Fluid handling – Processes – With control of flow by a condition or characteristic of a...
Reexamination Certificate
2000-03-22
2002-06-18
Buiz, Michael Powell (Department: 3753)
Fluid handling
Processes
With control of flow by a condition or characteristic of a...
C137S486000, C137S487500, C137S624110, C073S023310, C702S100000
Reexamination Certificate
active
06405745
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the injection of a precise quantity of gas into a system.
BACKGROUND OF THE INVENTION
The processes used to place a predetermined mass of a gas into a system have remained virtually unchanged for decades. Generally, the mass of a gas within a system is determined to one of a gravimetric process, a partial pressure process, or an analytical process.
The gravimetric process involves, for example, weighing an empty cylinder with a known volume in which the gas or gas mixture is to be placed. Gas is pumped into the cylinder and, thereafter, is weighed a second time. The weight of the empty cylinder is subtracted from the weight of the partially full cylinder to determine the mass of the gas contained within the cylinder. This partial filling of the container and weighing of the container is repeated until the cylinder contains the desired mass of gas. The concentration of the gas is derived from the mass of gas within the container, volume of the container, and the density of the gas. The accuracy of the determination of the mass of gas contained within the cylinder is dependent upon the accuracy of the weighing apparatus used and the accuracy of each measurement of mass. Multiple measurements of the partially filled container must be made for even relatively simple and common concentrations of gas. Obviously, this is a crude and a relatively time consuming method by which to place a precise amount of gas within a closed system, such as a cylinder. The accuracy of the gravimetric process is generally limited to a maximum of five percent for a typical gas. However, when it is desired to fill a cylinder with a very low mass of gas, or a low concentration of one or more gases, this method provides an accuracy of only about ten percent.
The partial pressure method involves Daltons law, which states that the total pressure of a mixture of gases is equal to the sum of the pressures of all of the component gases taken separately. Daltons law, however, holds true only for ideal gases. Furthermore, where a low concentration of one or more component gases in a cylinder is desired, it is difficult to obtain a high degree of precision using the partial pressure method since the pressure of a low concentration of gas in a given volume is relatively small and can be obscured by a gas present in higher concentration which, therefore, exerts a greater pressure.
The analytical process involves analytically monitoring the proportion or concentration of a gas within a system. Although this method provides an accurate measure of the concentrations of gases within the volume of a system, the concentration of the gases is determined after a mixture has been produced. If one or more of the concentrations of the component gases are not within a predetermined tolerance range of the intended concentration, the entire mixture must either be scrapped, reprocessed, or sold as a higher tolerance, and less profitable, mixture. Furthermore, the analysis of the mixture is a time consuming and expensive process, during the completion of which the quality of the product or mixture is unknown. Until the analysis is completed, the mixture can not be sold but rather must be stored by the manufacturer.
Mechanical critical flow orifice (CFO) kits are used to measure a flow of gas. CFO kits operate on a sonic nozzle principle. Critical orifice flow is achieved when the velocity of the gas through a CFO reaches the speed of sound (i.e., becomes sonic), and remains constant. Variables in critical orifice flow measurement calculations are avoided only if the flow through the CFO remains in the critical flow or sonic range. The critical rate of flow for a CFO is proportional to the ratio of the absolute static pressure at the nozzle inlet and the ambient temperature. For a given nozzle, there is a minimum ratio of pressure to temperature below which the flow rate of the gas through the CFO is no longer accurately predicted by the ratio of pressure to temperature under which the CFO is being operated. Manipulation of the inlet pressure is far simpler and more cost effective than varying the ambient temperature under which the gas is flowing through a CFO. Thus, in order to maintain the ratio of pressure to temperature above the minimum level at which the flow rate of the gas through the CFO is predictable, the pressure at the inlet of the CFO must be above a certain level. Typically, CFOs require an inlet pressure of at least about 20 to 30 psi in order to ensure operation of the CFO is critical. Such a relatively high inlet pressure makes it difficult to deliver a very low mass of gas to an external system, or cylinder. Furthermore, precisely machined and manufactured CFOs operating under critical conditions achieve a maximum flow rate accuracy of only about 1%.
A practical application of the ability to place a very low concentration of gas into a system arises in the testing of automobile emissions control systems. Automobile emissions are said to be the single greatest source of pollution in numerous cities across the country. Automobiles emit hydrocarbons, nitrogen oxides, carbon monoxide and carbon dioxide as a result of the combustion process. Evaporative emissions occur through the evaporation of gasoline in the engine and fuel tank. The automobile emissions control systems of today are so advanced and efficient that evaporative emissions, rather than emissions from the combustion process, can account for a majority of the total hydrocarbon pollution on hot days.
The Clean Air Act of 1970 and the 1990 Clean Air Act set national goals of clean and healthy air for all and established responsibilities for industry to reduce emissions from vehicles and other pollution sources. The 1990 law further tightened the limits on automobile emissions and expanded Inspection and Maintenance (I/M) programs to allow for more stringent testing of emissions. Standards set by the 1990 law limited automobile emissions to 0.25 grams per mile (gpm) non-methane hydrocarbons and 0.4 gpm nitrogen oxides. The standards are predicted to be further reduced by half in the year 2004.
Manufacturers of automobiles and emissions systems have risen to the challenge of reducing automotive emissions by designing Low-Emission Vehicles (LEVs), Ultra-Low-Emission Vehicles (ULEV), Super-Ultra-Low-Emission Vehicles (SULEVs) and Zero-Emission Vehicles (ZEVs). In particular, LEVs reduce the emissions by up to seventy percent, ULEVs reduce emissions by up to eighty-five percent, and SULEVs reduce emissions by up to ninety-six percent. For example, the emission requirement for a ULEV is that it emit no more than 0.04 grams of hydrocarbon per mile. A SULEV must emit no more than 0.01 gpm of hydrocarbons. The emission levels of these vehicles have been reduced to a level which even the most sophisticated equipment in a laboratory environment can not accurately measure. Furthermore, the emission levels have been reduced to a level which would require the I/M programs to use similarly sophisticated equipment at numerous testing locations, thereby rendering the I/M programs impractical and cost prohibitive. Corroborative of this fact is that Americas car companies have signed agreements with three Department of Energy national laboratories to develop prototype instruments which are capable of providing reliable, accurate, and high-speed measurement of the trace emissions from such vehicles.
These prototype instruments will require testing and calibration, a process which is rendered susceptible to inconsistent results and inaccuracies due to the minute levels of pollutants the instruments must detect. Typically, testing of instruments used in measuring emissions are themselves tested and/or calibrated by creating a flow of a precision mixture of gases, thereby simulating the exhaust of an ULEV vehicle, or by filling a Sealed Housing for Evaporative Determination (SHED) with a precision mixture of gas. The instrument under test is used to measure the known and precise mixture of gas and the measured results are then com
Kar Krishnendu
Semidey Robert
Buiz Michael Powell
Delphi Technologies Inc.
Krishnamurthy Ramesh
VanOphem John
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