A technique for measuring the volume of an incompressible liquid in a rigid tank involves measurement of the total volume of gas in those parts of the tank not occupied by the liquid. The volume of liquid is then computed by subtracting V from the total volume of the tank and the associated plumbing.
Unlike liquid-level-measuring techniques, this technique works whether or not a gravitational field is present and is unaffected by the shape of the liquid or tank. Even if bubbles of gas are present in the liquid or if the liquid has broken up into separate globules or pools, the measurement of the total volume of gas is unaffected.
The pressure in the tank is measured while the total volume of the tank is varied by use of a piston or bellows (see figure). It is assumed that the gas is a noncondensible ideal gas, that the alternating compression and decompression of gas is adiabatic, and that the variation in volume is a small fraction of the total volume of gas. Under these assumptions, the total volume (V) of gas in the tank is given by V = – γP(∆V/∆P), where γis the specific heat of the gas at constant pressure ÷ the specific heat of the gas at constant volume, P is the pressure, ∆V is the change in volume, and ∆P is the change in pressure that accompanies the change in volume. In a demonstration of this technique, the volume of water in a 94-liter tank was determined within 1 liter.
This work was done by Frank T. Hartley of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category. NPO-19211
This Brief includes a Technical Support Package (TSP).
Measuring Volume of Incompressible Liquid in a Rigid Tank
(reference NPO-19211) is currently available for download from the TSP library.
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Overview
The document presents a technical support package from NASA detailing a novel method for measuring the volume of incompressible liquids in rigid tanks, particularly in zero-gravity environments. Developed by Frank T. Hartley at the Jet Propulsion Laboratory, this technique addresses the limitations of traditional liquid-level measurement methods, which are unsuitable for spacecraft applications where gravitational forces are absent.
The method leverages the properties of gases and liquids: while liquids are relatively incompressible, gases are compressible. By using a closed bellows or piston arrangement connected to a sealed tank, the effective volume of the tank can be altered mechanically. When the volume of the tank is changed, the pressure of the gas within the tank changes accordingly. This pressure change is measured using a pressure transducer, allowing for the calculation of the gas volume. The volume of the liquid can then be determined by subtracting the gas volume from the total tank volume.
The document outlines the assumptions necessary for this method, including the ideal gas behavior of the gas in the tank and the adiabatic nature of the volume changes. It emphasizes that the technique is unaffected by the shape of the liquid or the tank, making it versatile for various applications. The method has been demonstrated to accurately measure the volume of water in a 94-liter tank within a margin of 1 liter.
The motivation behind this development stems from the need for accurate measurements of fuel and oxidant volumes in spacecraft, where previous methods based on estimated burn rates were found to be inaccurate. This new approach allows for precise liquid volume measurements, regardless of the liquid's distribution or the number of portions it consists of.
In summary, the document highlights a significant advancement in liquid measurement technology, particularly for aerospace applications. By utilizing gas pressure changes to infer liquid volumes, this method provides a reliable solution for measuring incompressible liquids in environments where traditional methods fail, thus enhancing the accuracy of propellant measurements in spacecraft.
