Low Pressure, Vacuum and Leak Measurements
NOTE: 1 Torr = 133.322 Pa
NOTE: Due to the time and effort required preparing vacuum instrumentation for calibration it is particularly important that they be known to be in proper operating condition when they are submitted to NIST. Equipment will be inspected upon receipt and the customer notified of any obvious damage. If the schedule permits, we will cooperate with the customer's efforts to repair or replace damaged equipment so that the calibration of their equipment can proceed. However, concealed damage or operational deficiencies most likely will not be detected before the instrument is operating on the vacuum system or the calibration has started; in such cases, if the equipment cannot be calibrated, we will charge 20% of the regular calibration fee for low-pressure transducers and 30% of the regular fee for spinning rotor and ionization gages.
u VCS (Pa) = (6e-5*P)^0.6 , where P is pressure in Pa, between 0.65 Pa, and 130,000 Pa.
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Low-Pressure Calibrations (30010C-30025C and 30040S)
Low-pressure gages and piston gages are calibrated by direct comparison to NIST UIM (Ultrasonic Interferometer Manometer) primary standards. Calibrations relative to vacuum are performed with either an oil UIM that has a range of 140 Pa (~1 Torr), or one of two mercury UIMs that have ranges of 160 kPa (1200 Torr) and 360 kPa (2700 Torr). Calibrations relative to higher reference pressures (up to 200 kPa) are performed with either the 360 kPa UIM or a low differential-pressure standard based on a 13 kPa (100 Torr) mercury UIM. The expanded (k=2) uncertainties, at pressure P(Pa), due to systematic effects of the UIM standards are:
Pressure instrumentation accepted for calibration generally falls into three categories. The first are absolute pressure transducers or differential pressure transducers that are operated relative to vacuum, such as capacitance diaphragm gages or quartz bourdon gages (Service ID Numbers 30010C and 30011C). The second category includes differential pressure transducers of a similar type intended for use with reference pressures near atmospheric pressure (Service ID Numbers 30020C and 30021C). The third category includes various types of piston gages and other deadweight testers such as ball gages (Service ID Number 30025C). Calibration of transducers relative to vacuum are performed in batches twice a year. Please call for deadline dates for the next calibration batch. Other calibrations are performed on request as NIST schedules and equipment availability permit.
Spinning Rotor Gages (SRGs, also called Molecular Drag Gages) are calibrated on a new Transition Range Standard of the orifice-flow type with an extended range from 10-4 Pa to 30 Pa and a relative expanded uncertainty between 0.3 and 1.0 %. The routine calibrations, 30029C-30031C, cover molecular-flow pressures below 0.1 Pa, where the SRG can, for all practical purposes, be characterized by a constant effective accommodation coefficient without a viscosity correction. Calibrations in this range can be performed using either the customer's controller or a NIST controller. The vacuum flange for the thimble assembly must be bakeable (2.75 in "Conflat" type preferred) unless special arrangements have been made.
These calibrations are performed with nitrogen, in batches, typically two or three times a year. Please call for the next scheduled calibration date, or to arrange for a gas other than nitrogen. Viscosity effects become increasingly important above 0.1 Pa. As a special service (30032S), SRGs can be calibrated up to 30 Pa on the Transition Range standard, and up to 100 Pa using an ultrasonic interferometer manometer. This requires that the ball, thimble, suspension head, and controller be calibrated as unit. Please call for scheduling and costs.
A standard of the orifice-flow type covers the pressure range from 10-1 Pa to 10-7 Pa (10-3 Torr to 10-9 Torr) for inert gases with a relative expanded uncertainty of 0.7 % or less in the range 10-5 Pa to 10-3 Pa, increasing to 2 % at 10-7 Pa. To be acceptable for calibration all gages must be bakeable to 250 °C and should be welded to "Conflat" type flanges. Standard procedure is to calibrate the gage and its control electronics as a package, although gages may be calibrated using NIST electronics by special arrangement. Unless specifically requested by the customer, all hot-cathode gages will be calibrated with 1 mA electron emission current and the preset bias voltages supplied by the customer's controller. Note that ionization gage controllers that do not regulate the emission current or deliberately change it are not considered suitable as transfer standards. After a gage has been calibrated via any of the Service ID Numbers 30034-30036C, calibration of the gage for additional gases or additional filaments (30037C) may be done for a reduced fee. Cold-cathode gages can be calibrate by special arrangement at the same fees. Ionization gage calibrations are generally performed twice a year; please call for scheduled dates. For an additional fee, NIST will provide glass-envelope Bayard-Alpert ionization gages with tungsten filaments and mounted on non-rotatable 2.75 in "Conflat" type flanges* (30038C). These will be calibrated with the user's electronics.
* 2.75 in Conflat flange is an industrial designation for connectors.
Instruments requiring special calibration procedures or prolonged testing can often be accommodated as a special test. This includes, as a complement to the 29000 Service ID Numbers, the determination of deadweight piston gage effective area using the NIST ultrasonic interferometer manometer as the reference standard. This test can be done in either the gage or absolute mode for a variety of gases. Please call for additional information.
Instruments requiring special calibration procedures or prolonged testing can often be accommodated as a special test. Please call for additional information.
Leak artifacts are calibrated in the range 1 x 10-6 mol/s to 1 x 10 -13 mol/s (2 x 10-2 std. cm3/s to 2 x 10-9 std. cm3/s at 0 °C). Flow rates are quoted at standard conditions for leak measurements of P = 101 325 Pa and T = 0 °C. When referenced to the specific temperature, std. cm3/s can be converted to mol/s by multiplying by 4.45 x 10-5. The calibration can be performed directly by the NIST primary leak standard (30061C) which has a range-dependent relative expanded uncertainty between 0.2 % and 4.5 %. For a lower fee, helium leak artifacts in the range 1 x 10-9 mol/s to 1 x 10-13 mol/s may be calibrated (2 x 10-5 std. cm3/s to 2 x 10-9 std. cm3/s at 0 °C) on a comparison system with respect to NIST-calibrated reference leaks (30062C). In both cases, the temperature dependence of the leak is measured and the Report of Calibration will include tabulated leak rates at 1 °C intervals from 0 °C to 50 °C. All leak artifacts submitted for measurement must be ultrahigh vacuum compatible and clean. The vacuum connection must have a standard 2.75 in "Conflat" type flange or 1/4 in VCR type fitting (30061C). An easily observable customer identification number or code must be engraved on the circumference of the vacuum flange or reservoir. By special arrangement (30060S), leaks can be calibrated with gases other than helium, such as argon and common refrigerants. Leaks can also be calibrated as a function of reservoir pressure. Calibrations using the NIST primary leak standard are performed once a year, usually in January. Comparison calibrations are performed throughout the year. Please call for further information.
High precision low-gas-flow instruments are calibrated in the range of (10-8 to 10-3) mol/s with inert gases and other gases by special arrangement. The calibration is performed by direct comparison to a NIST primary flow standard and can be accomplished with down stream pressures ranging from 10 Pa (vacuum) to 300 kPa. The relative expanded uncertainties in the measured flow are range dependent and vary from 0.05 % at a flow of 10-3 mol/s to 0.1 % at a flow of 10-8 mol/s. Gas flows higher than 10-3 mol/s are described under Service ID Numbers 18010C and 18050S. On-site proficiency tests may also be accomplished by special arrangement.
Development of a Low Differential-Pressure Standard, C. R. Tilford and A. P. Miiller, Proc. Natl. Conf. Stand. Lab. Ann. Workshop and Symp. (1997).
Measurement Performance of Capacitance Diaphragm Gages and Alternative Low-Pressure Transducers, A. P. Miiller, Proc. Natl. Conf. Stand. Lab. Ann. Workshop and Symp. (1997).
Pressure and Vacuum Measurements, C. R. Tilford, Chapter 2 in Volume VI of Physical Methods of Chemistry, W. Rossiter, J. F. Hamilton, and R. C. Baetzold, ed., John Wiley & Sons, New York (1992).
The NBS Ultrasonic Interferometer Manometer and Studies of Gas Operated Piston Gages, C. R. Tilford and R. W. Hyland, Metrology, Proc. 11th Triennial World Congress of the International Measurementation Confederation (IMEKO), Houston, TX, 16-21 Oct. 1988, W. C. Rutledge, ed. (Instrum. Soc. of America), Res. Triangle Park, NC (1988) p. 277.
New Developments In Barometric Range Pressure Standards, C. R. Tilford, Proc. 1988 Natl. Conf. Stand. Lab. Workshop and Symp. pp. 35-1 to 35-15 (1988).
The Speed of Sound in a Mercury Ultrasonic Interferometer Manometer, C. R. Tilford, Metrologia 24, 121 (1987).
Zero Stability and Calibration Results for a Group of Capacitance Diaphragm Gages, R. W. Hyland and C. R. Tilford, J. Vac. Sci. Technol. A 3, 1731 (1985)
Ultrasonic Manometers for Low and Medium Vacuum Under Development at NBS, P. L. M. Heydemann, C. R. Tilford, and R. W. Hyland, J. Vac. Sci. Technol. 14, 597 (Jan.-Feb. 1977).
Comparison of the standards for high and ultrahigh vacuum at NIST, NPL, and PTB, K. Jousten, A. R. Filippelli, C. R. Tilford, and F. J. Redgrave, J. Vac. Sci. Technol. A 15, 1 (1997).
Comments on the stability of Bayard-Alpert ionization gages, C. R. Tilford, A. R. Filippelli, and P. J. Abbott, J. Vac. Sci. Technol. A 13, 485 (1995).
Long-term stability of Bayard-Alpert gage performance: Results obtained from repeated calibrations against the NIST primary vacuum standard, A. R. Filippelli, and P. J. Abbott, J. Vac. Sci. Technol. A 13, 2582 (1995).
Influence of the filament potential wave form on the sensitivity of glass-envelope Bayard-Alpert gages, P. J. Abbott and J. P. Looney, J. Vac. Sci. Technol. A 12, 542 (1994).
PC-based spinning rotor gage controller, J. P. Looney, F. G. Long, D. F. Browning and C.R. Tilford, Rev. Sci. Instr.65 (9), 3012 (1994).
Behavior of commercial spinning rotor gages in the transition regime, J. Setina, and J. P. Looney, Vacuum 44, 577 (1993).
NIST Measurement Services: High Vacuum Standard and Its Use, S. Dittmann, Natl. Inst. Stand. Technol. Spec. Publ. 250-34 (1989).
The National Bureau of Standards Primary High-Vacuum Standard, C. R. Tilford, S. Dittmann, and K. E. McCulloh, J. Vac. Sci. Technol. A 6, 2853 (1988).
Low-Range Flowmeters for Use with Vacuum and Leak Standards, K. E. McCulloh, C. R. Tilford, C. D. Ehrlich, and F. G. Long, J. Vac. Sci. Technol. A 5, 376 (1987).
Long-Term Stability of Two Types of Hot Cathode Ionization Gages, S. D. Wood and C. R. Tilford, J. Vac. Sci. Technol. A 3, 542 (1985).
Sensitivity of Hot Cathode Ionization Gages, C. R. Tilford, J. Vac. Sci. Technol. A 3, 546 (1985).
References-Leak and Low-Flow
A critical evaluation of thermal mass flow meters , S. A. Tison, J. Vac. Sci. Technol. A 14, 2582 (1996).
Commercial helium permeation leak standards: Their properties and reliability , P. J. Abbott, and S. A. Tison, J. Vac. Sci. Technol. 14 (May-June 1996).
Using Characterized Variable Reservoir Helium Permeation Leaks to Generate Low Flows , S. A. Tison and P. Mohan, J. Vac. Sci. Technol. A 12, 564 (1994).
Experimental Data and Theoretical Modeling of Gas Flow Through Metal Capillary Leaks , S. A. Tison, Vacuum 44, 1171 (1993).
Transfer Leak Studies and Comparisons of Primary Leak Standards at the National Bureau of Standards and Sandia National Laboratories , R. W. Hyland, C. D. Ehrlich, and C. R. Tilford, J. Vac. Sci. Technol. A4, 334 (1986).
A Note on Flow Rate and Leak Rate Units , C. D. Ehrlich, J. Vac. Sci. Technol. A4, 2384 (1986).Calibrations Phone: 301-975-5454, Fax: 301-975-2950 NIST, 100 Bureau Drive, Stop 2300, Gaithersburg, MD 20899-2300