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Michael Bair Director of Pressure Metrology Fluke Calibration

An Examination of the Uncertainty in Pressure of Industrial Dead-Weight Testers Used For Pressure Calibrations in Different Environments. Michael Bair Director of Pressure Metrology Fluke Calibration. Introduction - Learning Objectives.

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Michael Bair Director of Pressure Metrology Fluke Calibration

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  1. An Examination of the Uncertainty in Pressure of Industrial Dead-Weight Testers Used For Pressure Calibrations in Different Environments Michael Bair Director of Pressure Metrology Fluke Calibration

  2. Introduction - Learning Objectives • What is an Industrial Dead Weight Tester (IDWT) and why is it being treated differently from a piston gauge? • What is the design of an IDWT? Need to know this for method and uncertainty. • What are the three methods of use and environmental limits? • What are the uncertainties? Note the uncertainties are not FC product uncertainties but something close to be able to express the concepts. 2012 NCSLI Workshop & Symposium July 30, 2012

  3. What is an IDWT (DWT)? • What’s in a name? A DWT works the same by any other name. • A DWT works under the same exact theory as what one might call a Dead Weight Pressure Gauge, Piston Gauge or a Pressure Balance. • Those devices are defined in existing technical references including • NCSLI RISP4 • OIML’s R110 Pressure Balances • EA 10/03 Calibration of Pressure Balances • The Pressure Balance, Theory and Practice by NPL 2012 NCSLI Workshop & Symposium July 30, 2012

  4. What is a DWT? • The difference is that a DWT is designed in such a way that it can be used with reasonable uncertainty with out use of the pressure equation described in the documents just mentioned. • The reason for this design is to simplify its operation for industrial applications, primarily industrial calibration of pressure gauges. • Because of a recently acquired responsibility of a DWT line, we decided to quantify product uncertainties for three different methods of use in an industrial environmental limit. The methods we decided to call… • Full correction • Partial correction • No correction 2012 NCSLI Workshop & Symposium July 30, 2012

  5. DWT Design Mass x Gravity Masses are rotated EQUILIBRIUM! Pressure x Area PRESSURE Pressure = (Mass x Gravity)/ Effective Area 2012 NCSLI Workshop & Symposium July 30, 2012

  6. DWT Design • The full correction method is what is referenced in the technical documents mentioned. • For partial and no correction methods, it is easier to understand if you understand the design. 2012 NCSLI Workshop & Symposium July 30, 2012

  7. DWT Design To create a DWT, we manufacture masses (weights) that will account for as many variables in the equation as possible. We start by removing the constants; surface tension and head correction; and assume no correction for piston-cylinder temperature, calculate a mid pressure effective area; and use what is left over. 2012 NCSLI Workshop & Symposium July 30, 2012

  8. DWT Design Then we plug in the variables to determine what pressure we will get for 1 kg. 2012 NCSLI Workshop & Symposium July 30, 2012

  9. DWT Design It then has to be converted to the requested pressure unit. In this example we will use psi. We then divide the Kl into the nominal weights we want. 2012 NCSLI Workshop & Symposium July 30, 2012

  10. DWT Design For the first pressure the same calculation is made for the carrier, then the piston mass is subtracted and corrections for surface tension, fluid buoyancy and head correction are applied by adjusting the mass. The head correction is applied to a convenient location such as the test port on the DWT. Reference level at Test port 2012 NCSLI Workshop & Symposium July 30, 2012

  11. DWT Design For DWTs that go to high pressure (20000 to 60000 psi), where deformation can be as high as 0.05%, the main masses are manufactured to be used in sequence to greatly reduce the uncertainty from the deformation of the piston-cylinder. 2012 NCSLI Workshop & Symposium July 30, 2012

  12. DWT Design And finally there are many DWTs where the same mass set is used for a high and a low range piston-cylinder. The one mass set must be made to work with both. This is called a ‘match’ and adds uncertainty. High range Low range 2012 NCSLI Workshop & Symposium July 30, 2012

  13. DWT Design ‘No correction’ and ‘partial correction’ are similar in the sense that they both depend on the nominal pressure values. ‘No correction’ is just as it sounds, there is complete dependency on the nominal pressures. ‘Partial correction’ depends on the nominal pressure values but includes a simple correction for gravity and piston-cylinder temperature. 2012 NCSLI Workshop & Symposium July 30, 2012

  14. DWT Design gl is where the DWT is going to be used. gc is the gravity the DWT was made for. Thermal expansion of the piston-cylinder effective area times the difference between the reference temperature and the presumed piston-cylinder temperature. • The calculation for ‘partial correction’ is as follows… and

  15. Uncertainties • The uncertainties listed in the paper are minimized for simplicity and include those that are significant. They are… • Gravity • Mass • Air buoyancy • Effective Area • P-C temperature • Level • Performance • Deviations (uncorrected bias) • Can’t go into detail in this presentation, but will hit the highlights. 2012 NCSLI Workshop & Symposium July 30, 2012

  16. Uncertainties • Gravity can vary as much as 0.4% in the normal industrial world. Gravity needs to be determined for all of these types of devices. • The difference is how we get it and the uncertainty. • For DWT it was decided to use an uncertainty of ±20 ppm primarily because of PTB’s gravity prediction web site and the fact it was international. • Other sources of gravity include National Geodetic Survey and the WGS84 gravity calculation. • A study was performed to look at uncertainties contributed by gravity. 2012 NCSLI Workshop & Symposium July 30, 2012

  17. Uncertainties 2012 NCSLI Workshop & Symposium July 30, 2012

  18. Uncertainties • There were two uncertainties for mass, one for the determination, and one for manufacturing, for no or partial correction methods. • Air buoyancy was only significant for high altitudes and for no or partial correction. 2012 NCSLI Workshop & Symposium July 30, 2012

  19. Uncertainties • Piston-cylinder temperature is significant only due to the assumption there was not a device to measure the piston-cylinder temperature and either ambient temperature was used, or there was no correction. • Environmental limits chosen for temperature were 18 to 28 ˚C (64 to 82 ˚F). Because in no correction there is not a temperature measurement the uncertainty was very significant. • There were three temperature tests performed to help with evaluating an estimation of using ambient air for the piston-cylinder temperature measurement. • Heating or cooling due to pressurizing or depressurizing • Fluctuations in an air conditioner • No air conditioning 2012 NCSLI Workshop & Symposium July 30, 2012

  20. Uncertainties Piston Head Upper MP Lower MP Low Range MP Extra MP 2012 NCSLI Workshop & Symposium July 30, 2012

  21. Uncertainties 2012 NCSLI Workshop & Symposium July 30, 2012

  22. Uncertainties 2012 NCSLI Workshop & Symposium July 30, 2012

  23. Uncertainties • With these tests we felt comfortable using the following for the uncertainties of the change in effective area due to piston-cylinder temperature. 2012 NCSLI Workshop & Symposium July 30, 2012

  24. Uncertainties • There are three uncertainties that ended up being evaluated as one. • These are called deviations and only apply to no or partial correction methods. • Mass Manufacturing. • Piston-cylinder deformation. • Piston-cylinder matches. • To determine this uncertainty the nominal pressures are compared to the calculated pressures, as in the difference between no and full correction methods. 2012 NCSLI Workshop & Symposium July 30, 2012

  25. Uncertainties 2012 NCSLI Workshop & Symposium July 30, 2012

  26. Conclusion The final uncertainty budget ended up looking something like this… 2012 NCSLI Workshop & Symposium July 30, 2012

  27. Conclusion Using a DWT in no or partial correction mode means that the entire DWT should be calibrated as a whole. Adjustments can be made to masses to account for changes in effective area. DWTs are very useful in an industrial environment. Ease of use is important in this environment. Using the no correction method you only need to know what the environmental temperature limits are and to be able to add nominal values. They are very stable and naturally control pressure to within their performance limits. This paper shows that uncertainties of the partial and no correction, in which DWTs are designed for, are sufficient for the applications with which they were intended to be used. Thank you! 2012 NCSLI Workshop & Symposium July 30, 2012

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