the first membrane 12 a flange not shown may be provided towards an apex of the outer second membrane 14 and may include a recess or other receiving formation for receiving means for detecting the volume of gas in the gas accumulation chamber 18 in one example the means for detecting the volume of gas in the gas accumulation chamber is an ultrasonic level detector which can be used to monitor the volume of gas in the gas accumulation chamber 18 without connecting the first and third membranes 12 16 thereby preventing stress on the first membrane 12 and reducing the possibility of damage to the first or third membranes 12 16 in use of the gas accumulator 10 gas to be stored is deposited in the gas accumulation chamber 18 by means of the valve 32 until the gas accumulation chamber 18 is full when gas is to be released from the gas accumulation chamber 18 the valve 32 in the gas accumulation chamber 18 is opened and air or another auxiliary gas is pumped into the pressurisation chamber 20 via valve 30 thereby pressurising the gas accumulation chamber 18 and causing gas to escape via the valve 32 air is preferred as the auxiliary gas as it is freely available but it will be understood.