Tank gauging apparatus and method

Apparatus for gauging the amount of liquid in a container of liquid and gas under low or zero gravity net conditions includes an accumulator and appropriate connector apparatus for communicating gas between the accumulator and the container. In one form of the invention, gas is removed from the container and compressed into the accumulator. The pressure and temperature of the fluid in the container is measured before and after removal of the gas; the pressure and temperature of gas in the accumulator is measured before and after compression of the gas into the accumulator from the container. These pressure and temperature measurements are used in determining the volume of gas in the container, whereby the volume of liquid in the container can be determined from the difference between the known volume of the container and the volume of gas in the container. Gas from the accumulator may be communicated into the container in a similar process as a verification of the gauging of the liquid volume, or as an independent process for determining the volume of liquid in the container.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to methods and apparatus for determining the 
amount of liquid in a container. More particularly, the present invention 
is related to techniques for gauging liquid quantities in containers in 
low or zero net gravity environments, such as in the case of satellites or 
other space vehicles. 
2. Description of the Prior Art 
The determination of the amount of liquid in a container in which both gas 
and liquid are present poses a considerable problem in zero, or near-zero, 
net gravity environments such as encountered in earth orbit or other space 
travel. For example, it may be necessary to gauge the amount of liquid 
fuel in a satellite or space station fuel tank. Typically, a tank for 
storing liquid that is removed therefrom as needed also contains a 
pressurant gas for maintaining the liquid under appropriate pressure to 
permit the liquid to be tapped off of the tank. However, due to the lack 
of gravitational forces, the liquid and gas are generally completely mixed 
throughout the volume of the tank, absent the use of any techniques to 
separate the two and gather the liquid toward one region of the tank. 
Consequently, the gauging of the amount of liquid in the container is not 
readily carried out. 
Several techniques are known for attempting to gauge liquid in a container 
under such circumstances. For example, the container may be accelerated to 
settle the liquid to a specific location in the container, then the level 
of the liquid at that location may be determined with a level detector, or 
liquid/gas interface sensor. However, so accelerating the container 
requires a complicated propulsive system to produce the accelerating 
force, which also affects the entire system of which the container is a 
part. Another technique utilizes the container, or tank, in a blowdown 
mode. After the tank containing liquid is pressured with the pressurant 
gas, the tank is isolated from the pressurant supply. Then, as liquid is 
withdrawn from the tank, the change in pressure within the tank is 
detected and the related expansion of the pressurant is determined to 
determine the quantity of liquid remaining within the tank. However, 
utilizing the tank in a blowdown mode results in a decrease in the liquid 
withdrawal rate as the pressure in the tank is reduced. Therefore, in 
order to achieve a desired flow rate at the diminished tank pressure, the 
initial tank pressure must be increased, resulting in a heavier initial 
tank weight. Furthermore, any gas leakage from the tank produces an error 
in the gauging of the liquid remaining. Yet another technique requires use 
of a dedicated pressurant tank from which gas may be supplied to the 
liquid container by means of a pressure regulator. The amount of liquid in 
the tank can thus be determined by gauging the quantity of pressurant gas 
added to the tank containing the liquid. However, such technique obviously 
requires a dedicated pressurant tank, and leakage of the pressurant gas 
may be mistaken for liquid loss. Further, of the aforementioned 
techniques, only the first technique resulting in settling of the liquid 
in one region of the container is usable for gauging cryogenic liquids. 
It is an object of the present invention to provide a method and apparatus 
for gauging the liquid in a container under zero, or near-zero, net 
gravitational force without the need for a special propulsive system, or 
any acceleration to be imposed on the container or the system of which the 
container is a part. 
It is a further object of the present invention to provide such a technique 
without the need for a dedicated gas pressurant source, and without the 
need for an increase in tank working pressure and weight to accommodate 
operation in a blowdown expulsion mode. 
SUMMARY OF THE INVENTION 
The present invention provides apparatus for gauging the amount of liquid 
in a container at zero, or near-zero, net gravitational conditions, and 
includes an accumulator and a compressor for compressing pressurant gas 
from the container into the accumulator. Appropriate connectors are 
provided for connecting the compressor and accumulator to the container, 
and sensors are included to measure the pressure and temperature in the 
container and in the accumulator. Gas from the compressor may pass through 
a radiator to remove heat of compression of the gas. Additionally, the 
compressor may have interstage cooling to maintain safe temperatures for 
reactive gases and vapors. A computer operated controller may be provided 
for receiving the sensor measurements of the pressure and temperature and 
for operating the compressor. 
In a method of the invention, an accumulator and associated compressor are 
provided with connectors to the container of liquid and pressurant gas. 
The pressure and temperature of fluid in the container are measured, and 
the pressure and temperature of gas in the accumulator are measured. Gas 
is then pumped from the container and into the accumulator by means of the 
compressor. A radiator may be provided to remove the heat of compression 
of the gas being pumped into the accumulator. The pressure and temperature 
measurements are taken again, and the gas law is applied to determine the 
volume of gas in the container, whereby the volume of liquid in the 
container may be determined. 
In a variation of the method of the invention, the pressure and temperature 
measurements are made, gas is communicated from the accumulator to the 
container and the pressure and temperature measurements are made again. 
Application of the gas law is then made to determine the volume of the 
liquid in the container.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A system for determining the amount of liquid in a container in zero, or 
near-zero, net gravitational conditions according to the present invention 
is shown generally at 10 in FIG. 1. The container, in the form of a tank, 
for example, initially holds liquid whose volume is to be determined, 
along with pressurant gas used to maintain a sufficient pressure within 
the tank to enable the liquid to be tapped off of the tank for appropriate 
use. Such a system may be employed, for example, as a liquid fuel supply 
in a satellite or other space vehicle operating in a net reduced 
gravitational condition. The gauging system 10 includes a compressor, or 
pump, 12 which is used in removing the pressurant gas from the tank (not 
shown) by way of a gas communication line 14. The compressed gas is 
communicated by a line 16 to a radiator 18, and gas output from the 
radiator is communicated by a line 20 to an accumulator 22. In general, 
the volume of the accumulator 22 may be considerably smaller than the 
volume of the main tank containing the liquid whose quantity is to be 
determined. Typically, the volume of the accumulator may be in the range 
of 0.001 of the main tank volume. Accordingly, the compressor may have a 
boost ratio on the order of 100:1, depending on the required accuracy of 
the measuring process and the system weight. Since the gas being pumped 
from the main tank to the accumulator is being compressed, the radiator 18 
is provided to remove the heat of compression from the compressed gas. In 
the case where compression ratio is high and temperatures must be limited 
for safe and reliable operation, interstage cooling of the compressor may 
be necessary. Since use of the compressor is intermittent, the necessary 
cooling may be achieved by jacketing specific areas and filling the 
jackets with phase-change materials (normally salts). Radiating fins may 
be added for reversal of the phase-change. Otherwise, more conventional 
cooling employing a heat transfer fluid, pump, and radiator may be 
employed. 
After appropriate measurements are made on the gas removed from the main 
tank, gas may be returned to the main tank from the gauging system 10 by a 
return line 24. The gas intake line 14 is provided with an isolation valve 
26. An isolation valve 28 is also provided in the gas return line 24. 
The system 10 may be provided with a controller 30 to operate the system. 
The controller 30, which may be a computer, or computer operated, thus 
controls the operation of a motor 32, which drives the compressor 12, by 
means of an appropriate power or operating signal line 34. Pressure and 
temperature conditions in the accumulator 22 and connecting lines, as well 
as pressure and temperature conditions in the main tank, must be 
determined. Appropriate sensors are provided for acquiring such 
information. The sensors may be connected to the controller 30 for 
communicating the pressure and temperature information directly to the 
controller which, in turn, may operate to carry out the calculations to 
determine the amount of liquid in the main tank, as discussed in detail 
hereinafter. The pressure and temperature sensing devices may be of any 
appropriate types, and which are generally known in the art and need not 
be described in detail herein. In general, such sensors provide electrical 
output signals reflecting the values of temperature and pressure sensed. 
Accordingly, a temperature sensor 36 is positioned to probe the 
temperature of gas in the accumulator 22, and conveys its output signal to 
the controller 30 by an appropriate communication line 38. A pressure 
sensor 40 is similarly situated, here in the output line from the 
accumulator 22, to determine the pressure of the gas in the accumulator 
and associated communication lines, and conveys its output signal to the 
controller 30 by an appropriate communication line 42. Temperature and 
pressure sensors 44 and 46 are positioned to determine the corresponding 
quantity values in the main tank (not shown), and appropriately 
communicate their respective output signals to the controller 30 over 
communication lines 48 and 50, respectively. It will be appreciated that 
any one or more of the signal communication lines 38, 42, 48 and 50 may be 
deleted and radio or other appropriate communication signals may be 
utilized to convey temperature and/or pressure information to the 
controller 30. 
Operation of the system 10 by means of the controller 30 is also a safety 
measure, since the controller may continuously monitor the pressure and 
temperature conditions in the system as well as the main tank, and cease 
operation of the compressor 12, for example, in the event of threatening 
pressure buildup. However, a relief valve 52 is provided as a backup to 
the controller 30 to prevent overpressure. The relief valve 52 
communicates with the compressor input line 14 as well as the compressor 
output line 16 by means of connecting lines 54 and 56, respectively. 
Generally, as the system 10 is being operated, the valve 52 is closed. 
However, in the event of an overpressure in the output side of the 
compressor 12, for example, the valve 52 may be set to open the 
communication line 56 to the input side of the compressor 54. 
To utilize the gauging system 10, the volume of the accumulator 22 and 
associated components and connecting lines, V.sub.a, is determined and 
considered a known quantity. For example, the volume of the accumulator 
22, the radiator 18 and all communication lines connected thereto from the 
compressor 12 to the isolation valve 28 would be considered to make up 
V.sub.a. Similarly, the volume of the main tank containing the liquid 
whose quantity is to be determined is known, and is designated herein as 
V.sub.M. Similarly, this quantity V.sub.M would include that of the main 
tank itself as well as any associated lines connecting the tank to the 
gauging system 10, for example, such as the lines 14 and 24 on the main 
tank side of the isolation valves 26 and 28, respectively. The gauging 
system 10, through the sensors 36, 40, 44 and 46, is used to determine the 
temperature and pressure conditions in the accumulator and the main tank 
before and after the compressor 12 is operated to remove gas from the main 
tank to the accumulator 22. These quantities are designated as follows: 
T.sub.ai =initial accumulator temperature 
T.sub.af =final accumulator temperature 
P.sub.ai =initial accumulator pressure 
P.sub.af =final accumulator pressure 
T.sub.mi =initial main tank temperature 
T.sub.mf =final main tank temperature 
P.sub.mi =initial main tank pressure 
P.sub.mf =final main tank pressure. 
The volume of liquid in the main tank, V.sub.1, is the difference between 
the tank volume V.sub.M and the volume of gas in the tank, V.sub.m. The 
volume of the liquid, V.sub.1, and the volume of the gas in the main tank, 
V.sub.m, are both constants. The total mass of the gas, M, is equal to the 
sum of the mass of the gas in the accumulator, M.sub.a, and the mass of 
the gas in the main tank, M.sub.m, and is a constant. However, as the 
system 10 is operated to pump gas from the main tank to the accumulator 
22, the mass of the gas in the main tank as well as the mass of the gas in 
the accumulator change. 
In operation, the main tank containing the liquid, whose quantity is to be 
determined, and pressurant gas is isolated from the gauging system 10 by 
the isolation valves 26 and 28, and the controller 30 operates to sense 
the initial values of temperature and pressure T.sub.ai and P.sub.ai in 
the accumulator, as well as the initial values of temperature and pressure 
T.sub.mi and P.sub.mi in the main tank. The combination of liquid and gas 
in the main tank may be expected to display uniform pressure throughout in 
the zero or near-zero net gravity conditions for which the system 10 is 
particularly advantageous. 
With the initial temperature and pressure measurements obtained, the 
compressor 12 is operated by the motor 32 with the isolation valve 26 open 
to pump pressurant gas from the main tank through the radiator 18 and into 
the accumulator 22. Since the accumulator is considerably smaller in 
volume than the main tank, it will be appreciated that the pressure of the 
gas removed from the tank to the accumulator 22 will be increased by 
operation of the compressor 12. The radiator 18 functions to remove the 
heat of compression of the gas pumped to the accumulator 22. When the gas 
has been removed to the accumulator 22, the isolation valve 26 is closed 
and the compressor 12 ceases operation. Final temperature and pressure 
measurements, T.sub.af, P.sub.af, T.sub.mf and P.sub.mf in the accumulator 
system and the main tank, respectively, are taken. Where the liquid vapor 
pressure is low, any liquid vapor being added to the pressurant gas may be 
ignored. Then, by operation of the controller/computer, for example, the 
gas law in the general form of Equation (1) is applied to determine the 
volume of liquid in the main tank. 
##EQU1## 
where R is the gas constant and T is the absolute temperature. Since total 
mass of the gas in the entire system is a constant, the mass of the gas 
removed from the tank is equal to the mass of the gas added to the 
accumulator. That is, 
EQU M.sub.mi -M.sub.mf =M.sub.af -M.sub.ai (2) 
where 
M.sub.mi =mass of gas initially in the main tank 
M.sub.mf =mass of gas in the main tank after gas is transferred 
M.sub.ai =mass of gas initially in the accumulator system 
M.sub.af =mass of gas in the accumulator system after gas is transferred. 
Applying Equation (1) to each of the mass quantities of Equation (2) yields 
##EQU2## 
All of the quantities on the right-hand side of Equation (3) are known or 
measured. Consequently, Equation (3) yields the volume of gas in the main 
tank, so that the volume of liquid in the main tank is given by 
EQU V.sub.1 =V.sub.M -V.sub.m. (4) 
It will be appreciated that the application of the gas law through the 
above equations, for example, may be carried out to determine the volume 
of liquid in the tank not only in a process of removal of gas from the 
tank as described above, but also by a process of adding gas to the tank 
from the accumulator 22. The same types of measurements described above 
are taken, that is, the pressure and temperature conditions in the 
accumulator system and in the tank are determined before the gas is vented 
from the accumulator to the tank, and after the gas has been communicated 
to the tank. It will further be appreciated that in such a process, the 
initial pressure of the gas in the accumulator 22 is higher than the 
initial gas pressure in the tank, and the gas need only be vented to the 
tank through the isolation valve 28 over the return line 24 without the 
need for the operation of a compressor. However, a compressor may be 
utilized to raise the level of the final tank pressure above that in the 
accumulator system. Additionally, cooling of the gas upon expansion into 
the tank may be accommodated by an appropriate heat exchanger, or the 
like. In any event, final pressure in the main tank is greater than its 
initial pressure. 
The two above-described processes may be used to obtain two measurements of 
the temperature and pressure conditions before and after in each of the 
two segments of the system to make two determinations of the volume of 
liquid in the tank. This can be done as a verification of the operation of 
the gauging system, for example. In such a circumstance, for example, the 
gas may initially be moved from the tank to the accumulator 22 in the 
first above-desribed process to obtain a value of the volume of liquid in 
the tank. Thereafter, the isolation valve 28 may be opened and the 
compressed gas vented back to the tank, and additional temperature and 
pressure measurements made both in the accumulator system and the tank. 
The aforementioned equations are again applied, and a second determination 
of the volume of liquid in the tank is obtained. When the above two-cycle 
process is complete, the tank is essentially back to its initial 
conditions, with no fluid having been wasted. 
In the case of a cryogenic tank, some of the liquid may be expected to 
vaporize and a small amount of such vapor be carried into the accumulator 
system as the gas is compressed by operation of the pump. Vapor carried 
into the gauging system may affect the outcome of the process of 
determining the volume of liquid in the tank. However, if the gas return 
process is carried out, a different value for the volume of liquid in the 
tank will be obtained. Consequently, the discrepancy due to the vaporized 
liquid in the first half of the cycle will be detected. 
For cases where the liquid vapor pressure is a significant portion of the 
total gas pressure in the tank, and consequently where the vapor will 
condense in the accumulator, an adjustment can be made to the process by 
substracting the vapor pressure of the liquid from the initial and final 
values of the pressure in the tank, that is, P.sub.mi and P.sub.mf, before 
application of the gas law. The volume of liquid occupied by the condensed 
vapor in the accumulator may be calculated, but will normally have 
negligible effect on the result. 
The effects of high vapor pressure liquid on operation of the gauging 
system 10, for example where a cryogenic liquid is in the main tank, may 
be avoided by utilizing the gauging system in the mode wherein pressurized 
gas in the accumulator 22 is communicated to the main tank, with 
temperature and pressure conditions determined before and after the gas 
transfer, to obtain a value for the volume of liquid in the tank, as 
discussed above. 
The ability to operate the gauging system 10 in either direction, that is, 
by pumping gas into the accumulator 22 from the tank, or venting gas from 
the accumulator to the tank, allows multiple determinations of the volume 
of liquid in the tank even as the liquid is being withdrawn from the tank, 
or more liquid added thereto. For example, after one such measuring cycle 
is carried out to determine the quantity of liquid in the tank, such as 
the process in which gas is pumped from the tank to the accumulator 22, 
the isolation valves 26 and 28 may remain closed and the system 10 not 
operated until some later time after which liquid has been withdrawn from 
the tank or added thereto. Then, the gas return cycle can be carried out 
and another determination of the volume of liquid in the tank obtained. 
This process may be repeated as needed. It will also be appreciated that 
subsequent liquid volume determinations can be carried out in this fashion 
to determine whether liquid has leaked from the tank system, for example. 
It will be appreciated that the liquid and gas within the main tank are 
randomly distributed in the zero or near-zero net gravity environment. 
Consequently, in order to withdraw the gas from the tank, it may be 
necessary to separate the gas from the liquid within the tank or to 
provide some means for presenting the gas at the outlet of the tank so 
that it may be withdrawn therefrom. 
A separator system for removing gas from an enclosure containing both 
liquid and gas in a zero or near-zero net condition is shown generally at 
110 in FIG. 2. A tank 112 for containing the liquid and gas combination 
features a vortex impeller suction tube 114 extending from one interior 
wall of the tank to and through the opposite wall of the tank as 
illustrated. Liquid and gas within the tank 112 may enter the interior of 
the suction tube 114 through several passages, or slots, 116 (four are 
illustrated) angled as shown to assist fluid movement as discussed more 
fully hereinafter. A fluid communication line 118 extends into the end of 
the suction tube 114. An impeller 120 is mounted with its blades 
circumscribing the end of the fluid communication line 118 within the 
suction tube 114, and is rotatable relative to the communication line. The 
fluid line 118 may continue on to the gauging system 10 of FIG. 1, and 
particularly to the inlet line 14 thereof. The gauging system 10 outlet 
line 24 may enter the tank 112 of FIG. 2 at any convenient location (not 
shown). Also, the temperature and pressure sensors 44 and 46, 
respectively, may be positioned to detect the corresponding quantities 
within the tank 112. 
An encapsulated permanent magnet rotor 122 is also located within the 
suction tube 114 and generally circumscribing the fluid line 118, and is 
fixed to the impeller 120. Appropriate bearing mountings (not shown) are 
used to rotatably attach the impeller 120 and/or the rotor 122 to the 
fluid line 118, with the bearings being lubricated by the liquid in the 
tank 112. A stator 124 is positioned about the exterior of the suction 
tube 114, and aligned with the rotor 122, so that electrical energy 
provided to the stator in the normal fashion of a motor will cause the 
rotor, and the attached impeller 120, to rotate about the fluid line 118 
within the suction tube. This arrangement of the rotor 122 and stator 124 
separated by a pressure containing barrier is generally of the type 
provided in "canned" pumps. Such construction avoids the use of a drie 
shaft passing through the wall of the suction tube 114 requiring seals to 
prevent fluid leakage about the shaft. As an alternative, a magnetic drive 
of the impeller 120 may be utilized in the manner commonly known in 
construction and operation of industrial pumps. 
The end of the fluid communication line 118 at the impeller 120 is open to 
receive fluid flow from the interior of the suction tube 114. In general, 
with the impeller 120 stationary, the gas and liquid within the tank 112 
are randomly distributed throughout the tank, including the interior of 
the suction tube 114. When the impeller is caused to rotate, fluid within 
the suction tube 114 is rotated accordingly, resulting in the formation of 
a vortex along the longitudinal axis of the tube. With the vortex 
established within the suction tube 114, liquid in the vortex will tend to 
move toward the wall of the tube 114, and gas within the tube will occupy 
the center of the vortex leading to the impeller, at which point the gas 
may enter the open end of the fluid communication line 118. The impeller 
120 may incorporate screen elements (not shown) surrounding the entrance 
to the fluid communication line 118. When the impeller 120 is in motion, 
the screen elements will deflect and sling away spattered liquid, 
preventing droplets from entering the fluid communication line 118. 
A liquid vent circuit, or return line, 126 branches off from the suction 
tube 114 in the vicinity of the impeller 120 outside the tank 112, and 
returns to the tank at a location outside the suction tube. Liquid drawn 
toward the impeller 120 within the suction tube 114 by the vortex action 
will generally move along the interior surface of the suction tube wall to 
the return circuit 126, and move back to the interior of the tank 112 
outside the tube 114 by means of the return line. As fluid, both liquid 
and gas, within the suction tube 114 is moved out of the suction tube by 
action of the impeller 120, additional liquid and gas within the tank 112 
may enter the suction tube 114 through the tube slots 116 to enter the 
vortex. Fluid enters the tube 114 through the slanted slots 116 in a 
direction tangential to the inner wall of the tube, exhibiting a rotation 
to help sustain the vortex caused by the impeller 120. To assure continued 
fluid circulation the impeller may be of compound design, with an axial 
fan inducer element supplying fluid to a primary centrifugal element. 
A three-way, solenoid operated valve 128 is positioned along the fluid 
communication line 118, and controlled, through a signal communication 
line 130, by a controller 132, which may be computer operated. The 
controller 132 monitors the fluid communication line 118 by an ultrasonic 
liquid detector 134 in the line, sending signals to the controller by an 
appropriate communication line 136. Similarly, a pressure sensor 136 
detects the pressure within the tank 112 and conveys its pressure output 
signal to the system controller 132 by an appropriate communication line 
138. A liquid bypass line 140 extends from the valve 128 to a point in the 
return line 126 at which a venturi throat 142 is provided in that return 
line. 
Generally, the valve 128 is positioned to close its outlet port to the 
fluid communication line 118 to the right, as viewed in FIG. 2, and to 
open its port to the bypass line 140. This is the configuration of the 
system when no gas is being withdrawn from the tank 112. When gas is to be 
withdrawn from the tank 112, the valve 128 is positioned to close the 
bypass port to the bypass line 140 and to open the outlet port to the 
fluid communication line 118. The impeller 120 is actuated to rotate with 
the valve 128 so positioned to allow passage of gas along the 
communication line 118. As the vortex within the suction tube 114 is 
established by the impeller 120, gas will enter the low pressure vortex 
core at the nearest point to the impeller that is in contact with gas 
space, and will travel toward the impeller. Gas will be drawn into the 
fluid communication line 118 to the gauging system 10, as liquid moving 
along the interior of the suction tube 114 moves through the return line 
126 back to the interior of the tank. It will be appreciated that this 
circulation of the liquid, caused by the establishment of the vortex, will 
cause random circulation of all fluid in the tank to help bring the 
temperature of the fluid within the tank to equilibrium and promote 
movement of the gas distributed throughout the tank at random toward the 
suction tube 114. 
During operation of the separator system 110, if liquid enters the fluid 
communication line 118, it may be sensed by the liquid detector 134. Then, 
in response to a liquid-indicating signal from the detector 134, the 
control unit 132 may operate the valve 128 to close the outlet port 
thereof to the fluid communication line 118, and to open the bypass port 
to the bypass line 140. Then, all fluid entering the fluid communication 
line 118 will move back through the bypass line 140 to the venturi throat 
142. Liquid flow along the return line 126 establishes pressure at the 
venturi throat 142 lower than the pressure at the impeller 120 to ensure 
circulation of liquid and gas through the bypass line 140. When the liquid 
detector 134 no longer senses liquid in the fluid communication line 118, 
the controller 132 may reopen the valve port to the fluid communication 
line to the gauging system 10, at the same time closing off the bypass 
port to the bypass line 140. 
Once a vortex is established within the suction tube 114, flow of gas from 
the tank 112 along the line 118 may provide a sufficient vortex effect in 
the tube 114 to sustain the system operation without continuous use of the 
impeller 120. Thus, the impeller motor may be turned off and need not be 
restarted unless the vortex diminishes to an unsatisfactory extent. In any 
event, the impeller 120 may be turned off when only gas is present at the 
impeller to prevent overheating of the bearings and to conserve energy. 
The controller 132 may perform this function after a specified time 
without detecting liquid in the line 118 has elapsed. In general, the 
controller 132 may be preset to stop the gas removal when a specified 
pressure within the tank 112 has been reached, as determined by the 
pressure sensor 136. 
Use of the tube 114 permits the establishment of a vortex within the tube's 
limited confines to draw off the gas rather than establishing a vortex 
throughout the tank 112, which would require more energy to generate. The 
interior of the vortex tube 114 may be polished or coated for smoothness 
to minimize friction between it and the liquid, thus increasing the 
efficiency of the generation of the vortex. The outside of the tube 114 
may be coated with polytetrafluoroethylene, for example, to make it 
unwettable, thereby facilitating the movement of gas into the tube. The 
outside of the fluid line 118 within the suction tube 114 may be similarly 
coated to be non-wettable. 
Depending on the physical arrangement of components of the particular 
gauging system, additional sensors may be employed to determine the mass 
of gas contained in various segments of the system according to the 
techniques disclosed herein. 
It will be appreciated that the gas removal system 110 operates with little 
or no liquid loss into the fluid communication line to the gauging system 
10. Additionally, the gas removal system 110 operates to separate the gas 
from the liquid without the need, for example, of accelerating the tank 
112 to move the liquid toward one side thereof. 
The gauging system of FIG. 1 requires no propulsive forces to be applied to 
the system as a whole to operate, and, therefore, no external propulsive 
system is required. The gas separation system 110 likewise imposes no need 
for such propulsive forces. Further, the present invention requires no 
dedicated gas source, nor any increase in tank working pressure and weight 
to accommodate use in a blowdown expulsion mode. All operative components 
of the gauging system are outside the tank, and thus there is no 
requirement for tank entry for repair or replacement of a component. The 
present invention is effectively a closed system with the tank, wasting no 
precious fluids that must be resupplied from earth, for example. Further, 
the system uses only electrical energy, which can be acquired in orbit. 
The foregoing disclosure and description of the invention is illustrative 
and explanatory thereof, and various changes in the method steps as well 
as in the details of the illustrated apparatus may be made within the 
scope of the appended claims without departing from the spirit of the 
invention.