Process Measurements Division Calibration of Hydraulic Piston Gauges Operating in Differing Fluids Dana R. Defibaugh and Douglas A. Olson National Institute of Standards and Technology with Greg Driver and Jeff Kelley NCSL International Workshop and Symposium August 1, 2007

Process Measurements Division Background Oil piston gauge (PG) pressure standards are designed for specific fluids NIST PG primary standards operate in Spinesstic oil, which is no longer made. NIST PG working stds use Spinesstic Most NMIs and newly made PGs use Di(2-ethylhexyl) sebacate (DOS) Due to longevity, older PGs will be used for many years Oil properties affect operation. Effective area?? Fluid Density (kg/m 3 ) Viscosity (Pa.s) Maximum P (MPa) Spinesstic DOS Oil Properties at 23 C and 1 atm

Process Measurements Division Background and Objective NIST customers will desire PG calibrations in Spinesstic and other oils Key Comparison CCM.P-K7 (2002 to 2005) specified DOS in transfer standard –Driver and Olson used DP cell to separate fluids, calibrated zero drift vs. pressure Need method to measure effect of oil on PG effective area –For NIST PGs –For customer PGs Objective: develop method for calibrating piston gauges with the reference gauge and test gauge using different hydraulic oils

Process Measurements Division Traditional Piston Gauge Crossfloat Calibration with Different Fluids Calibration: equilize P r and P t by adjusting masses Fall rate method: open valves, oils mix. Density and head correction? Does oil R get into PG T? DP Cell: isolates oils. Is zero offset correct? Is there density effect? Requires skilled operator Variable Volume Valve Isolation Valve Variable Volume Valve Pressure Balance REF Pressure Balance TEST Oil T Oil R DP Cell PRPR PTPT

Process Measurements Division Transducer Assisted Crossfloat (TAC) for Two Fluids Pressure balances brought into approximate equilibrium CVV REF and TEST never open simultaneously Balances connected sequentially to pressure transducer; transducer measures difference in pressure Variable Volume Valve Constant Volume Valve REF Variable Volume Valve Pressure Balance REF Pressure Balance TEST Constant Volume Valve TEST Pressure Transducer Fluid mixing in common line Oil R Oil T PRPR PTPT

Process Measurements Division TAC for Single Fluids Hydraulic single fluid use up to 280 MPa –Kobata and Olson, Metrologia 42 (2005) S231-S234 –Olson and Kobata, Proc. of NCSL (2002) –Used by NMIJ in CCM.P-K7 Pressure equivalence not required –|P T -P R |/P < 100 ppm: mass trimming eliminated Measurement cycle and data taking is automated Air-operated constant volume valves with solid state relays Pressure transducer w/electronic signal –Need resolution and short-term uncert. of 1 in 10 6 –Vibrating quartz crystal type, integral counter Electronic piston-height monitors Variable volume valves to adjust piston height Head correction via  g  h

Process Measurements Division Measurement Protocol: 6 REF, 5 TEST, 5  P Time (s): Start Meas. REF 6X Switch CVVs, adj. REF, wait Meas. TEST 6X Switch CVVs, adj. TEST, wait Repeat End 15.5 min

Process Measurements Division

TAC for Single Fluid 2 to 3 ppm agreement between TAC and other methods Comparison of A from TAC to A from DP Cell and FR

Process Measurements Division TAC for Two Fluids 18 hrs in container. Diffusion very slow Sebacate (DOS) Spinesstic Keep oils in proper piston gauge Account for head correction: know oil density in vertical lines Prevent mixing; limit to horizontal lines

Process Measurements Division

Common Line Design for 2 Fluids 1/8 in tubing, 30 cm long, 2 cm D coils Drain line Adjustable stands for CVVs Small diameter tubing to minimize diffusion, mixing, and restrict flow Set common line (mixing zone) at same elevation as REF PG Set same reference levels of PGs REF and TEST

Process Measurements Division TAC operation for 2 fluids Flush both lines to common drain prior to calibration Always keep 1 CVV closed Keep P T - P R small to avoid “compressibility pumping” –Use TAC to compute makeup mass, adjust until  P TAC = P T - P R < 200 Pa PG Ref, P REF hAhA hBhB hChC hDhD PG Test, P TEST PRPR PTPT

Process Measurements Division TAC Measurement Eqn + Head Correction, Test or Ref Pressure equation valid if no mixing in vertical lines We will evaluate method by looking at A ref /A test for different fluids h1h1 h2h2 P REF P TEST

Process Measurements Division Method Evaluation Ref PG: GEIS , 140 MPa, 7.11 mm 2 “J364” Test PG: DHI 7302, 200 MPa, 4.90 mm 2 “DH479” Both designed for DOS h 1 = mm, h 2 = 0.7 mm Measure A ref /A test at 10 pressures, 10 MPa to 125 MPa Transducer is 140 MPa full scale When changing oil: drain and flush line, base, and pump; remove & clean piston CrossfloatJ364 OilDH479 Oil 1DOS 2 Spinesstic 3 4 DOS 5

Process Measurements Division Area Ratio, A J364 /A DH479 Agreement within 5 ppm No “tare” effects: fluids isolated and  gh corrected properly

Process Measurements Division Fall Rate as Diagnostic of Oil Isolation Oil J364DH479 FR hh hh Spinesstic DOS Fall Rates (FR, mm/min)) at 125 MPa  h (mm) in 3 min at 125 MPa

Process Measurements Division Change in A J364 /A DH479 from Test 1 (Both DOS) Ratio of area ratios: no type B unc. of primary standard or masses Allows investigating parameter on single PG Effects of oils on these PGs same order as reproduceability both PGs: Spinesstic vs DOS J364: Spinesstic vs DOS DH479: Spinesstic vs DOS Reproduceability of method and both PGs (Both DOS)

Process Measurements Division Type A Uncertainty from TAC  P TAC uncertainty due to repeatability of  P, stability of each PG Relative uncertainty less than 1 ppm above 30 MPa

Process Measurements Division Change in Area Ratio compared to TAC unc Unc shown does not include reproduceability of PGs or of method At most a 1 ppm fluid effect in each PG

Process Measurements Division Oil Effect on PG41 Test PG: 104 MPa FS, 16.8 mm 2, re-entrant, NIST working standard “PG41” –Test with Spinesstic and DOS –Used with Spinesstic in NIST cal service –Fall rate varies little with pressure –High FR in DOS is problematic for TAC Ref PG: J364. DOS Oil J364 at 125 MPaPG41 at 104 MPa FR (mm/min)  h in 3 min (mm) FR (mm/min)  h in 3 min (mm) Spinesstic DOS ~2~6 Meas. REF 6X Switch CVVs, adj. REF, wait Meas. TEST 6X Switch CVVs, adj. TEST, wait Meas. REF 6X Time (s): Start End 15.5 min

Process Measurements Division PG41, Spinesstic vs DOS PG41: DOS increases average Ae by 3.8 ppm;  = 4.6 ppm Area Ratio, A PG41 /A J364 PG41 Area Ratio, A DOS /A Spin

Process Measurements Division TAC Uncertainties, PG41 v J364 Relative std. unc. less than 1.3 ppm above 20 MPa Increase in abs. unc. above 100 MPa due to PG41 instability

Process Measurements Division Conclusions Hydraulic piston gauges can be calibrated in differing fluids using TAC method –Minimize differences in reference levels –1/8 in diameter tubing in common line to reduce mixing –Operate TAC at  P  200 Pa Two recently manufactured PGs show at most 1 ppm difference between DOS and Spinesstic NIST working standard PG41 operates in DOS, but FR high. Average A increases by 3.8 ppm vs Spinesstic Method can be used to quantify fluid effect on PGs, or error in calibrating with substitute fluid