Bar to MPa - Hydraulic Circuit Pressure Conversion

A Hydraulic Circuit Quoted in Bar, Specified in MPa

Hydraulics never settled on one unit. The test gauge screwed into the pump outlet is marked in bar, the machine's service manual quotes relief settings in bar, and then the valve block, the hose and the cylinder in the parts catalogue are all rated in MPa. Nothing on the bench tells you which number belongs to which column, so the conversion has to happen before a setting is trusted.

Factor: 1 bar = 0.1 MPa, so the MPa figure is the bar figure with the decimal point moved one place left. An excavator main relief set to 320 bar is 32 MPa — exactly the number printed against that valve in an ISO-format catalogue.

Where the Two Units Meet on One Machine

Pump nameplates

A variable piston pump carries a continuous rating and a peak rating. European mobile plates often show both in bar; the same pump in an OEM datasheet appears as, say, 25 MPa continuous and 35 MPa peak.

Relief and sequence valves

Cartridge valves are ordered against a spring-range code — 7, 21 or 35 MPa ranges are typical — while the setting you dial in with a gauge on the tee is read off in bar.

Hose and fitting ratings

Hose is sold against a maximum working pressure with a burst figure normally four times higher. Whichever unit the sleeve carries, the circuit's peak has to sit under it with margin.

Accumulator data

Bladder and piston shells are stamped with a maximum working pressure in MPa, while the nitrogen pre-charge you actually set with a charging rig is read in bar on the gauge block.

From the Test Point to the Parts Book

The order of work is the same whether you are commissioning a power unit or arguing a warranty claim about an over-pressure event.

Enter what the test gauge showed

Put the bar reading in the left box — 210, 320, 27.5, whatever the needle or the digital tester settled on. The MPa value builds up as you type, and a comma is accepted in place of the decimal point if that is how your keypad is laid out.

Line it up against the component rating

Compare the MPa result with the valve, hose or cylinder rating in the catalogue. A circuit measured at 340 bar is 34 MPa, which already overruns a 31.5 MPa directional valve — a mismatch that stays invisible while the two numbers live in different units.

Take the plain figure into the paperwork

Each field has its own copy button and hands over the number alone, with no unit and no spaces, which is what a commissioning sheet cell or a maintenance ticket wants. Pressing Ctrl + C inside a field does the same job.

Run it the other way for an MPa spec

When the datasheet is the starting point, the swap arrow (↔) turns the page into MPa → bar so you know what to dial on the gauge. By hand it is ten times the MPa figure: a 21 MPa power unit means a 210 bar setting.

Gauge position changes the answer: readings taken at the pump outlet, at the valve bank inlet and at the cylinder port differ by the line losses between them, so record where the tee was fitted alongside the pressure.

Working Pressure by Class of Hydraulic Equipment

Circuits cluster into a handful of pressure bands, and knowing which band a machine belongs to tells you straight away whether a reading is plausible. These are the levels a mobile or industrial technician meets most weeks, in the bar the gauge shows and the MPa the component list uses.

Circuit / equipment class	Typical working pressure (bar)	Same value (MPa)	Note
Closed-loop charge pump	25 bar	2.5 MPa	Keeps the loop fed and the case cool
Pilot / control circuit	30 bar	3 MPa	Feeds joystick pilot valves and clutch packs
Accumulator nitrogen pre-charge	90 bar	9 MPa	About 90 % of the minimum circuit pressure
Tractor / open-centre auxiliary	175 bar	17.5 MPa	Loader and implement services
Industrial power unit	210 bar	21 MPa	The classic general-machinery band
Log splitter ram circuit	250 bar	25 MPa	Two-stage pump drops to low flow at pressure
Excavator implement main relief	320 bar	32 MPa	Often with a power-boost step above this
Port / anti-shock relief, same machine	350 bar	35 MPa	Overload protection, not an operating level
High-tonnage press power pack	400 bar	40 MPa	Low flow, long dwell, heavy tube and fittings

The whole working range spans a factor of about sixteen, from a 25 bar charge circuit to a 400 bar press — yet in MPa the same span is a tidy 2.5 to 40, one reason component engineers prefer it for stress and wall-thickness work.

What the Page Gives a Hydraulics Technician

Gauge value and catalogue value together

Both boxes stay editable and follow each other as you type, so you can step a relief setting up in 10 bar increments and watch the MPa column track it without clearing anything.

Reverse for an ISO datasheet

One press of the swap arrow turns the direction around when the specification arrives in MPa and the adjustment has to be made against a bar-marked gauge.

Any unit a workshop runs into

Searchable dropdowns on both sides carry all 26 pressure units in eight groups, enough for a psi-marked American test kit or an old kg/cm² dial met on the same visit.

Figures fit for a commissioning record

Results run to as many as eight decimals with thousands spaced for legibility, and copying strips everything but the number so nothing needs retyping.

Questions From the Hydraulics Bench

Should the relief valve be set at the machine's working pressure?

No — it sits above it. A main relief cracked at the level the circuit works at will chatter and dump flow over heat every time the load peaks. Common practice is a margin of roughly 10 to 15 % above the highest pressure the work really needs, so a 210 bar (21 MPa) duty is usually protected around 230 to 240 bar, that is 23 to 24 MPa. Set it against a gauge at the pump with the circuit deadheaded, never by counting turns on the screw.

Why do component catalogues list MPa when my gauge is marked in bar?

MPa is the coherent SI unit for the calculations a component designer performs: stress, wall thickness and bolt loading all come out in newtons per square millimetre, and 1 MPa is exactly 1 N/mm². Bar survived on the shop floor because it sits close to atmospheric pressure and gives whole-number dial scales. Manufacturers keep both audiences by printing MPa in the engineering tables and bar on the gauge.

What pressure does the pilot circuit run at compared with the main circuit?

Roughly a tenth of it. A mobile machine whose implement circuit peaks near 320 bar (32 MPa) typically feeds its joystick pilot valves from a reduced supply around 30 bar (3 MPa), taken through its own reducing valve and fine filter. That is why a machine can lose all control response while the main pump still makes full pressure: the fault lives in the small circuit, not the big one, and the two need separate test points.

How high can a pressure spike climb above the relief setting?

Much higher than most people expect. A relief valve needs milliseconds to open, and a moving column of oil stopped by a fast-closing spool can transiently reach two to three times the setting — a 250 bar (25 MPa) circuit briefly seeing 500 bar or more. A mechanical dial gauge is far too slow to show it; only a pressure transducer with a fast logger will. Anti-shock port reliefs, ramped spool timing and an accumulator on the pressure line are the usual cures.

What nitrogen pre-charge should a bladder accumulator get?

For energy storage the usual starting point is about 90 % of the minimum system pressure, so a circuit that never falls below 100 bar takes roughly a 90 bar (9 MPa) pre-charge. Too little and the bladder bottoms out against the poppet; too much and it slams the shell every cycle. Charge with dry nitrogen only, set it with the hydraulic side vented, and check it cold, because the gas pressure moves with temperature.

Bar to Megapascals