Measuring and testing – Instrument proving or calibrating – Volume of flow – speed of flow – volume rate of flow – or mass...
Reexamination Certificate
1999-03-26
2002-03-26
Patel, Harshad (Department: 2855)
Measuring and testing
Instrument proving or calibrating
Volume of flow, speed of flow, volume rate of flow, or mass...
Reexamination Certificate
active
06360579
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to systems that are used in calibrating flowmeters for purposes of assuring accuracy from the meter which is calibrated. More specifically, the system uses statistical analysis to calibrate volumetric flowmeters, mass flowmeters, densitometers, and viscosimeters in situations where the meter being calibrated can be of the same type as the standardized meter.
PROBLEM
It is often desirable to perform periodic maintenance upon flow meters that are placed in service. One aspect of this maintenance is to calibrate the meters for the purpose of ensuring accurate and reliable measurement data. As used in the discussion below, the term “standard meter” is hereby defined to mean a meter that has been calibrated according to precise standards where this calibration permits the meter to be used as a standard measurement tool for use in calibrating other meters. The term “service meter” refers to a meter that is normally in use obtaining measurement data for a specified purpose, but periodically requires calibration to ensure the accuracy of this measurement data. A standard meter is also a service meter in the sense that the standard meter is normally in use obtaining measurement data for the purpose of calibrating other meters, and a standard meter itself requires periodic calibration.
The purpose of a meter calibration effort is to ascertain a flow calibration factor that is used to convert electronic signals to direct measurements of mass, volume, and other information from the meter under test. Coriolis meters and positive displacement meters are known in the art as linear meters, i.e., the flow calibration factor is a constant with respect to flow rate. Other meters including orifice meters, magnetic flow and vortex meters are nonlinear meters where the flow calibration factor varies with flow rate.
The calibration process typically entails removing the meter from service for shipment to a test facility where the meter is cleaned, repaired as needed, and subjected to test measurements. The most common calibration measurements usually involve the use of a gravimetric diverter system to flow through the meter under test a standard fluid having precisely known intrinsic and extrinsic fluid properties, e.g., temperature, density, viscosity, and volume. The meter under test performs flow measurements on this fluid, and these measurements are cross checked against the known fluid properties. Gravimetric diverter systems may be designed for testing purposes across a wide range of flow rates, but the additional structure that is required to provide this functionality is so large as to make transportation of these systems impracticable.
The use of gravimetric diverter systems to test flowmeters is relatively time consuming and expensive. The gravimetric diverter systems themselves occupy relatively large volumes of space. The loss of time, space and money can be reduced by calibrating very precise meters, i.e., standard meters, against gravimetric standards for subsequent use in calibrating other meters under test. During the course of calibration tests, these standard meters are connected in series with the meter under test to perform simultaneous flow measurements. The measurement data from the meter under test is used in calculations with measurement information from the standard meter on the same fluid volume to provide or confirm a flow calibration factor for the meter under test.
Flowmeters can never be relied upon to provide measurement data that is completely accurate because there are always small uncertainties in the meter output. For example, many Coriolis flowmeters sold by Micro Motion, Inc., of Boulder, Colo., are specified to be accurate within 0.1 percent of the total mass flow rate within a selected operating range of flow rates. Many of these meters are capable of even more outstanding accuracy down to less than 0.01 percent within subportions of this range. There is no one single flowmeter that provides this outstanding level of accuracy across all flow rates. Coriolis flowmeters have been designed to perform mass flow rate measurements on flows ranging from less than 0.1 lbs/min to greater than 25,000 lbs/min.
Coriolis effect mass flowmeters are well known and have been described in numerous patents, e.g., in U.S. Pat. Nos. 4,444,059, 4,491,025, and 4,422,338 to Smith, which all describe mass flow rate meters that use vibrating tubes to impart measurable Coriolis effects which are related to mass flow rate. U.S. Pat. No. 4,491,009 to Ruesch describes a vibrating tube densitometer based upon the structure of a Coriolis mass flowmeter. The ability of Coriolis effect mass flowmeters to measure density permits the determination of a volumetric flow rate by a simple division of the density value into the mass flow rate value. It is also well known that Coriolis effect mass flowmeters can be operated as viscosimeters.
The total level of uncertainty in a flow measurement arises from random uncertainties and systematic uncertainties in the meter together with its environment of use. The metering industry has generally considered these uncertainties and published official guidelines for quantifying and managing meter uncertainties, as in ISO-5168, which is hereby incorporated by reference to the same extent as though fully disclosed herein.
The metering industry uses a rule of thumb advantage that requires the uncertainty in output from a standard meter to be at least three times better that the manufacturer's accuracy specification of a meter under test. Thus, a service meter that is specified as being accurate to 0.1 percent of a flow rate requires a standard meter that is accurate to 0.033 percent for calibration purposes.
Coriolis mass flow meters are the most accurate type of meter known for practical use through many flow regimes. The meters are generally insensitive to flow profile, and calibration factors that are developed using liquid fluids work equally well when applied to gas fluids in service. There is no known or readily available metering technology with superior accuracy for use as a measurement standard against Coriolis effect mass flow meters.
Some problems could arise when using Coriolis meters as the standard meters to calibrate Coriolis service meters. Where the two Coriolis meters have similar or identical manufacturer's accuracy specifications, it becomes impossible to gain the rule of thumb advantage requiring uncertainty in the standard meter to be three times better than the manufacturer's accuracy specification in the meter under test. This situation digresses into a requirement for relatively expensive gravimetric testing of the service meter.
There exists a need for a compact flowmeter calibration system including standard meters that are operable across a wide range of flow rates with sufficient accuracy for use as a standard metering system. The system should be operable for calibrating linear and nonlinear meters. The system should also be modular so it can be manufactured, shipped and installed easily. The system can also be transported for calibration of meters at remote sites where meters are in service, as opposed to the present practice of removing the meters from service and shipping the meters to a flow laboratory for calibration purposes.
SOLUTION
The present invention overcomes the problems that are outlined above and advances the art by providing a compact flowmeter calibration system including a plurality of standard meters with sufficient accuracy for use as standard meters across a wide range of flow rates. The system is operable for calibrating both linear and nonlinear meters through a wide range of flow rates. The system is also easily transported to remote locations for calibration testing, and can be broken down into modular components for further ease of transportation and storage.
As used herein, the term “calibration” is defined to mean a flow measurement test that provides data which is used to either improve the accuracy of a flowmeter or to verify the accu
Buttler Marc Allan
De Boom Robert J.
Gniffke Julie
Longo Joe J.
Chrisman Bynum & Johnson, P.C.
Micro Motion Inc.
Patel Harshad
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