe meter in a partially filled drain. Specify the structure and flow regime, or investigate a qualified area-velocity system. Measure head at the required upstream location.
Explore the buying guidance →THE OTHER PHYSICS THAT MATTERS
A good signal needs a suitable installation.
Elbows change the flow
Swirl and uneven velocity profiles can change a sensor’s sampled signal. Straight-run requirements belong to the exact meter and installation.
HydraulicsPressure is not flow
A closed valve can leave high pressure and zero flow. Pressure loss is an energy cost across a restriction; it is not the same as the line’s working pressure.
Specialist principlesVortices and other process meters
A bluff body sheds vortices whose frequency can track velocity within a qualified operating range. Differential-pressure systems instead infer flow from a calibrated pressure difference. Neither is a universal replacement for a water consumption meter.
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