Extending the Range of Gas Piston Provers
(Presented at 7ISFFM 2009)

Traditionally, constant-pressure gas provers have been used in the range of 5 sccm to 50 slm. Our most recent viscous-sealed provers have extended the range upward to 500 slm and downward to 0.5 sccm.

Over the traditional range of 5 sccm to 50 slm, we have published uncertainty analyses yielding combined standard uncertainties of approximately 0.06% and have rated our commercial products at 0.15%. However, the new provers have greater uncertainties when calibrated by dimensional means. The 500 slm prover is subject to significant uncertainty from piston rocking, as the piston width of 152 mm and the piston-cylinder gap of approximately 0.1 mm create a significant uncertainty with respect to the measured path length of 200 mm. Both provers are also subject to significant uncertainty caused by the difference of piston and cylinder diameters being significant with respect to the absolute diameter. For these reasons, we specified a conservative 0.25% combined standard uncertainty for the highest and lowest range provers. In both cases, however, the uncertainty can be improved by using gravimetric gas calibration instead of direct dimensional or, in the case of the larger prover, water-gravimetric calibration.

Here, we present our uncertainty analyses for the extended-range provers using different calibration methodologies and we discuss the cost-uncertainty tradeoffs for each.

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Ultra Low Flow Fast Primary Gas Prover
(Presented at MSC 2009)

Traditionally, constant-pressure gas provers have been used in the range of 5 sccm to 50 slm. Our recent work with viscous-sealed provers has extended the range upward to 500 slm. However, there has been a growing demand for primary standards in the sub - 1 sccm range.

We have built prototype provers designed for flows of approximately 0.1 to 20 sccm. Using a quartz tube and piston of 2.5 mm diameter and 5 micron diametric difference, we are initially experimenting with a measured piston path of 5 cm. This will allow 1 sccm flows to be measured in approximately 21 seconds.

Our initial experiments show a standard deviation (repeatability) of about 0.02% and a piston leakage (tare) of about 0.0002 sccm. Using a target combined standard uncertainty of 0.25% or better, our pending uncertainty analysis will determine whether we can shorten the piston travel to affect faster readings.

We present data to date, along with preliminary production designs.

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Dissemination of Primary Gas Flow Standards
(Presented at IMEKO International Measurement Conference 2008)

Increasing the world's wealth requires dissemination of increasingly accurate measurement standards, both for field use and for comparison of geographically dispersed laboratories and NMIs. For some units of measure, such as gas flow, this has been difficult to achieve. The primary devices used at NMIs, such as mercury-sealed piston provers and bell provers, are virtually impossible to ship in a calibrated state, while secondary standards such as laminar flow elements and sonic nozzles lack the stability or dynamic range of primary standards.

To answer this need, a series of primary piston flow provers have been developed that utilize viscous seals. They are small, stable and readily shippable. The instruments presently cover the flow range of 1 sccm to 500 slm and are conservatively represented as having combined standard uncertainties of 0.15% to 0.2%.

These provers have been used to informally compare laboratories on multiple continents, in one instance obtaining a correlation of 0.02% between the NMIs on two continents. Using this equipment, our laboratory has been accredited by NVLAP to ISO 17025 at a combined standard uncertainty of less than 0.07% at flows from 5 sccm to 50 slm.This paper will present a design and development overview, summary uncertainty analysis, inter-laboratory comparisons and validation data for the viscous-sealed provers. In addition, it will discuss automation of flow calibration for mass flow controllers and validation of primary provers, themselves, using these instruments.

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Comparisons of Viscous-sealed provers with LNE and Studies of Piston - Cylinder Leakage
(Presented at 6ISFFM Mexico 2006)

There has long been a need for fast, cost-effective primary (dimensionally traceable) flow calibration systems for use with mass flow meters and mass flow controllers in the 1 sccm to 50 slm range. Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty on the order of 0.07%. These formed the basis of the ML-500 system under discussion here. For the ML-500, a shorter measurement path was employed. Temperature and absolute pressure transducers were added to allow standardization of the readings.

Although we designed for 0.5% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.35%, with 0.30% possible if three or more readings are averaged. Maximum standardized single-reading uncertainty was 0.40%.

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Uncertainties and Inter-Laboratory Comparisons of Dry Piston Gas Flow Provers
(Presented at Flomeko 2004)

There has long been a need for fast, cost-effective primary (dimensionally traceable) flow calibration systems for use with mass flow meters and mass flow controllers in the 1 sccm to 50 slm range. Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty on the order of 0.07%. These formed the basis of the ML-500 system under discussion here. For the ML-500, a shorter measurement path was employed. Temperature and absolute pressure transducers were added to allow standardization of the readings.

Although we designed for 0.5% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.35%, with 0.30% possible if three or more readings are averaged. Maximum standardized single-reading uncertainty was 0.40%.

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Inter-Laboratory Comparisons and Application Uncertainties of Dry Piston Provers
(Presented at MSC 2004)

There has long been a need for fast, cost-effective primary (dimensionally traceable) flow calibration systems for use with mass flow meters and mass flow controllers in the 1 sccm to 50 slm range. Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty on the order of 0.07%. These formed the basis of the ML-500 system under discussion here. For the ML-500, a shorter measurement path was employed. Temperature and absolute pressure transducers were added to allow standardization of the readings.

Although we designed for 0.5% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.35%, with 0.30% possible if three or more readings are averaged. Maximum standardized single-reading uncertainty was 0.40%.

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Constant-Pressure Provers from the Standards Lab to the Shop Floor
(Presented at NCSL 2003)

Metrological devices are continually evolving, in both usability and accuracy. Ruggedized and made simpler to use, precision instruments can now be implemented on the manufacturing floor. In essence, one generation's laboratory instrument becomes the next generation's field instrument.

For example, field-rugged primary gas flow measurement devices in the 1 sccm to 50,000 sccm flow range have long been needed for diverse applications, such as calibration of environmental air samplers at 1% accuracy, and the manufacture or field service of mass flow meters and flow controllers at 0.2% accuracy.

Historically, primary flow calibrators had the advantages of direct traceability, minimal drift mechanisms; however, they were large, delicate and slow to operate (a practical limit to dynamic range). More portable and user-friendly primary provers were highly desirable. Here we trace the history of constant-volume provers and describe a version of the piston prover that has evolved to reduce the primacy/portability tradeoff. Using a viscous-sealed, virtually frictionless piston, this design achieves uncertainties below 0.2% while remaining small, fast and portable.

The analysis that follows shows that our goals have been met. In fact, a specification of ± 0.15% may be possible after further empirical verification.

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Uncertainty Analysis of a High-Speed Dry Piston Flow Prover
(Presented at MSC 2002)

There has long been a need for fast, cost-effective primary (dimensionally traceable) flow calibration systems for use with mass flow meters and mass flow controllers in the 1 sccm to 50 slm range. Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty on the order of 0.07%. These formed the basis of the ML-500 system under discussion here. For the ML-500, a shorter measurement path was employed. Temperature and absolute pressure transducers were added to allow standardization of the readings.

Although we designed for 0.5% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.35%, with 0.30% possible if three or more readings are averaged. Maximum standardized single-reading uncertainty was 0.40%.

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Development of a 0.2% High-Speed Dry Piston Prover
(Presented at MSC 2003)

There has been a long-standing need for economical, high-speed primary standards for use in the manufacture and recalibration of mass flow controllers (MFCs) and mass flow meters in the 5 sccm to 50 slm flow range. It is especially desirable to obtain a dynamic range (turndown) greater than the 10:1 ratio typical of existing devices.

Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty approximately 0.07% [1]. These later formed the basis of the 0.4% ML-500 system [2], with temperature and absolute pressure transducers added to allow standardization of the readings.

The prover described here is intended for use in the 0.2% range. The production version of the ML-500 was used as a starting point. The cylinder was lengthened to increase the volumetric repeatability, but the biggest improvements were in the standardization system. A special highaccuracy, high-speed transducer system was added to permit unique dynamic standardization techniques to be developed.

This paper is necessarily an uncertainty analysis as well as a description of our design. Although we designed for 0.2% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.085% to 0.12%, depending on flow.

That being said, this is a preliminary analysis of an experimental system. We will now build several units for better statistical verification of the reproducibility and for inter-laboratory, intermethodology testing. We have great confidence that the subsequent uncertainty analysis will confirm that our goal of 0.2% has been met.

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High Speed Prover Systems For Cost- Effective mass Flow Meter and Controller Calibration
(Presented at NCSL 2002)

There has long been a need for fast, cost-effective primary (dimensionally traceable) flow calibration systems for use with mass flow meters and mass flow controllers in the 1 sccm to 50 slm range. Exhibiting turndown ranges of hundreds to one, the provers described here use a clearance seal between a graphite piston and borosilicate glass cylinder. They are small, portable, fast, and contain no toxic materials. Reading cycle time is on the order of seconds. Used in conjunction with a stable flow generator, they are also very suitable for calibration of flow meters.

We have previously reported on clearance-sealed volumetric Laboratory Master Provers with an expanded uncertainty on the order of 0.07%. These formed the basis of the ML-500 system under discussion here. For the ML-500, a shorter measurement path was employed. Temperature and absolute pressure transducers were added to allow standardization of the readings.

Although we designed for 0.5% expanded uncertainty (at 2X), the analysis that follows shows typical single-reading results of better than 0.35%, with 0.30% possible if three or more readings are averaged. Maximum standardized single-reading uncertainty was 0.40%.

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