Foster-Miller Creates First Accurate Flow Meter for Cryogenic Liquids

Note to those ready to buy the item under development: The company developing this product – Foster-Miller Inc. – reports that their "research and development project was interrupted and no final completed product resulted."

Foster-Miller Inc. has made perhaps the first flow meter that can accurately monitor the flow of cryogenic fluids. Until now, the two-phase nature of cryogens has prevented precise measurement because bubbles within the liquid interfere with the operation of conventional flow meters. Foster-Miller's compact device uses microwave sensing to non-invasively measure flow. By actually using the bubbles, it permits accurate and immediate determination of mass fluid transfer for applications ranging from rocket fuel loading to liquid nitrogen delivery. Key to making the device small was a data acquisition processor (DAP) card, an analog-to-digital (A/D) converter with an on-board microprocessor. The card permits data acquisition at more than 800 kHz, high enough to capture the incoming microwave signals. More importantly, its microprocessor reduces the raw data by a factor of 7500:1 before it is passed to a second CPU for signal processing. "Because the DAP handles preprocessing, a relatively low-end CPU can handle our proprietary routines and no hard drive is necessary for data storage," says Tom Lovell, Senior Engineer for Foster-Miller. "This enabled us to minimize the size and cost of the device."

Cryogenic Flow Measured Using Real-Time Signal Analysis

“We were very happy to find the DAP board because this one small device handles signal collection, analog-to-digital conversion, and preliminary signal analysis. ...Another benefit came from having real-time data reduction performed on the DAP.”
Tom Lovell, Senior Engineer, Foster-Miller, Inc.

Foster-Miller Inc., Waltham, Massachusetts, is a privately held engineering and technology development firm located on Rte. 128 / Interstate 95, "America's Technology Highway." The company maintains more than 170,000 square feet of offices, laboratories, and shops and has a staff of more than 250 composed of mechanical, electrical, thermal, chemical, nuclear, aerospace and materials engineers as well as metallurgists, physicists, mathematicians, chemists and biologists. In addition to the engineering business, Foster-Miller has the following subsidiaries and affiliates: Aztex, which works on improving the properties of composites; Foster-Miller Technologies, developers of biomedical devices, machinery, and measurement systems; Gridcom, designers and manufacturers of voltage and current sensors; LAST Armor, manufacturers of composite appliqué tiles for military use.

Measurement problems with cryogens

The need to accurately measure cryogenic flow is important in applications ranging from routine liquid nitrogen delivery to pumping rocket fuel in zero gravity. As Lovell explains, "Currently when people deliver liquid nitrogen in trucks, they estimate the amount they pumped or determine the exact amount based on weight," he says. "Unless they actually weigh the product on-site, they cannot give the customer an accurate reading or invoice at the time of delivery. The problem is worse for NASA when they need to transfer rocket fuel between tanks in outer space. A dipstick doesn't work in zero gravity since there is no level, yet knowing how many gallons have been pumped is critical for planning."

Until now, it has been impossible to accurately measure the flow of cryogenic fluids-such as liquid carbon dioxide, the refrigerant HFC-134a, liquid nitrogen, ammonia, liquid oxygen, and some rocket fuels-because they are always at saturation temperature and thus on the verge of boiling. As a result, the smallest pressure drop or heat leak in the storage and transport system causes bubbles to form. Even the friction of fluid against pipe walls causes enough pressure drop to create bubbles. "Bubbles interfere with the operation of conventional flow meters, which are designed to handle pure vapor or pure liquid, but not two-phase, flow," says Lovell. Turbine meters, for example, get knocked off their bearings when flow alternates between bubbles and liquid. A rotameter, a vertical clear column with a steel ball inside that rises in response to the flow, is also inaccurate with two-phase flows because the intermittent bubbles disturb the motion of the ball. Some people attempt to measure flow with a meter that uses vibration to create a Coriolis effect. "By measuring the Coriolis effect, you can measure mass flow, even in theory two-phase mass flow. With many two-phase flows, however, we have found it doesn't work consistently. The device is also somewhat complicated and requires a 'loop' inserted into the piping. These features disqualify such meters from many applications," Lovell notes.

To meet the need for accurate flow metering of cryogenic fluids, Foster-Miller invented a non-invasive device that uses microwave sensing to measure both mass flow and flow quality, the fraction of the flow weight that is vapor. "This device is significant because, to our knowledge, there has never been a simple non-invasive meter that can measure two-phase cryogenic flow without the need to separate the vapor and liquid components," says Lovell. The general principle underlying Foster-Miller's device is that when a microwave signal is transmitted through the pipe in which cryogenic liquid is flowing, the flow will modify the signal. The received signal is analyzed by means of Foster-Miller's patented signal processing technology, called Dynamical Instruments, that characterizes the degree and the type of modification to the signal. This determines the velocity of the flow and the void fraction, which is the percentage of vapor by volume. "By combining those two parameters with other flow information, we can compute mass flow and quality," explains Lovell. The device requires only a six-inch section of smooth bore pipe where the microwave transmitter and receiver are placed. "Nothing intrudes into the flow," Lovell adds.

In designing the system, Foster-Miller engineers kept size in mind, realizing that space is limited on both delivery trucks and NASA vehicles. Their goal was to create a modular device that was small and lightweight yet included all the necessary functionality such as analog-to-digital (A/D) conversion, raw data processing, proprietary signal processing technology, and a graphical display. One possible configuration would have been a PC equipped with an A/D board, hard drive, and monitor. "In that type of system, the A/D board would send the raw data to the hard drive. Then the PC's processor would have run our own code to process signal," says Lovell. "But that would have made a larger, more expensive, and slower device than we wanted."

Thinking small

One of the keys to making the system compact turned out to be a DAP board from Microstar Laboratories, Bellevue, Washington. The unique capability offered by DAPs is an onboard microprocessor that executes processor-intensive routines in real time. "We were very happy to find the DAP board because this one small device handles signal collection, analog-to-digital conversion, and preliminary signal analysis," says Lovell. "This way, we don't need a hard drive, which takes up a lot of space. Also, we can use a relatively low-end processor to run our proprietary routines because the DAP board converts millions of data points that would otherwise be put onto a disk."

During the development of the cryogenic flow meter, Lovell installed the DAP in a PC on the PCI bus, which is how the card is typically used. In that configuration, the DAP's multitasking, real-time operating system was important because the fact that Windows is not a real-time operating system can be a problem in data acquisition applications. To move the high volume of microwave signal data that is collected in real-time, computing power has to be available when it is needed, every millisecond or so. Operating systems like Windows use up many cycles on the PC platform, and when they take control of the CPU they hold onto it for a long time in data acquisition terms. The DAP's operating system made it possible to move data through the system without involving the Windows or the PC's CPU. That way, data flow was not interrupted when Windows was occupied with other tasks. "Although the operating system issue wouldn't be a concern with our final device, the fact that the DAP doesn't rely on Windows was a real benefit to us during development," says Lovell. "Another benefit came from having real-time data reduction performed on the DAP. Since we didn't have to manipulate data off-line, we got the results of our test runs faster. This is ultimately reducing the time to market for the device."

Foster-Miller is now developing the version of the flow meter that will be used in real-world applications. It is built around the DAP 5200a/626, which has an onboard 400 MHz AMD K6-III+ processor and 32 megabytes of DRAM onboard memory. It provides 14-bit A/D converter resolution with a sampling rate of more than 800 kHz. Although the board is user-programmable, Foster-Miller hired Microstar to write the C-code that was compiled and loaded onto the board for signal pre-processing. "The DAP board's real-time pre-processing allows us to deliver a real-time flow meter quite easily," says Lovell. "It collects and instantly processes the raw data, using our algorithms to reduce data by a factor of 7500:1." The final device will have a PCI bus where the DAP board will reside. A second processor will perform final signal processing. A small touch screen will serve as the user interface, for example, to allow switching between flow measurement and totalizer modes.

A data acquisition card with an on-board microprocessor allowed Foster-Miller to design an accurate flow meter for cryogenic fluids that is compact enough for use in delivery truck and space vehicles. "With the DAP card handling data collection, A/D conversion, and data preprocessing, we don't need additional data storage and the system can be kept very small," says Lovell. "The DAP is key to getting a compact and inexpensive final product."