A pressure sensing system for volume measurements, comprising: a container having an unknown available space volume V 2 that is to be determined and having a container opening; a reference container with a known reference volume V 4 having a container opening; a chamber having a known chamber volume V 1 and defining a chamber opening configured to sealably couple the reference container volume V 4 or the unknown available space volume V 2 and the chamber volume V 1; a pressure changing device coupled to the chamber, the pressure changing device configured to apply a predetermined pressure of a gas substance to the common volume V 1 + V 4 of chamber and the reference container when the reference container opening of the reference container is sealably coupled to the chamber opening, and to apply the predetermined pressure of a gas substance to the common volume V 1 + V 2 of the chamber and the container having the unknown available space volume V 2 when the container opening is sealably coupled to the chamber opening after the reference container is replaced by the container having the unknown available space volume V 2; a pressure sensing device configured to measure a reference pressure P 4 of the gas substance contained within the common volume V 1 + V 4 of the chamber and the reference container, and to measure a second pressure P 2 of the gas substance contained within the common volume V 1 + V 2 of the chamber and the container after the reference container is replaced by the container having the unknown available space volume V 2; and a processor configured to determine the available space volume V 2 of the container by using the second pressure P 2, the reference pressure P 4, the chamber volume V 1, and the reference volume V 4 in the equation V 2 = [(V 1 +V 4)P 4 /P 2] - V 1, such that a temperature is not used to determine the available space volume V 2 of the container. a container having an unknown available space volume V 2 that is to be determined and having a container opening; a reference container with a known reference volume V 4 having a container opening; a chamber having a known chamber volume V 1 and defining a chamber opening configured to sealably couple the reference container volume V 4 or the unknown available space volume V 2 and the chamber volume V 1; a pressure changing device coupled to the chamber, the pressure changing device configured to apply a predetermined pressure of a gas substance to the common volume V 1 + V 4 of chamber and the reference container when the reference container opening of the reference container is sealably coupled to the chamber opening, and to apply the predetermined pressure of a gas substance to the common volume V 1 + V 2 of the chamber and the container having the unknown available space volume V 2 when the container opening is sealably coupled to the chamber opening after the reference container is replaced by the container having the unknown available space volume V 2; a pressure sensing device configured to measure a reference pressure P 4 of the gas substance contained within the common volume V 1 + V 4 of the chamber and the reference container, and to measure a second pressure P 2 of the gas substance contained within the common volume V 1 + V 2 of the chamber and the container after the reference container is replaced by the container having the unknown available space volume V 2; and to apply a predetermined pressure of a gas substance to the common volume V 1 + V 4 of chamber and the reference container when the reference container opening of the reference container is sealably coupled to the chamber opening, and to apply the predetermined pressure of a gas substance to the common volume V 1 + V 2 of the chamber and the container having the unknown available space volume V 2 when the container opening is sealably coupled to the chamber opening after the reference container is replaced by the container having the unknown available space volume V 2; a pressure sensing device configured to measure a reference pressure P 4 of the gas substance contained within the common volume V 1 + V 4 of the chamber and the reference container, and to measure a second pressure P 2 of the gas substance contained within the common volume V 1 + V 2 of the chamber and the container after the reference container is replaced by the container having the unknown available space volume V 2; and a processor configured to determine the available space volume V 2 of the container by using the second pressure P 2, the reference pressure P 4, the chamber volume V 1, and the reference volume V 4 in the equation V 2 = [(V 1 +V 4)P 4 /P 2] - V 1, such that a temperature is not used to determine the available space volume V 2 of the container.