A method for measuring the volume of liquid, such as ink or solvent, remaining in a container, such as a replacement cartridge for a continuous ink jet printer, uses a reservoir enclosing an internal space having a variable volume for storage. The reservoir is adapted to provide a reduction in the pressure of the internal space, the reduction substantially monotonically increasing in magnitude as liquid is drawn into the printer, such that the volume of liquid remaining may be calculated from knowledge of the minimum withdrawal pressure required to draw further liquid from the reservoir into the printer. Containers for use with the method have a liquid dispensing port adapted to allow liquid to be dispensed when a withdrawal pressure at the exterior of the port is less than the pressure of the internal space and adapted to prevent the ingress of air into the internal space of the reservoir as liquid is dispensed.