The state of the art of pressure gauging has been advanced by the development of a new technique, and of a nonintrusive gauge based on the technique, for measuring the pressure of a fluid in the same segment of pipe in which the mass-flow rate of the fluid is also measured. NASA anticipates the first use of the technique in support of the High Flow Test Facility at White Sands Test Facility, and Kennedy Space Center has expressed interest in applying the technique on the X-33 aerospace launch vehicle. Nonintrusive pressure and flow gauges based on the present technique could also supplant older pressure and flow gauges in a variety of commercial processes and pressure systems in which the intrusion of gauges is known to affect local fluid dynamics.
Prior flowmeters and pressure gauges are subject to some limitations:
- Frequently, pressure gauges of prior design are attached to the outside walls of straight pipe segments, but intrusive wall taps are needed to couple fluid pressure to the transducers in the gauges. A wall tap can disturb the local flow profile and introduce fluid-entrapment zones and fluid-containment volumes.
- Straight-tube mass-flow meters of prior design do not measure pressures along with mass-flow rates.
Gauges for simultaneous measurement of pressure and flow rate would be useful on NASA's space shuttle. An important consideration in the design of such gauges as retrofit items is that they are not allowed to intrude into the plumbing carrying the fluid to be gauged. Therefore, the present technique for measurement by an externally mounted, nonintrusive gauge was conceived. The technique offers the advantage of eliminating not only flow-disturbing intrusions into the plumbing but also the complications associated with implementation of pressure taps.
The present technique involves the use of a piezoelectric transmitting transducer, mounted on the outside of a fluid-filled pipe, to excite vibrations in the pipe. The transducer is driven by a voltage-tunable electronic power oscillator. A receiving transducer, also attached to the exterior surface of the pipe, is used to measure the frequency and phase of the vibrations. An external phase-locked-loop control circuit ensures that the frequency of the power oscillator automatically tracks the resonance frequency of the selected vibrational mode of the pipe. The frequency output of the gauge can be coupled to external readout equipment by use of an optical fiber. The pressure of the fluid is then computed by use of the correlation between the fluid pressure and the resonance frequency (the pressure varies approximately linearly with the frequency).
Because the only part of the gauge that comes in contact with the fluid is the pipe segment, which is already part of the plumbing, the nonintrusive pressure gauge can coexist with a mass-flow gauge that utilizes or is mounted in or on the same pipe segment. Although the sensitivity of the present nonintrusive pressure gauge is lower than that of a typical intrusive gauge, the very fact of its nonintrusiveness enhances its potential utility. Once the issue of sensitivity is addressed, the gauges based on the present technique could be expected to become tools of choice in commercial as well as aerospace applications.
This work was done by W. C. Smith of Honeywell for Johnson Space Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Johnson Space Center
Refer to MSC-22738.