25 October 2001
Flowmeter for two-phase mixtures
by Bob Felton
High dynamic range, quick response, and good sensitivity are possible.
A new flowmeter pulls extra duty controlling phase quality or measuring velocity, level, humidity, or density for two-phase mixtures. Engineers at Kennedy Space Center developed the technology to control cryogenic flows of liquid nitrogen, oxygen, and hydrogen.
The meter uses a capacitance probe to measure the dielectric constant of a two-phase liquid and gas flow stream. The dielectric constant of the stream varies according to the relative volume of liquids and gas. Because quality is the ratio of the volume of a liquid or gas to the total volume of flowing material, changes in the dielectric constant are indicative of changes in quality and provide a basis for its determination.
Generally, capacitance-based meters rely on a probe inserted into the flowing material, and they are susceptible to errors introduced by electrical noise, temperature changes, and pressure changes. According to the inventors, "The present invention seeks to overcome the drawbacks of prior art capacitance-based quality meters and flowmeters by providing a device which eliminates the need for a high-frequency oscillator circuit to measure capacitance changes. Instead, the invention employs circuitry that accurately measures the time required to charge a probe capacitor to threshold voltage. Timer and counter circuits are employed to measure the elapsed time between the start of capacitor charging and the attainment of the threshold value, resulting in a digital value which is fed directly into a microprocessor and is linearly proportional to the capacitance value. The beauty of this arrangement is that it effectively generates a high-resolution digital value based upon analog measurements without use of an expensive, high-resolution analog-to-digital converter. In addition, this arrangement is extremely noise tolerant since it does not rely upon frequency changes to convert a capacitance measurement to a digital value."
Like conventional designs, the new probe passes the material between two charged plates. The designers added a twist that minimizes disturbances due to the instrument itself, however.
"The Quality Flow Meter system consists of an insulated pipe section with an inner solid probe held in place by insulating standoffs. The probe is electrically isolated from the inside of the pipe. The probe has a wire attached to each end. These wires are electrically isolated from each other. The wires pass through the pipe wall to a connector."
Further, the probe body is "split at its center to provide two separate, spaced probes for making the capacitance measurements. A flow-directing tube surrounds the probes and has an increased diameter along the probe body length to maintain the flow cross-section constant around the probes. The two probes and the flow-directing tube combine to form first and second coaxial measurement capacitors, with the exterior surfaces of the probes forming a center plate for each measurement capacitor and the inner surface of the flow-directing tube forming the outer, ground plate for each measurement capacitor."
To add utility to the meter, the design incorporates a vacuum jacket that surrounds the tube and minimizes heat losses. Additionally, the designers built temperature and pressure sensors into the probe assembly in order to generate signals that can compensate for pressure- and temperature-induced measurement errors. Further, the device measures flow in two directions.
An additional benefit of the design is that by continually monitoring both of the capacitors and measuring the time offset between identical readings, engineers can determine flow velocity.
Additional information about the meter is available from the National Technology Transfer Center, 316 Washington Avenue, Wheeling, WV 26003; (800) 678-6882. The meter was granted patent number 5,861,755. You can view the document online. IT
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