An apparatus for measuring the rate of flow and the mass density of a liquid or slurry includes a special section of pipe instrumented with microwave and sonic sensors, and a computer that processes digitized readings taken by the sensors. The apparatus was conceived specifically for monitoring a flow of oil-well-drilling mud, but the basic principles of its design and operation are also applicable to monitoring flows of other liquids and slurries.

The Height of the Fluid relative to the antenna is determined from differences between the phases of stepped-frequency microwave signals transmitted to, and reflected from, the top surface of the fluid.

In one configuration, a special section of pipe is located immediately upstream of the point of discharge of the flow to be monitored. The special section of pipe must be large enough that the pipe can accommodate the entire flow of interest (in contradistinction to a small diverted sample flow), that the flow remains laminar at all times, and that the pipe is never entirely full, even at the maximum flow rate.

In another configuration, the apparatus does not measure the rate of flow or the density directly: Instead, it (a) measures the height of the fluid in the special section of pipe and computes the flow rate as a predetermined function of the height and (b) measures the speed of sound in the fluid and computes the density of the fluid as a predetermined function of the speed of sound in the fluid. To enable the apparatus to perform these computations, one must calibrate the apparatus, prior to operation, by measuring the flow rate as a function of height and the mass density as a function of the speed of sound for the drilling mud or other fluid of interest.

In the second configuration, the velocity of the fluid can be measured subsurface using a set of one transmitter and two receivers to measure differential phase shifts. This second configuration can be used within a filled or unfilled closed pipe to measure volume flow. The microwave portion of the apparatus (see figure) includes a broadband swept-frequency (more precisely, stepped-frequency) transmitter/receiver pair connected, via a directional coupler, to an antenna aimed downward at the liquid. Transmitted- and received-signal data are processed by an algorithm that uses a modified Fourier transform to compute the roundtrip propagation time of the signal reflected from top of the fluid. The height of the fluid is then computed from the round-trip travel time and the known height of the antenna. A sonic sensor that operates alongside the microwave sensor gives an approximate height reading that makes it possible to resolve the integermultiple- of-2π phase ambiguity of the microwave sensor, while the microwave sensor makes it possible to refine the height measurement to within 0.1 in. (≈2.5 mm).

Ultrasonic sensors on the walls near the bottom of the special section of pipe are used to measure the speed of sound needed to compute the density of the fluid. More specifically, what is measured is the difference between the phase of a signal of known frequency at a transmitting transducer and the phase of the same signal at a receiving transducer a known distance away. It may also be necessary to resolve an integer-multiple-of- 2π phase ambiguity. This can be done by using two sonic frequencies chosen according to a well-established technique. Alternatively, one could use a single sonic frequency low enough not to be subject to the phase ambiguity, albeit with some loss of density resolution. Simulations indicate that a density accuracy measurement of 0.25 percent (0.0025) can be attained with a single-tone system.

This work was done by G. D. Arndt and Phong Ngo of Johnson Space Center and J. R. Carl and Kent A. Byerly, independent consultants.

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
(281) 483-0837.

Refer to MSC-23311.


NASA Tech Briefs Magazine

This article first appeared in the June, 2004 issue of NASA Tech Briefs Magazine.

Read more articles from the archives here.