Spinning Rotor Gauge Calibrations

1 Page 1 NIST Special Publication 250-93 NIST Calibration Services for Spinning Rotor Gauge Calibrations Robert F. Berg James A. Fedchak This publication is available free of charge from: http://dx.doi.org/10.6028/NIST.SP.250-93 Open at page → 2 Page 2 NIST Special Publication 250-93 NIST Calibration Services for Spinning Rotor Gauge Calibrations Robert F. Berg James A. Fedchak Sensor Science Division Physical Measurement Laboratory This publication is available free o... Open at page → 3 Page 3 Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or... Open at page → 4 Page 4 iii NIST Special Publication 250-93 2014 NIST Calibration Services For Spinning Rotor Gauge Calibrations Robert F. Berg and James A. Fedchak Contents List of tables .......................................................... Open at page → 5 Page 5 iv List of tables 1. Changes in accommodation coefficient for NIST check standards and customer rotors ....8 2. Flanges located on the vacuum chamber ......................................................................... Open at page → 6 Page 6 1 Introduction “Vacuum” covers an enormous range of pressures below one atmosphere (10 5 Pa). Uses of vacuum vary from building infiltration tests (only 50 Pa below one atmosphere) to high energy particle accelerators (1... Open at page → 7 Page 7 2 uncertainty budget. This document describes only the medium-range standard that is used for all of the SRG Standard Calibrations and the majority of the SRG Special Calibrations. Two books with useful general informati... Open at page → 8 Page 8 3 Figure 1 . The analogy between the voltage V = IR generated by an electrical current I flowing through a resistance R, and the pressure p = qpV/C generated by a gas flow qpV through an orifice of conductance C. Another... Open at page → 9 Page 9 4 U P L p R p  . (3) Here we use pU instead of pstd since pU represents the measured upstream pressure and not the calculated standard pressure. Since the SRG decrement is linear for p < 0.1 Pa, RP does not depend on th... Open at page → 10 Page 10 5 moment were aligned with the rotation axis.) However, if the principle moment of inertia is too large, the residual drag will have a large frequency dependence. This can be taken into account mathematically, but it may... Open at page → 11 Page 11 6   -1 -1 -1 -3 universal gas constant (8.314772 J mol K) mean thermal velocity of the gas (m/s) gas temperature (K) gas molar mass (kg mol ) rotor density (kg m ) rotor diameter (m) / / no R v T M d DCR df dt f   ... Open at page → 12 Page 12 7 Above 0.1 Pa, the decrement is no longer a linear function of pressure and Eq. (5 ) is no longer valid. This is not an issue for Standard Calibrations (C-test) since σ is determined at a single pressure < 0.1 Pa, nomin... Open at page → 13 Page 13 8 Figure 3 . Comparison of SRG pressure readings to a calibrated CDG. The SRG pressure readings agree well with the CDG to about 0.8 Pa. At 1Pa, the CDG and SRG disagree by about 0.4 %. At 10 Pa (data not shown), the dis... Open at page → 14 Page 14 9 Figure 4 . Histogram showing percent change in the accommodation coefficient between SRG calibrations. With careful use, the accommodation coefficient of most SRGs will change no more than 2 % over a 2-year period. Thi... Open at page → 15 Page 15 10 Figure 6 . History of the accommodation coefficient  for two SRG rotors. The vertical scale is 3 times larger than for Figure 5 . Chang and Abbott [22 ] studied possible causes for the irreproducibility of SRGs, incl... Open at page → 16 Page 16 11 Figure 7 . Accommodation coefficient as a function of head tilt Figure 8 . The residual drag (RD) as a function of the head tilt angle. Apparatus This section discusses the three main parts of the apparatus: the vacuu... Open at page → 17 Page 17 12 An aluminum frame supports the chamber and a surrounding table, which is made of ceramic fiber board covered by sheet aluminum. A large oven may be placed on the table that encloses the chamber for baking. Baking is n... Open at page → 18 Page 18 13 Figure 10 . View of the upper half of the vacuum chamber. The 11 mm orifice plate is in place. Table 2. The number and function of the (2.75 inch CF or DN40) flanges located on the top of the vacuum chamber and on the... Open at page → 19 Page 19 14 At the bottom of the upper chamber, the orifice is mounted in the horizontal plate that separates the two halves of the chamber. The mounting scheme allows the orifice to be changed without opening the vacuum chamber.... Open at page → 20 Page 20 15 1.5 The flow meter 1.5.1 Principle of operation Two constant-pressure flow meters are used to generate the gas flows needed to calibrate vacuum gauges and vacuum leak artifacts. Both the “piston flow meter” (PFM) and... Open at page → 21 Page 21 16 nRT t p V   . (10) Here t = 1000 s is the typical duration of a run, and the volume displaced by the piston is V = 12.9 cm 3 for the “inch” piston and 2.0 cm 3 for the “cm” piston. In other words, the ratio betwee... Open at page → 22 Page 22 17 pressure 0.017 0.066 0.101 0.323 outgassing uoutgas 0.001 0.001 0.000 0.155 leak past sliding seal uleak 0.010 0.010 -- 0.000 gas purity upurity 0.010 0.010 0.010 0.010 miscellaneous umisc 0.014 0.014 0.010 0.010 tota... Open at page → 23 Page 23 18 Clausing factor orifice area average molecular speed orifice diameter gas constant temperature molecular weight K A v d R T M        The Clausing factor, K < 1, is the transmission probability for a molecule to... Open at page → 24 Page 24 19 sphere potential and pressures in the range 1 2Kn   (corresponding to 2.7 PaP nitrogen through the 11 mm orifice) can be summarized as       1/2 1 0 11 1 22 U LL U LP CP A AA dM A C P Kn P Kn P P R RT   ... Open at page → 25 Page 25 20 Operation of the standard The measurement equations for determining the effective accommodation coefficient can be derived by dropping the last term in Eq. (5 ) and combining it with Eq. (6):   1/2 0 12 10 std RT d... Open at page → 26 Page 26 21 5. The rotor must be mounted and suspended at least one day before any reliable data can be recorded. This ensures that the rotor is in temperature equilibrium with the chamber. 6. Record residual drag data for at lea... Open at page → 27 Page 27 22 obtained with the medium range pressure standard. Measurements made using the two orifices agree to within the measurement uncertainty. The values acquired with the 11 mm orifice differ by only 0.07 %, which is consis... Open at page → 28 Page 28 23 Figure 14 . The accommodation coefficient σ as a function of pressure for 9 rotors. The black lines represent linear fits to the data for p > 0.1 Pa. The slopes for all of the rotors except T792 were approximately 0.0... Open at page → 29 Page 29 24   1 2 11 2 * 1 1 exp AKn AA Kn               , (19) Here A1 and A2 are empirical constants, and the Knudsen number Kn is defined by 1/2 2 8 RT Kn pM        , (20) Note t... Open at page → 30 Page 30 25 0 1 fixed constant with no uncertainty molar flow rate through chamber orifice conductance for molecular flow pressure coefficient for orifice conductance chamber pressure ratio SRG de P K n C R DCR         0... Open at page → 31 Page 31 26 P R u : From Fedchak [9], the uncertainty in the pressure ratio is 0.37 % P R u and Rp ~ 26.2, therefore / 0.014% P RP uR  . 0 DCR u : Since the residual drag can strongly vary, it is better to estimate the absolute... Open at page → 32 Page 32 27 T u : The temperature uncertainty comprises two components: that due to the temperature gradient across the chamber, and that due to the calibration uncertainty. The standard uncertainty associated with the calibratio... Open at page → 33 Page 33 28 Quality control NIST uses the following methods to maintain the quality of the SRG calibration service.  Adhering to the NIST quality system, which is described briefly in the next section.  Participating in interna... Open at page → 34 Page 34 29 2. S. Dittmann, High vacuum standard and its use, NIST Special Publication SP-250-34, (1989). 3. S. Dittmann, K.E. McCulloh, C.R. Tilford, “Vacuum calibrations using the molecular drag gage”, textbook for course taugh... Open at page → 35 Page 35 30 26. A.W. Hartman, “Report of measurement: Measuring specified dimensions of two supplied rings”, NIST memorandum to J.P. Looney (1990). 27. R.F. Chang, “Experimental determination of conductance for mid-range 2 mm ori... Open at page → 36 Page 36 31 11. Appendix: Example calibration report Open at page → 37 Page 37 32 Open at page → 38 Page 38 33 Open at page → 39 Page 39 34 Open at page → 40 Page 40 35 Open at page → 41 Page 41 36 Open at page → 42 Page 42 37 Open at page → 43 Page 43 38 Open at page →