Metering Pump Accuracy Explained: Steady-State, Repeatability and Linearity for Hydraulic Diaphragm Pumps

Metering pump accuracy is not one number. The "±1%" printed on most hydraulic diaphragm pump datasheets is usually steady-state accuracy: at a fixed setting under stable conditions, delivered flow stays within that band, and only over the stated adjustment range, often 10 to 100% of rated flow. Repeatability and linearity are separate specs that answer different questions. This guide is for process engineers, equipment specifiers and bid reviewers who need to read a spec sheet correctly and avoid comparing numbers that were measured different ways.

How hydraulic diaphragm pump flow range is stated

A hydraulic diaphragm metering pump uses a plunger to move hydraulic oil, and the oil moves the diaphragm. Pressure is nearly balanced across the diaphragm, which is why these pumps reach high discharge pressures. Flow range is normally given as two figures: maximum flow per head and turndown ratio.

Some figures from the literature we hold:

• Milton Roy Milroyal B: up to 2,616 L/h per head, 250 bar maximum discharge pressure, 10:1 turndown, ±1% steady-state accuracy.
• Milton Roy Milroyal D: 0.11 to 345 L/h across the series, which offers both hydraulic diaphragm and plunger liquid ends.
• SEKO Nexa: up to about 9,000 L/h and 200 bar; SEKO Nyva: about 660 L/h and 124 bar.

A 10:1 turndown means the metering pump accuracy claim covers 10 to 100% of maximum flow. Below 10% the pump still delivers, but the datasheet no longer promises that accuracy. Flow range and accuracy come as a pair. Quoting accuracy without the range it applies to tells you very little.

Three metering pump accuracy specs and what they mean

Steady-state accuracy

Fix the stroke setting, hold pressure, temperature and fluid constant, and measure flow repeatedly. Steady-state accuracy is how far those readings stray from their average. Milton Roy's Primeroyal literature states ±1% steady-state accuracy from 10 to 100% of rated flow. It is the metering pump accuracy figure most datasheets lead with, and it answers one question: once set, does the pump hold its rate?

Repeatability

Move the stroke from 50% to 80%, then back to 50%. Does flow return to the original value? Plants change settings as load changes, so repeatability decides whether metering pump accuracy survives all that adjustment.

Linearity

Is the relationship between the stroke dial and actual flow a straight line? With good linearity, 50% on the dial is close to half flow. With poor linearity you need a calibration curve. SEKO's literature says its mechanical and hydraulic pumps meet API 675 for linearity and repeatability.

Do not mix the three. If one supplier quotes steady-state accuracy and another quotes linearity, a side-by-side comparison of the numbers is meaningless.

Test conditions behind a metering pump accuracy figure

Every figure has conditions attached: rated or specified back pressure, a set temperature, a defined test fluid (often water), good suction conditions, and the stated adjustment range. Move away from those and field results will differ from the datasheet.

Hydraulic diaphragm pumps add one more factor: the oil chamber. If the oil holds air or the volume runs low, part of each plunger stroke is lost compressing it, and the diaphragm travels less. That is why Milton Roy highlights the MARS automatic mechanical oil refill system on Milroyal B, and why SEKO says its automatic refill keeps oil level constant to protect linearity and repeatability. Asking how a pump refills and vents its oil tells you more about long-term metering pump accuracy than the headline number.

For field causes such as low back pressure, siphoning and suction air, see why metering pump dosing is off.

Comparing ProMinent metering pump accuracy with other brands

Many buyers use ProMinent as a benchmark and ask for its accuracy figure. The only ProMinent document in our files covers the DUODOS air operated pump, with no hydraulic diaphragm accuracy data, so we will not quote a number here.

When you compare any two brands, ask:

1. Is the figure steady-state accuracy, repeatability or linearity?
2. Over what range does it apply: 10 to 100% of rated flow, or something else?
3. At what pressure, fluid and temperature was it measured?
4. Is the percentage of rated flow or of set flow?

The last question is the one people skip. If ±1% is relative to rated flow, a pump running at a 10% setting can be off by roughly ten times that relative to the setpoint. For points that run at low load for long periods, choose a smaller pump.

Sizing for metering pump accuracy

Work out normal, minimum and maximum dose, and size the pump so normal duty sits in the middle of its range. Then check that back pressure and fluid temperature are close to the datasheet test conditions. For high-pressure, toxic or zero-leak duties, see how a hydraulic diaphragm metering pump works, and for real flow bands look at our SEKO Nexa and Nyva hydraulic diaphragm pump guide.

Safety note: before calibrating or servicing a hydraulic diaphragm pump, stop it, close the isolation valves and relieve discharge pressure. Never break a high-pressure fitting under pressure.

Conclusion

Metering pump accuracy only means something together with flow range and test conditions. Flow range comes from maximum flow per head and turndown; accuracy holds only inside the stated range. Steady-state accuracy tells you whether the rate holds, repeatability whether it comes back after adjustment, and linearity whether the dial tracks flow. Put suppliers on the same basis before you compare. Browse our hydraulic diaphragm metering pumps and the Milton Roy brand page, then send us your flow range, pressure and fluid and contact our engineers to check a model.

FAQ

Does a ProMinent pump reach ±1% metering pump accuracy?

It depends on the series and how its datasheet defines the figure. We do not hold ProMinent hydraulic diaphragm accuracy data, so ask the supplier for the model datasheet and its test conditions.

Is ±1% of setpoint or of rated flow?

Check the wording. Many datasheets use rated flow, as in "±1% over 10 to 100% of rated flow", so error relative to setpoint grows at low settings.

Will the pump stay accurate at a 5% stroke setting?

Not guaranteed. Outside the stated turndown the pump still runs, but error grows noticeably. For long-term low-end duty, pick a smaller pump.

Does metering pump accuracy drift with age?

It can, through hydraulic oil condition, the refill valve and check valve wear. Pumps with automatic refill and venting hold up better, but recheck with a calibration column regularly.

How do I verify metering pump accuracy on site?

Use a calibration column at actual back pressure. Repeat one setting several times for steady-state accuracy, then change the setting and return to check repeatability. Keep the records.