55 is the volume of the cylinder and not weight. So its 55 liters and not kg.
55 liters means 55 liters of water can be filled into it.
weight of 1 liter of water = 1 kg
weight of 55 liter of water = 55 kg
weight of 1 liter of cng = 0.18 kg (approximately)
weight of 55 liter of cng = 55 x 0.18 kg = 9.9 kg (max pressure 200 bar)
Please note that these cylinders are not manufactured for pressures above 200 bar, so it is not safe to go for a full tank if the gauge at the cng station reads above 200 bar.
Please also note that :-
1 liter cng = 0.18 kg at very low temperatures. Normally at cng stations and especially at ones where there is alot of rush, the gas does not get much time to cool down so the weight per liter decreases to like e.g.
1 liter = 0.15 kg
So its normal if a 55 kg of cylinder fills up with 8 kg.
Some times your cylinder fills up with oil due to which the volume can decrease may be from 55 liters to 50 liters. Oil comes from the cng pump where oil mixes with the gas during pressure build up. This oil is usually separated by high pressure oil filters before filling into the car but if the pump owner does not change these oil filters on time then oil will pass into your car cylinder and reduce its volume.
Hamid Bilal Younis