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A Method for Verifying Traceability in Effective Area for High Pressure Oil Piston-Cylinders Michael Bair Director of Metrology-Pressure Fluke Calibration

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Traceability in pressure, like temperature, cannot be “built up” by measuring in parallel. Traceability in pressure is dependent upon effective area and the elastic properties of piston-cylinders used with pressure balances (piston gauges). Fluke Calibration’s reference (method) for effective area/ pressure is the Piston- Cylinder Pressure Calibration Chain. With this Fluke Calibration’s uncertainty in pressure is ± 0.0028% at 200 MPa (30,000 psi) which is very low when reviewing NMI’s CMC uncertainties for that range. Because there is extrapolation of the effective area and elastic deformation coefficient verification is required at high pressure. Calibration chain was re-characterized in 2010. This paper shows 3 methods of verification, one being an attempt to use the Dadson single piston method. 2011 NCSL International Workshop & Symposium2 Introduction 24 Aug 2011

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One Minute Piston Gauge Primer Mass x Gravity Pressure = (Mass x Gravity)/ Effective Area EQUILIBIUM! Masses are rotated 3 Effective Area Pressure x Area Effective Area Changes with Pressure

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Primary standard for effective area from 5 kPa to 500 MPa (<1 psi to 72500 psi). Re-characterized in 2009/2010. 4 th modern re-characterization. Original traceability is defined using dimensional measurements and the Dadson method on a 50 mm diameter piston-cylinder. Traceability is transferred to higher pressures/ smaller effective areas using the “Base Ratio” crossfloat. Each level has some portion of the range that extrapolates elastic deformation to support the next higher range. Uncertainties increase as pressure gets higher. 2011 NCSL International Workshop & Symposium4 Calibration Chain 24 Aug 2011

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2011 NCSL International Workshop & Symposium5 Calibration Chain 24 Aug 2011

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Comparison with Houston facility primary to 280 MPa (40,000 psi) Comparison with SN 27D, an old/dormant 200 MPa range piston- cylinder with original traceability with NIST and LNE (France). A alternate method based on a basic principal discussed by Dadson called the single piston method. 2011 NCSL International Workshop & Symposium6 Calibration Chain Verification 24 Aug 2011

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Comparison with Houston facility primary to 280 MPa (40,000 psi) 2011 NCSL International Workshop & Symposium7 Calibration Chain Verification 24 Aug 2011

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Comparison with SN 27D, an old/dormant 200 MPa range piston- cylinder with original traceability to NIST and LNE (France). 2011 NCSL International Workshop & Symposium8 Calibration Chain Verification 24 Aug 2011

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A alternate method based on a basic principal discussed by Dadson called the single piston method. –Not used this time as a complete characterization of the pressure balance but just as a verification tool for the results of the CalChain. –Attractive because of the equation used for an incompressible fluid to determine average gap and the fact a known viscosity characterization existed for the test fluid (Vergne). 2011 NCSL International Workshop & Symposium9 Calibration Chain Verification 24 Aug 2011

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Determine the effective area of SN 1488 controlled clearance piston gauge using the calibration chain and base ratio method. Dimensionally characterize SN 1488 2.5 mm piston at NIST. Perform drop rate tests to determine the average gaps at various pressures. Using the zero pressure gap and dimensioned piston, calculate the effective area at zero pressure and 20˚C and compare to what was determined from the crossfloats. 2011 NCSL International Workshop & Symposium10 Single Piston Method Procedure 24 Aug 2011

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2011 NCSL International Workshop & Symposium11 Single Piston Method 24 Aug 2011

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Two pistons were sent to NIST for dimensioning, one was for SN 1488, the other a slightly smaller piston for future use. Two different orthogonal planes ±16.5 mm to cover float range 24 total diameter measurements 2011 NCSL International Workshop & Symposium12 Single Piston Method 24 Aug 2011

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2011 NCSL International Workshop & Symposium13 Single Piston Method Average diameter = 2.4980883 mm Diff of 90 ˚ [nm] -5 -8 5 27 5 15 35 15 16 37 5 40 24 Aug 2011

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Single Piston Method - Gap Determination Performed gap determinations for three different controlled clearance pressures, 0, 25 and 50% of measured pressure. For each CCP performed drop rates for at least 5 pressures. Tried to get 5 drop rates for each CCP/measured pressure combination. Performed 92 drop rate tests. Calculated gap for each drop rate test. Plotted gap to get zero pressure gap to be used with the diameter. 2011 NCSLI Workshop & Symposium1424 Aug 2011

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Single Piston Method - Gap Determination Where –h = average gap between piston and cylinder. –L = engagement length of piston cylinder –R = radius of the piston –V fl = volume flow calculated by piston drop rate –P gauge = gauge pressure of the fluid 2011 NCSLI Workshop & Symposium1524 Aug 2011

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Single Piston Method - Gap Determination 2011 NCSLI Workshop & Symposium1624 Aug 2011

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Single Piston Method - Gap Determination Keys to performing the drop rate tests. –Started with high end industrial drop indicator. Kept getting stuck. Contact reduced rotation times. Found out that the internal non-contact position sensor performed very well if calibrated every day – 2 minute procedure. –It was essential to perform the tests from approximately +1.5 to -1.5 mm to match the dimensional tests on the piston. –Could not have ANY air in the system, went through extensive purge procedure. –Temperature had to be very stable. Used piston-cylinder temperature for the temperature of the media. –Isolated pressure directly outside the piston gauge to reduce environmental temperature influences on the fluid. (changes were usually less than 0.02 deg for each test) –Leveled as best as possible for each test. 2011 NCSLI Workshop & Symposium1724 Aug 2011

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Single Piston Method - Gap Determination 2011 NCSLI Workshop & Symposium18 Using the average piston radius and the average of the gaps at zero pressure, the result effective area at 20˚C and zero pressure is 4.9033956 mm 2. +1.4 ppm from CalChain Determination 24 Aug 2011

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Single Piston Method - Gap Determination Result was actually much better than the uncertainty analysis. 2011 NCSLI Workshop & Symposium19 k=1SensitivityUnc (h) k=1 [% relative][% of 0.550 µm][µm] V fl 0.234%0.330.00043 R0.034%0.330.00006 L0.050%0.330.00009 P gauge 0.250%0.330.00046 η3.000%0.330.00549 h Combined0.0055 k=1SensitivityU radius k=1 [µm][µm/µm][µm] Piston Diameter0.02150.50.0108 h (from above)0.00550.50.0028 h std error of fit0.00760.50.0038 Combined0.0117 Expanded (k=2)0.0235 Radius0.0019% effective area0.0038% 24 Aug 2011

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Conclusion Results are good due to the keys described earlier. All methods of validation show that the CalChain to 200 MPa is within stated uncertainties. Would like to move forward with FEM analysis, Heydemann and Welch model and also verify using a smaller piston in the same cylinder. The 100 – 500 MPa part of the CalChain will be re-characterized at the end of 2012. Will attempt to repeat this process. Thanks to Ken Kolb and NIST. 2011 NCSLI Workshop & Symposium2024 Aug 2011

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