Megapascals to Bar

A 70 MPa Tank and a 700 Bar Nozzle Are the Same Thing

Gaseous vehicle fuelling is split down the middle by units. Standards, cylinder labels and vehicle documentation talk in megapascals — H70, H35, NWP 70 MPa. Station hardware, compressors, cascade storage and the display on the dispenser almost always talk in bar. A technician commissioning a hydrogen or CNG site spends the day translating one into the other, and the arithmetic is mercifully simple.

Conversion factor: 1 MPa = 10 bar exactly, since a bar is 100 000 Pa and a megapascal is 1 000 000 Pa. An H70 receptacle rated at a nominal working pressure of 70 MPa is a 700 bar connection, and the H35 nozzle beside it is 350 bar.

The Numbers on a Gaseous Fuel System

Nominal working pressure anchors everything

NWP is the settled pressure at a reference temperature, usually 15 °C. Every other figure on the label — fill allowance, protection setting, test, burst — is a fixed multiple of it.

A fill deliberately overshoots

Tanks may be filled to 125 % of service pressure provided the settled value at 15 °C comes back to NWP, so an H70 tank can legitimately see 87.5 MPa, or 875 bar, at the end of a fast fill.

Test pressure is a separate stamp

Cylinder qualification is done well above anything the vehicle sees — commonly 1.5 × NWP for the hydrostatic test and a burst requirement several times higher again for composite tanks.

CNG sits a decade lower

Natural-gas vehicles run on 20 or 25 MPa service pressure — the familiar 200 and 250 bar classes — so CNG hardware and hydrogen hardware are never interchangeable despite looking similar.

Reading a Cylinder Label Against the Dispenser

The usual task is checking that a component rated in one unit really does cover what the station will put through it in the other.

Enter the megapascal rating from the label

Type 70, 35, 25 or 20 in the left box and the bar figure appears straight away. Decimals matter here — 87,5 with a comma is read exactly the same as 87.5.

Apply the multiple before you convert

Work out 1.25 × NWP for the fill ceiling or 1.5 × NWP for the test value first, then convert. For an H70 system that gives 87.5 MPa and 105 MPa, which land at 875 bar and 1 050 bar.

Search the dropdown when a spec sheet uses psi

North American documentation still quotes hydrogen as 10 000 psi and CNG as 3 000 or 3 600 psi. Both dropdowns are searchable and carry every pressure unit, so a psi rating can be checked on the same screen.

Reverse it when the reading comes off the station

Press the swap button (↔) to run bar → MPa and turn a cascade-bank gauge into the unit the vehicle standard uses. Divide by ten to do it in your head: 900 bar is 90 MPa.

Never rate a component by a converted number alone: hydrogen service also brings embrittlement, permeation and cycle-life requirements. Matching the pressure is necessary but nowhere near sufficient for approving a part.

Service, Fill and Test Pressures Across Gaseous Fuel Systems

The pressures that appear on hydrogen and CNG equipment, with the megapascal figure the standards use and the bar figure printed on station gauges and European hardware.

System or condition	Basis	Pressure (MPa)	Pressure (bar)
H11 refuelling connector	Material handling, working pressure	11 MPa	110 bar
CNG, 200 bar class	Service pressure at 15 °C	20 MPa	200 bar
CNG, 250 bar class / H25 connector	Service pressure at 15 °C	25 MPa	250 bar
CNG cylinder hydrostatic test	1.5 × 20 MPa service	30 MPa	300 bar
H35 nozzle and receptacle	Working pressure, bus and truck fleets	35 MPa	350 bar
H70 nozzle and receptacle	Working pressure, light-duty cars	70 MPa	700 bar
H70 end-of-fill allowance	125 % of service pressure	87.5 MPa	875 bar
Station high-pressure bank	Typical cascade storage	90 MPa	900 bar
H70 dispenser protection ceiling	140 % of service pressure	98 MPa	980 bar
H70 cylinder test pressure	1.5 × service pressure	105 MPa	1 050 bar
H70 composite tank, minimum burst	2.25 × service pressure	157.5 MPa	1 575 bar

Notice how far apart the two fuels sit. A CNG cylinder's hydrostatic test at 300 bar is still less than half the everyday working pressure of a hydrogen car, and the hydrogen tank's burst requirement is more than five times the CNG test. That gap is why the connectors are keyed and why an H70 vehicle can take an H35 nozzle but never the other way round.

What Makes the Ten-to-One Step Easy to Live With

Ten to one, live in both boxes

Enter a megapascal rating or a bar gauge reading — whichever you have in front of you — and the other side updates keystroke by keystroke, with 1 MPa loaded by default.

Point it at the dispenser instead

Swapping the direction gives bar → MPa, which is the way round a commissioning engineer works when the station gauge is the source and the standard is the reference.

Psi ratings on the same screen

All 26 units are grouped and searchable on both sides, so a 10 000 psi hose assembly and a 700 bar receptacle can be compared without leaving the page.

Awkward multiples stay exact

Values like 87.5 or 157.5 MPa keep their decimals, thousands are spaced for readability, and copying hands over the digits alone for a commissioning sheet.

Hydrogen and CNG Fuelling Questions

What separates an H70 fill from an H35 fill?

The dispensing pressure, and therefore how much gas ends up on board. H70 dispenses at 70 MPa (700 bar), H35 at 35 MPa (350 bar). A car built around a 70 MPa tank can be fuelled from an H35 nozzle, but it will only take a partial fill and its range drops accordingly. A vehicle designed for 35 MPa must never be put on an H70 dispenser, which is why the connectors are physically distinct.

Why does the tank read above 700 bar during a fill and then fall back?

Because the rating is a settled pressure at a reference temperature. Filling is allowed up to 125 % of service pressure — 87.5 MPa or 875 bar on an H70 system — as long as the gas returns to 70 MPa once it has cooled to 15 °C. A gauge glimpsed at 800 bar mid-fill is normal behaviour, not an over-pressure event, and the dispenser's own protection setting sits higher again at 140 % of service pressure.

Why does the gas get hot during a fast fill, and what is done about it?

Compressing gas into a tank raises its temperature, and hydrogen adds a small warming effect of its own as it expands through the throttling in the line. Left alone, a three-minute fill would push the tank material past its permitted temperature and leave the tank short once the gas cooled. Stations answer this by pre-cooling the hydrogen and by following a controlled pressure ramp, so the density on board matches what 70 MPa settled is supposed to mean.

How does a 200 or 250 bar CNG system compare with a hydrogen one?

CNG service pressure is 20 or 25 MPa, an order of magnitude below hydrogen's 70 MPa. That difference cascades through the whole site: compressor stages, storage vessel wall thickness, valve and hose ratings, and cylinder construction. Because hydrogen is far less dense per unit volume, it needs that much higher pressure to store a useful amount of energy on board, which is why hydrogen station equipment cannot simply be adapted from natural-gas practice.

On a cylinder label, how do service pressure and test pressure relate?

Service, or nominal working, pressure is the settled value the cylinder is meant to hold in use. Test pressure is a qualification figure, typically 1.5 times that — 30 MPa (300 bar) for a 20 MPa CNG cylinder, 105 MPa (1 050 bar) for a 70 MPa hydrogen one — applied hydrostatically during manufacture or periodic requalification. Composite tanks carry a third figure, a minimum burst pressure of about 2.25 times service for hydrogen, which is a design proof rather than anything a cylinder is ever taken to in service.