High Pressure Helical Gear Flowmeter for Hydraulic Test Benches and Leak Testing

Designed for test stands where a component is judged good or bad by a few millilitres of oil per minute at 40 MPa. The meter measures true displaced volume, holds its calibration from cold oil to hot oil, and gives a pulse output your PLC can totalise without a viscosity correction table.

High pressure helical gear flowmeter, 40 MPa body with heavy duty flanges.

Second view: compact measuring chamber sized for leak flows from 0.05 L/min.

Why Test Benches Cannot Live With an Inferred Reading

A hydraulic test bench asks one question repeatedly: did this valve, pump or cylinder leak more than the limit? At 35 MPa the pass band is often 0.05–0.5 L/min against a background of oil that changes temperature by 40 °C during a shift. Any instrument that infers flow from rotor speed or pressure differential will drift across that band, and the drift becomes false rejects, retested parts and arguments with production.

Turbine meters are the usual first choice and the usual first failure: bearing drag rises as oil viscosity climbs at low temperature, so the K factor learned during a summer acceptance run no longer matches a winter morning. Differentials across an orifice plate flowmeter suffer the opposite problem, because viscosity changes the discharge coefficient itself. Neither instrument tells the operator whether the bench or the part is out of tolerance.

A helical gear meter counts displaced volume. The volume trapped between the helical rotors and the chamber wall is fixed by geometry, so the same pulse count corresponds to the same millilitres at 20 °C and at 70 °C. What changes with viscosity is the pressure needed to turn the gears, not the measurement — and on a bench that runs at 40 MPa, pressure is the one thing already available in abundance.

Two Working Conditions, Measured Side by Side

How Helical Gear Metering Holds Accuracy at Low Flow and High Pressure

Two helical rotors mesh inside a precision chamber. Product pressure turns them; the meshing line seals inlet from outlet, so a known volume is carried around the chamber wall on every rotation. Because the rotors are helical rather than straight-cut, the transfer of volume is continuous and gradual: there is no sudden opening and closing of a pocket, which is what produces the low pulsation and low noise this design is known for on a bench where a pressure gauge is being watched.

Each rotation generates a fixed number of pulses from a non-contact pick-up, and the K factor in pulses per litre is established on a gravimetric calibration rig at five points including the low-flow end where leak tests actually run. At 0.05 L/min the meter is turning slowly but still turning, and the trapped volume per revolution never changes — which is why the turndown reaches 1:200 without a technology change.

Pressure handling comes from the housing and the bearings, not from the measuring principle. The chamber wall is thick-sectioned and the rotor pins run in tungsten carbide bushes, so 40 MPa continuous and 50 MPa peak do not distort the measuring gap. A small internal leakage across the meshing line does exist and rises with differential pressure; it is characterised on the certificate, so the leak-test software can subtract it or work against a corrected curve.

Structural Design for Pressure Spikes, Contamination and Continuous Duty

Test benches are hostile in three specific ways, and the mechanical design answers each one:

• Pressure spikes and relief-valve events. A thick-walled chamber, bolted end plates with metal-to-metal seals and a 50 MPa peak rating keep the measuring gap stable when a directional valve closes abruptly.
• Contaminated oil. Hardened tool-steel rotors and tungsten carbide bearing pins tolerate the occasional particle that a stainless-on-stainless design would score. We specify 10 µm absolute filtration upstream and most benches already provide it for the component under test.
• Continuous three-shift duty. The rotor set and bearings are supplied as a matched pair, so wear is corrected on site by replacing the pair and recalibrating, rather than scrapping the whole instrument.

Six Advantages on a Test Bench

Selection, Filtration and Calibration for Bench Integration

Start from the smallest flow you must resolve. For leak detection at 0.05–2 L/min choose DN6; for a dyno rig running 5–80 L/min choose DN20. If a single bench must do both, install the small meter on the leak branch and the larger one on the return line — see the rest of our positive displacement range for the sizes in between — two instruments cost less than the rejects caused by stretching one meter across a 1:2000 range.

Mounting is simple because there is no straight-run requirement: the meter can sit on the manifold block, upstream of the test coupling, with elbows on both sides. Two rules apply. Keep the flow arrow aligned with actual flow, and for horizontal installation keep the rotor shaft axis horizontal so air cannot be trapped after an oil change. For benches that flush in both directions, order the bi-directional version with dual pulse output.

Calibrate with ISO VG 46 mineral oil at 40 °C on a gravimetric rig, record five points, and publish a viscosity correction curve for the operating range so the PLC can compensate if the bench runs cold or hot. If the bench tests fluids other than mineral oil, we calibrate with your fluid instead — the K factor is a property of geometry, not of marketing claims.

Integration layout (schematic drawing): leak measurement and full-flow flush on one meter.

Application Case 1: Hydraulic Valve Manufacturer, Italy

Application Case 2: Hydraulic Component Remanufacturer, Chile

Technical Specifications

Frequently Asked Questions

Why not use a turbine or a Coriolis meter on a hydraulic test bench?

A turbine meter relies on bearing-supported blades that slow down as oil viscosity rises with temperature, so a cold start at 20 °C and a warm run at 65 °C give different K factors unless you correct continuously. On a leak bench where the pass criterion is often a few millilitres per minute, that drift can be larger than the leak being measured. Coriolis meters are accurate and measure mass directly, but the smallest units cost several times more, are sensitive to pump pulsation, and are over-specified for routine pass/fail testing. A helical gear meter holds volumetric accuracy across the viscosity range, accepts 40 MPa working pressure, and produces a pulse output the PLC totalises without any conversion layer.

How does oil contamination affect accuracy and service life?

Hard particles are the main life limiter. Gears and bearings run with clearances measured in micrometres, so a 50 µm particle can score a rotor face and change the trapped volume permanently. Bench pressure is normally logged by a pressure transmitter on the same manifold, and the two channels should be calibrated against the same reference. We specify 10 µm absolute filtration upstream, which most test benches already provide for the component under test. Our rotor set uses hardened tool steel with tungsten carbide bearing pins, which tolerates occasional fines far better than stainless-on-stainless designs. When accuracy drifts more than 0.3% against your reference, the gear pair and bearings are replaced as a matched set on site and the meter is recalibrated — the housing and sensor stay in service, so the repair takes hours, not weeks.

Can the meter be mounted directly on the test bench manifold?

Yes, and that is how most of our customers install it. Because the measuring principle counts trapped volumes rather than inferring velocity, no upstream or downstream straight run is required, so the meter can sit close to a manifold block, a quick-release coupling or a directional valve. Two conditions apply: keep the flow direction arrow aligned with actual flow, and for horizontal mounting keep the rotor shaft axis horizontal so air cannot be trapped after an oil change. Where a bench flushes in both directions, order the bi-directional version with dual pulse output. Send us the manifold drawing before the order is released and we will confirm orientation, clearance and connection thread in writing.

What calibration fluid and conditions are used?

We calibrate with ISO VG 46 mineral oil at 40 °C unless your specification says otherwise, on a gravimetric rig with a traceable balance and temperature correction. Five points are recorded across the flow range, including the low-flow end where leak tests run. The certificate states the K factor in pulses per litre, the deviation at each point, and the ambient and fluid temperature during the test. Because viscosity and temperature shift the K factor slightly, we also publish a correction curve for your operating range so the PLC can compensate when the bench runs at 30 °C or 70 °C. Calibration with your own fluid is available when the bench handles fluids other than mineral oil.

Related Products and Further Reading

Test bench instrumentation is usually specified together, so these pages are the ones engineers ask for next:

• Positive displacement flowmeters — the full range, including the wet-type gas and disc versions.
• Reciprocating piston flowmeter — high precision in horizontal or vertical mounting for calibration benches.
• Orifice plate flowmeters — differential pressure measurement where the flow is steady and the range is fixed.
• Pressure transmitters — the pressure channel that normally sits alongside the flow channel on a bench.
• Case study: cooling water flow in a steel plant — a closed-loop installation with the same pulsation and filtration constraints.

Send us the manifold drawing and the leak limit you must resolve, and we will confirm size, connection and K factor in writing before the order is released.