A Sperryn G940 Mk 3, a compact domestic meter regulator: the one you will find on the inlet of most G4 meters fitted in recent years. It is a two-piece aluminium casting, 78 mm across and 100 mm tall, and maintenance free: the lid is crimped down over the body, so it cannot be taken apart (the exploded view is only to show the parts). Gas at about 30 mbar arrives from the ECV through the meter connector on the left (the same part as in the ECV figure, 400 mm overall: a BS 746 female swivel nut on the ECV, a crimped collar, 20 mm corrugated stainless hose with its yellow ON / OFF tape and a black sleeve at the regulator end, and a 3/4 in BSP taper male tail screwed into the regulator's Rp 3/4 side port). It leaves from the bottom through the meter union: a liner screwed into the Rp 3/4 outlet and a BS 746 female swivel nut that goes on the meter's 1 in BSP inlet. The whole figure is drawn to one scale.
How it works. Gas enters the inlet chamber, passes down through the seat's port (the large "metering diameter") and past the valve bob, an acetal disc on the end of an aluminium spindle that closes up against a nitrile seat. The spindle runs up through the insert tube and plate, where a small secondary diaphragm seals it, to the main diaphragm. An impulse passage in the wall feeds outlet pressure to the chamber under the main diaphragm. The stainless loading spring pushes the diaphragm down to open the valve; outlet pressure under it pushes back up. More demand pulls the outlet down a little, the spring wins and the valve opens further, so the outlet sits near 23 mbar at low flow and about 20 mbar at 6 m³/h, the G940's rating. With no demand the outlet rises until the bob seats and the regulator locks up, at about 25 mbar. The space above the diaphragm is kept at air pressure through a breather in the lid. In the cross section the blue darkens with pressure.
Setting it. The G940 is set at the factory for about 21 mbar at the meter outlet and can be set between 17 and 37.5 mbar. To adjust it you break the wire seal, pull off the blue top cap and turn the slotted spindle: clockwise squeezes the spring harder and raises the outlet pressure, anticlockwise lowers it. Only the gas transporter, or someone authorised to act for it (in practice an approved meter installer), may break the seal. Where you'll meet it: checking working pressure at the meter outlet test point (19 to 23 mbar with appliances running) and standing pressure (no more than 30 mbar with everything off). The outlet here turns amber when it leaves those limits.
Two failures can be shown. A ruptured diaphragm can no longer hold outlet pressure against the spring, so the regulator fails open: the valve goes wide, the outlet rises to nearly the inlet pressure, and gas fills the spring chamber and escapes from the breather. Where you'll meet it: a smell of gas at the meter box with a high standing pressure.
A valve that comes off its spindle falls away from the seat, because the gas flow and gravity both push it down, and on a regulator sitting on the meter inlet it can drop through the outlet into the meter (as happened with the plastic discs on the Donkin 225 Type 2). With nothing to close the valve the outlet climbs to nearly the inlet pressure and the regulator neither regulates nor locks up. Where you'll meet it: working and standing pressures both close to the supply pressure.
The internal passages are simplified from the datasheet's cutaway, valve travel is drawn larger than life, and the turns per millibar are illustrative.