Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
2002-11-12
2004-05-11
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
Reexamination Certificate
active
06732596
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to improvements in gas flow measurement, and more specifically to use of critical flow nozzles for improved accuracy and/or for gas flow measurement at high mass flow rates.
By way of background, the measurement and control of the mass flow of gases is important in a wide variety of industrial processes. This includes the control of reactant gases used in semiconductor processes and pharmaceutical applications as well as the precise measurement of gases to create known gas blends. A variety of devices exist to measure and control mass flow and these devices need to be regularly calibrated with reference standards to assure their accuracy. The potential accuracy of the devices is continually improved in response to process needs and therefore the accuracy of the standards available to calibrate them must continually improve.
Perhaps the closest prior art includes the American National Standard entitled “Measurement of Gas Flow by Means of Critical Flow Venturi Nozzles”, ASME/ANSI MFC-7M-1987 et ISO 9300:1990 sponsored and published by The American Society of Mechanical Engineers, incorporated herein by reference, and commonly assigned patent U.S. Pat. No. 5,445,035 entitled “PRECISION GAS MASS FLOW MEASUREMENT APPARATUS AND METHOD MAINTAINING CONSTANT FLUID TEMPERATURE IN THIN ELONGATED FLOW PATH” issued Aug. 29, 1995 to Pierre R .Delajoud, also incorporated herein by reference.
The above mentioned U.S. standard, hereinafter referred to as “the U.S. standard”, discloses critical flow nozzles, in which the gas pressure upstream of the nozzle's throat is great enough relative to the downstream pressure to ensure that the gas flow velocity at the throat reaches “critical flow” or “sonic flow”, i.e., reaches the local value of the speed of sound (acoustic velocity). The speed of sound in a critical flow nozzle is a limiting speed that the gas flow cannot exceed for given upstream conditions. The U.S. standard discloses equations that allow precise computation of the gas mass flow rate based on the nature of the gas, the throat diameter, the upstream pressure and temperature and certain thermodynamic characteristics of the flowing gas. In a critical flow nozzle, enough pressure is applied upstream from the nozzle throat relative to the downstream pressure to ensure that the gas flow velocity at the throat becomes critical, i.e., attains the speed of sound and cannot be increased further. The gas mass flow rate then becomes proportional to the density of the gas upstream of the nozzle, i.e., to the pressure upstream of the nozzle. The mass flow rate then can be precisely computed on the basis of the nature of the gas, the gas temperature and the pressure upstream of the nozzle.
The above mentioned U.S. standard describes use of critical flow nozzles to measure gas mass flow rates, and specifies or recommends all of the conditions for various uses of a critical flow nozzle, including the diameter of the upstream passage compared to the diameter of the nozzle, where the pressure connection should be located to read the upstream gas pressure, where the temperature probe should be positioned upstream of the nozzle, and various other parameters.
However, a problem of critical flow nozzles disclosed in the U.S. Standard is that for low gas flow rates, if the temperature of the incoming gas is substantially different than the ambient temperature, the difference can result in inaccurate measurement of the gas mass flow rate, because the temperature of the gas between the temperature probe and the critical flow nozzle is affected due to thermal conduction and radiation of the tube. Specifically, the U.S. standard teaches that the gas mass flow computations may be inaccurate if the temperature of the incoming gas is more than 5 degrees Centigrade different than the ambient temperature.
The U.S. standard recommends exact adherence to its published recommendations, and does not provide any suggestion of how more precise measurement of gas mass flow rates might be achieved.
Above mentioned U.S. Pat. No. 5,445,035 discloses a mass flow meter that includes a body having a cylindrical bore and an elongated cylindrical piston positioned in the bore concentrically with the body. An elongated annular fluid flow channel of uniform depth is bounded by a cylindrical surface of the piston and a surface of the bore, and the gas flows through the channel in the laminar flow regime. A first pressure measuring probe in fluid communication with an upstream equalization chamber measures fluid pressure in the upstream equalization chamber, and a second differential pressure transducer in fluid communication between the upstream equalization chamber and a downstream equalization chamber measures differential fluid pressure between the two equalization chambers. The difference between the pressures measured in the two equalization chambers represents the mass flow of the fluid through the channel. The simple, near-ideal geometric shapes of the bore, piston, and ferrules supporting the piston interact so as to permit simple, accurate mathematical modeling of corrections to account for changes in pressure, temperature, and thermal gradient. By design, the temperature of the gas in the flow path assumes the temperature of the body in which the bore is located so that the gas temperature can be determined by measuring the temperature of the body.
However, the mass flow meter described in U.S. Pat. No. 5,445,035 cannot provide accurate gas mass flow rate measurements at gas mass flow rates greater than approximately 30 standard liters per minute (slm). Even using the latest technology, the upper limit for gas flow rates that can be accurately measured using the laminar flow technology described in the '035 patent is approximately 100 slm. This is because as flow rate increases, the velocity of the gas in the flow path increases, and the gas is not in the flow meter body for a sufficient amount of time for the gas to precisely assume the temperature of the body.
The setup and exploitation of critical flow nozzles to measure gas mass flows are precisely defined by the above U.S. standard. The objective of the setup recommendations in the above U.S. standard is to be able to predict the gas flow from the diameter of the throat of the nozzle by means of the recommended calculations. The calculations use tables of value of the critical flow function for various gases as a function of the pressure and temperature upstream from the nozzle and a calculation of the discharge coefficient as a function of the Reynolds number of the gas flow. The calculation of the discharge coefficient is applicable only for Reynolds numbers greater than 1*10. Those skilled in the art will understand that a lower limit Reynolds number prevents use of the recommended calculations for low gas flow rates.
The problem faced by the applicant was how to use the well-known principles described in the above prior art, especially the U.S. standard, to achieve more precise, and especially more repeatable, gas flow rate measurements in the range of flow from less than a standard liter per minute to 5000 standard liters per minute.
Thus, there remains an unmet need for an improved mass flow meter that is capable of making accurate gas flow measurements which are more precise and especially more repeatable than the prior art mass flow meters at any gas flow rate, especially at gas flow rates up to approximately 5000 standard liters per minute or more.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a mass flow meter capable of providing accurate, precisely repeatable gas flow measurements over a very wide range.
It is another object of the invention to provide a mass flow meter capable of providing accurate, precisely repeatable gas flow measurements at flow rates up to approximately 5000 liters per minute.
It is another object of the invention to provide a single mass flow meter with a very wide useful range or “rangeability”, i.e., 10:1.
It is another object of the
Cahill von Hellens & Glazer, P.L.C.
CalAmerica Corp.
Lefkowitz Edward
Thompson Jewel V.
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