Volume Measurement Using Gas Laws

Abstract

A system and method for measuring fluid volume and changes in fluid volume over time, using a simple, low cost architecture is described.

1 . A method of measuring a volume of liquid in a flexible container, comprising:
 placing the flexible container within a first rigid container of known volume, the first rigid container containing an inflatable bladder;   pressurizing the inflatable bladder with a gas;   pressurizing a second rigid container of known volume with a gas, the pressure of the gas in the bladder being approximately equal to the pressure of the gas in the second rigid container;   delivering a first molar quantity of gas to the bladder to cause a measurable increase in pressure in the bladder;   delivering a second molar quantity of gas to the second rigid container to cause a measurable increase in pressure in the second rigid container, the first molar quantity of gas being approximately equal to the second molar quantity of gas;   measuring the increase in pressure in the bladder;   measuring the increase in pressure in the second rigid container;   calculating the volume of gas in the bladder using the known volume of the second rigid container, the measured increase in pressure in the bladder, and the measured increase in pressure in the second rigid container; and   calculating the volume of liquid in the flexible container by subtracting the calculated volume of gas in the bladder from the known volume of the first rigid container.   
   
   
       2 . The method of  claim 1 , further comprising:
 subtracting the known volume of incompressible materials within the first rigid container from the known volume of the first rigid container.   
   
   
       3 . The method of either one of  claims 1  and  2 , wherein said gas is air. 
   
   
       4 . The method of any one of  claims 1 - 3 , wherein said gas is delivered to said first and second rigid containers with a pump. 
   
   
       5 . The method of  claim 5 , wherein said pump is not a precise metering device. 
   
   
       6 . The method of any one of  claims 1 - 5 , further comprising:
 prior to delivering said first and second molar quantities of gas, adjusting the pressure in one or both of said bladder and said second rigid container.   
   
   
       7 . The method of any one of  claims 1 - 6 , further comprising:
 monitoring the pressure in said bladder; and   calculating a flow rate of liquid exiting the flexible container.   
   
   
       8 . A method for calculating a flow rate of liquid exiting a flexible container contained within a rigid container of known volume, the rigid container containing an inflatable bladder, the inflatable bladder pressurized with a gas to urge the liquid out of the flexible container, said method comprising:
 calculating the volume of gas in the inflatable bladder;   determining the initial pressure in the inflatable bladder;   monitoring the pressure decay in the inflatable bladder over time until the change in pressure reaches some preselected threshold value;   calculating the rate of pressure decay; and   calculating the flow rate using the rate of pressure decay, the calculated volume of gas in the inflatable bladder, and the initial pressure in the inflatable bladder.   
   
   
       9 . A fluid delivery system, comprising:
 a pressure frame ( 10 ) of known total volume;   an inflatable bladder ( 20 ) within said pressure frame;   said pressure frame adapted to receive a flexible bag ( 30 ) containing a liquid to be infused ( 40 );   a calibration tank ( 60 ) of known volume;   a pump ( 50 ) fluidically coupled to said bladder and said calibration tank for selectively delivering a gas to said bladder and said calibration tank;   a first pressure sensor ( 202 ) coupled to said bladder for sensing the pressure of a gas in said bladder;   a second pressure sensor ( 204 ) coupled to said calibration tank for sensing the pressure of a gas in said calibration tank;   a processing unit ( 700 ) coupled to said first and second pressure sensors and said first and second temperature sensors for storing pressure and temperature information from said first and second pressure sensors and said first and second temperature sensors;   said processing unit coupled to said pump for controlling operation of said pump; and   said processing unit further including means for calculating one or both of:
 a volume of liquid in the flexible container; and 
 a flow rate of fluid exiting the flexible container. 
   
   
   
       10 . The fluid delivery system of  claim 9 , further comprising:
 a first vent valve ( 108 ) fluidically coupled to said bladder for selectively venting gas within said bladder; and   a second vent valve ( 104 ) fluidically coupled to said calibration tank for selectively venting within said calibration tank.   
   
   
       11 . The fluid delivery system of either one of  claims 9  and  10 , further comprising:
 a first inlet valve ( 106 ) fluidically coupled to said bladder and said pump; and   a second inlet valve ( 102 ) fluidically coupled to said calibration tank and said pump.   
   
   
       12 . The fluid delivery system of any one of  claims 9 - 11 , further comprising:
 a first temperature sensor ( 302 ) coupled to said bladder for sensing the temperature of the gas in said bladder; and   a second temperature sensor ( 304 ) coupled to said calibration tank for sensing the temperature of a gas in said calibration tank.   
   
   
       13 . The fluid delivery system of any one of  claims 9 - 12 , wherein the gas is air. 
   
   
       14 . The fluid delivery system of any one of  claims 9 - 13 , wherein said pump is not a precise metering device.