A previously developed quality/flow meter has been redesigned to improve its performance as a device for mea- suring the quality (but not the flow) of two-phase (liquid + vapor) oxygen or nitrogen flowing in a pipe. As used in engineering disciplines concerned with two-phase flows, “quality” denotes, loosely, volume fractions of liquid or gas. Like some other quality meters, the previously developed meter and the present meter are based on a capacitance-measurement concept: The fluid flows through a space between electrodes, the capacitance between the electrodes is measured, and the volume fractions of liquid and gas are estimated from the effective permittivity, using the known relationships (a) between the effective permittivity and the capacitance and (b) between the volume fractions and the effective permittivity. The estimate of quality can be refined by use of additional data from pressure and temperature sensors.
The previously developed quality/flow meter was described in “Quality/ Flowmeter for Two-phase Fluid” (KSC-11725), NASA Tech Briefs, Vol. 21, No. 7 (July 1997), page 62. That instrument was built around a section of pipe that constituted a common outer electrode for two capacitive sensors. The other elec- trodes for the capacitive sensors were metal rods mounted coaxially within the pipe on electrically insulating radial spacers.
A major difference between the previously developed and present instruments lies in the shapes of the electrodes and the flow cross section. The present instrument contains flat electrodes in a square flow cross section (see figure). In comparison with the previous configuration of concentric round electrodes in a round flow cross section, the present configuration should afford greater accuracy in the measurement of quality because the sensing electric field generated by flat electrodes is more nearly uniform across the flow path.
The design of the present instrument also incorporates a number of hardware improvements to minimize leakage, provide a rugged connection between an electrical-feedthrough pin and the central plate electrode, position the feedthrough to minimize condensation, and facilitate assembly and disassembly. In addition, the design ensures compatibility of the instrument with liquid oxygen: for this purpose, all parts in contact with liquid oxygen are made of either stainless steel, poly(tetrafluoroethylene) or (in the feedthrough) ceramic.
The technique used to measure the capacitance is somewhat complicated but offers more stability and accuracy than do simpler techniques. First, a circuit that includes timer integrated circuits is used to convert a change in capacitance to a change in the duration of a digital pulse. The duration is then measured by use of an oscillator and a counting circuit. These circuits yield a 12-bit digital counting signal, which is fed to a digital-to-analog converter to generate an analog signal indicative of the measured capacitance and thus the quality. The circuit parameters are chosen so that if, for example, the fluid is a mixture of liquid and gaseous nitrogen, the analog output potential ranges from ≈1 V when only vapor is present to ≈4 V when only liquid is present. The circuitry is capable of sampling the capacitance about 2,000 times per second, but the speed of response is limited by the 0.03-second time constant of a filter in an operational amplifier that process the output of the digital-toanalog converter. This time constant is acceptably short in the original intended application of the quality meter.