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NIST Handbook 143 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Gravimetric | | | |
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| ³ 1 L | ³ 2 000 L or ³ 500 gal | ||
| 1 L to 100 mL | 100 L < V < 2 000 L or 25 < V < 500 gal | ||
| 100 mL to 1 ?L | 100 mL to 1 ?L | £ 100 L or £ 25 gal | |
| Volume Transfer | | | |
| ³ 1 L or 1 qt | ³ 2 000 L or ³ 500 gal | ||
| 100 mL < V < 1 L or 1 gill < V < 1 qt | 100 L < V < 2 000 L or 25 < V < 500 gal | ||
| ··· | £ 100 L or £ 25 gal | ||
7.3.1.1 Volumetric units are derived from mass units. Volume calibrations may be determined by either a gravimetric (weighing procedure) or a volume transfer (comparative) method. The two methods require different technical requirements and both are defined here. The measurement of volume by metering methods (and meter calibration) is outside the scope of this document.
7.3.1.2 The gravimetric procedure is based on the conservation of mass principle where a determination of the mass of water contained in or delivered from the vessel that is being calibrated is used to define volume. The mass values are determined in air, are corrected for air buoyancy effects, and are corrected to appropriate reference temperatures. The accuracy and precision will vary depending on balances used, the purity of the water, the ability to make accurate temperature measurements, the nominal value of the volume standard being tested, and the ability to make adequate mass measurements.
7.3.1.3 In the volume transfer procedure, water is delivered from a primary volume standard to the vessel under test. Temperature corrections are made to compensate for the cubical coefficients of thermal expansion of the standard, test vessel, and water to a specified reference temperature. The accuracy and precision will vary considerably depending on the presence of a meniscus, the cleanliness and drain characteristics of the container, the cleanliness and purity of the water, and the ability to make adequate temperature measurements.
7.3.2 References
[1] ANSI/ASTM E 287, Standard Specification for Burets.
[2] ANSI/ASTM E 288, Standard Specification for Volumetric Flasks.
[3] ANSI/ASTM E 438, Standard Specification for Glasses in Laboratory Apparatus.
[4] ANSI/ASTM E 542, Standard Practice for Calibration of Volumetric Ware.
[5] ANSI/ASTM E 694, Standard Specification for Volumetric Ware.
[6] ANSI/ASTM E 969, Standard Specification for Volumetric (Transfer) Pipets.
[7] OIML IR #4, Volumetric Flasks.
[8] OIML IR #40, Graduated Pipettes for Verification Officers.
[9] OIML IR #41, Standard Burettes for Verification Officers.
[10] OIML IR #43, Standard Graduated Glass Flasks for Verification Officers.
[11] NIST Handbook 105-2, 105-3, 105-4 Specifications and Tolerances for Reference Standards and Field Standard Weights and Measures; Specifications and Tolerances for Field Standard Glass Flasks, Open-neck Provers, and LPG Volumetric Provers.
[12] API Manual of Petroleum Measurement Standards, Chapter 4 - Proving Systems (Field Standards, Tank Provers).
7.3.3 Statistical Process Control
7.3.3.1 Appropriate measurement control programs should be in place and available for review for each measurement type (based on procedures) and nominal volume range for which calibration data is provided. Note Table 12 for appropriate measurement control programs for each measurement type. Appropriate data includes standard deviations and range values that represent process variation and well characterized check standard values.
7.3.3.2 Measurement control techniques should exhibit results consistent with the procedures used to perform calibrations and should be integrated into the measurement to accurately reflect the measurement process. For those situations where statistical information is not inherent to the process, i.e., simple measurements without built-in redundancy checks, additional measurements should be made to provide experimental characterization of the measurement sufficient for an adequate estimation of the process uncertainty. Those data should be available for review.
7.3.4 Accommodation and Environment
7.3.4.1 To be deemed capable of making adequate measurements, calibration laboratories should provide a facility with adequate environmental controls appropriate for the level of measurements to be made, according to procedure types as shown in Table 10. Lower relative humidity may increase measurement error due to evaporation.
Table 10. Environmental facility guidelines for volumetric calibration
| | | |
| I Gravimetric | 20 °C to 23 °C, set point ± 2 °C maximum change 1.0°C/h | 30% to 55% |
| II Volume Transfer | 18 °C to 25 °C, maximum change 1.0 °C/h | 20% to 60% |
7.3.4.2 The environment in which testing activities are undertaken should not invalidate the results or adversely affect the required accuracy. Particular care should be taken when such calibration practices are undertaken at sites other than the permanent laboratory facility to minimize the effects of uncontrolled environments.
7.3.4.3 Vibration, air currents, rapid temperature fluctuations, and other environmental concerns should not diminish the accuracy and precision of volume transfer methods or the performance of precision balances or scales when gravimetric methods are used.
7.3.4.4 The quality of water used as a calibration medium should be of adequate purity (potable) and cleanliness, and should be free from excess air entrapment. For gravimetric procedures the density should be calculated/measured to the nearest 0.00001 g/cm3.
7.3.5 Equipment and Reference Materials
7.3.5.1 Mass standards used as reference standards should be traceable to a national laboratory (such as NIST) and be available at each class and range, for which the laboratory is recognized, as recommended in Table 12. Sufficient historical data and uncertainty analysis should be available to support the standards used.
7.3.5.2 Volume standards used as reference standards in the laboratory should be traceable to a national laboratory (such as NIST) and the laboratory should have appropriate procedures in place for verification and recalibration. The accuracy of the primary volume standards of the laboratory should be appropriate for the accuracy class of services provided.
7.3.5.3 Gravimetric methods, which generally use water as calibration media, require the verification of an adequate supply of deionized or distilled water.
7.3.5.4 Gravimetric methods require the use of weighing equipment with adequate accuracy and precision for the uncertainty of the measurement procedure. Appropriate control charts or range charts should be maintained to verify the measurement process.
7.3.5.5 Gravimetric methods require the means to adequately measure barometric air pressure, air temperature, water temperature, and relative humidity of the laboratory environment. Volumetric methods require temperature measurements. Environmental measuring equipment should be available with the accuracy indicated in Table 11. Relative humidity may need to be monitored more closely if evaporation is a concern.
Table 11. Environmental equipment accuracy (expanded uncertainty)
| | Barometric Pressure | water/air | |
| Gravimetric | ± 135 Pa (1.0 mm Hg) | ± 0.1 °C / ± 0.5 °C | ± 10% |
| Volume Transfer | Not essential | ± 0.5 °C | Not essential |
7.3.6 Calibration Methods
7.3.6.1 The algorithm chosen for the measurement, the reference standard to be used, and the equipment to be used for a particular calibration should be correct for that calibration. A documented procedure should be available in the laboratory to determine the correct algorithm. (Examples are provided in NIST Handbook 145 Standard Operating Procedures, SOPs.)
7.3.6.2 Computer programs should have passed software quality analysis. Computer programs should be documented in detail. The documentation should include technical references that provided the basis for the algorithm, any weighing equations, and data set used to test the program for errors.
7.3.7 Handling of Calibration Items
7.3.7.1 The laboratory should have documented procedures to ensure adequate chain-of-custody of calibration items if required by law.
7.3.7.2 Appropriate procedures should be documented to ensure adequate tracking of calibration items that are appropriate for glass or metal volumetric standards.
7.3.8 Certificates and Reports
7.3.8.1 Calibration reports should describe the volume standards mentioned in the report with sufficient detail to avoid any ambiguity. Additional items to be included on a test report are: volume, uncertainty, reference temperature, material, thermal coefficient of expansion (assumed or measured), construction, any identifying markings, and any tolerances if appropriate.
7.3.8.2 Environmental parameters measured during the test should be provided on the test report as appropriate. These include laboratory temperature, volume standard temperature, barometric pressure and relative humidity.
7.3.8.3 Volume standards being tested should meet the appropriate classifications such as NIST, ASTM, API, or OIML, if required by laboratory customers. It is the responsibility of laboratory customers to determine acceptable accuracy levels for their needs.
7.3.8.4 The calibration item (volume standard) should be free of any sign of abuse or damage. Signs of abuse or misuse include dents, chips, improper draining due to lack of cleanliness, and dirty sight gages. Out of tolerance conditions should be reported.
Table 12. Summary of technical criteria for volume calibration
(single page version of table)
| | Measurement Control | Reference Standard | Minimum Traceability | Calibration Methods |
| I Gravimetric | · Process control charts · Check standards · Surveillance of all standards used to provide measurement services · Proficiency testing -On site assessment -Round robin participation | Appropriate mass standards · ASTM Class 2 or 3; or · OIML Class F1 or F2; or · Calibrated Balance | · NIST, or national level calibration periodically, based on independent historical data verification or · Test of weighing equipment using correct methods of calibration and adjustment with traceable mass standards | · Documented comparison calibration procedure or · NIST Handbook 145, double substitution (for example) or · Use of calibrated balance |
| II Volume Transfer | · Process control charts · Check standards · Surveillance of selected standards · Proficiency testing -On site assessment -Round robin participation | Primary volume standards with accuracy and repeatability characteristics acceptable for the type of service provided | · Original NIST calibration and periodic independent verification or · Calibration by accredited or recognized Echelon I laboratory, if uncertainty requirements can be met | Documented volume transfer (or water draw) procedure recognized by NIST, OIML, ASTM, or API |
7.1 Technical Criteria for Dimensional Laboratories
7.2 Technical Criteria for Mass Laboratories
Table 8. Summary of Mass Requirements
7.3 Technical Criteria for Volume Laboratories
Table 12. Summary of Volume Requirements
7.4 Technical Criteria for Hydrometer Laboratories
7.5 Technical Criteria for Thermometer Laboratories
7.6 Technical Criteria for Moisture Laboratories
Table 14. Summary of Environmental Facility Guidelines
Preface, Acknowlegements and History
1. Program Summary
2. Glossary
3. General Information and Operational Requirements
4. Recognition Process
5. General Technical Requirements for Calibration Labs
6. Quality Assurance Requisition for Measuring and Test Equipment
7. Specific Technical Guidelines
8. References
Appendix A
Appendix B
Appendix C Part 1
Appendix C Part 2
Appendix D
Uncertainties Chart
Last Update (12/04/02)
Your comments and suggestions are solicited: Linda.Crown@nist.gov.