Electromagnetic MeasurementsVoltage MeasurementsDC Voltage Measurements and StandardsTechnical Contacts: June E. Sims Yi-hua Tang Denise D. Prather Please contact the administration and logistics staff before shipping instruments or standards to the address listed below. Mailing Address:
back to top of page | back to index of electromagnetic measurements General Information-DC Voltage Measurement StandardsThe service described in this section provides for the calibration of standards of direct voltage, saturated and unsaturated standard cells, and solid-state standards. The U.S. Representation of the Volt is maintained by monitoring the emfs of several groups of saturated standard cells in ovens on a monthly basis using the ac Josephson effect. Customer cells are calibrated by measuring the difference between their emfs and those of working groups of standard cells using automated systems comprised of low thermal emf, computer-controlled switches and high-resolution digital voltmeters. back to top of page | back to index of electromagnetic measurements Special DC Voltage Measurements, by Prearrangement (53110S)The evaluation, testing, or calibration of prototype dc voltage standards and measuring apparatus or unique voltage measurements are provided by this service. These measurements are performed only when deemed reasonable by the appropriate technical staff and serving the best long-term interests of the client, the measurement community, and NIST. Table 9.5. Expanded Uncertainties of NIST DC Voltage Measurement
back to top of page | back to index of electromagnetic measurements Saturated Standard Cells (53130C-53140C)Routine calibrations of saturated standard cells involve the following considerations: A. Saturated standard cells of the unshippable type should always be transported by messenger because such cells should never be tipped from an upright position by more than 45 ° in any direction. Unshippable saturated cells contained in portable, temperature-regulated enclosures should also be transported by messenger and with the enclosure activated or under power if possible. B. Saturated standard cells of the shippable type housed in portable thermo-regulated enclosures should be packed carefully and shipped under power if possible. Any liquid-in-glass thermometer mounted in such a device should be removed and provided with additional rigid packing for protection against breakage. Enclosures having a nominal cell temperature of 28 °C or lower should not be transported during the summer due to the danger of overheating. Enclosures should not be energized by using the ac power mains while they are in shipping containers as heat from the transformer will cause them to go over-temperature. C. Saturated standard cells, which are maintained continuously at their nominal temperature of use during shipment, will undergo test starting 1 week after receipt for a period not to exceed 6 weeks, unless other arrangements are made. If such cells perform abnormally, the owner will be notified. Arrangements for further testing may be made at that time if desired. Cells will be returned as soon as possible after calibration. D. Saturated cells arriving at a temperature other than their nominal temperature of use will be brought to their use temperature as soon as possible after receipt. Starting 1 month after they initially attain use temperature, daily readings will be taken to observe the stability of the cells. When the cells stabilize, 10 daily readings will be taken and averaged to assign values to them. This process will not exceed 90 days without special arrangements being made. E. For an additional fee, the temperature of air bath enclosures for saturated standard cells will be determined using a calibrated NIST platinum resistance thermometer (Service ID Number 53140C ). Daily readings are taken and reported. The reported cell emfs are assumed to correspond to the mean of the temperatures measured on the same days as the emf readings were taken. The client must understand that, when this is done, the uncertainties of the reported emfs include the emf equivalent of the uncertainty of the measured temperatures in terms of the International Temperature Scale of 1990 (ITS-90). Moreover, estimates of the uncertainties of any voltage measurements made by clients using these cells as a reference must include corresponding uncertainties of their own temperature measurements. F. NIST accepts cells used in oil baths for calibration in NIST oil baths maintained at 28 °C. Calibration relative expanded uncertainties generally range from 0.15 x 10-6 to 0.50 x 10-6. The stated uncertainties are those of the NIST-measured average values, i.e., they do not reflect long-term behavior of the cells, transportation effects, etc. back to top of page | back to index of electromagnetic measurements Unsaturated Standard Cells (53150C)Unsaturated cells require approximately 3 weeks for a complete calibration. The emfs of such cells are read daily for a minimum period of 10 days. These cells are compared with NIST saturated cells using a precision digital voltmeter to measure the difference emf directly. The calibration relative expanded uncertainty is 0.005% of the measured voltage unless the cell is abnormal. If the measured emf fluctuates unduly or is unusually low, or if the cell behaves abnormally, the reported uncertainty will be increased appropriately. Unsaturated cells are not likely to be injured by normal transportation (mail or express) if they are packed carefully. Because of the possible hazard from freezing, shipment during extremely cold weather should be avoided. back to top of page | back to index of electromagnetic measurements Solid-State Voltage Reference Standards (53160C and 53161C)NOTICE: Beginning in January 2010, NIST will accept zener reference standards for calibrations at regular prices during two periods of each year. The two calibration periods are January-February and August-September. Zeners must arrive at NIST prior to January 15 to be included in the winter calibration round, and prior to August 15 for the summer round. Calibrations will be complete by February 15, and by September 15. Solid-state voltage standards with outputs in the range from 1 V to 10 V are calibrated using a self-calibrating automated system which scales to any multiple up to 10 V of 1.018 V from the emf of a working group of NIST saturated standard cells. It then measures the difference between the emf of the standard under test and the emf of its own output closest in voltage to that of the standard being measured and computes its emf. Measurements are taken daily for 12 to 15 working days and the mean value of the results reported. It has been reported that solid-state voltage standards are subject to changes in their output values that are due to environmental temperature, barometric pressure, and relative humidity changes. The reported results are referenced to environmental conditions at NIST when the measurements are taken. Adjustments may be necessary for customer based on known pressure, temperature coefficients of the solid-state voltage standard measured separately. Due to the limited battery life of many commercial standards, special shipping arrangements are advisable and can be made by contacting the Quantum Electrical Metrology Division. Many solid-state standards have multiple outputs; to ensure proper testing, the outputs to be calibrated should be specified on the shipping papers as well as on the purchase order. Voltmeter calibrators, multirange instruments with up to eight decimal digits of adjustability, are not considered by NIST to be standards and are not to be submitted routinely for calibration under this test category. Likewise, NIST will not accept component solid-state devices for routine calibration. However, new, state-of-the-art devices and instruments may be accepted for test under special circumstances (see Service ID Number 53110S) at the discretion of NIST technical staff. The NIST calibration service for voltage is directly tied to NIST Josephson-junction voltage-standard arrays. This 1-V standard fabricated from niobium trilayer resists the effects of cycling between its operating temperature of liquid helium and room temperature better than previous designs. back to top of page | back to index of electromagnetic measurements References-Voltage Measurements and StandardsGuidelines for Implementing the New Representation of the Volt and Ohm Effective January 1, 1990, N. B. Belecki, R. F. Dzuiba, B. F. Field, and B. W. Taylor, Natl. Inst. Stand. Technol., Tech. Note 1263 (June 1989). NBS Measurement Services: Solid-State DC Voltage Standard Calibrations, B. F. Field, Natl. Bur. Sand. (U.S.), Spec. Publ. 250-28 (Oct. 1987). NBS Measurement Services: Standard Cell Calibrations, B. F. Field, Natl. Bur. Stand. (U.S.), Spec. Publ. 250-24 (Oct. 1987). The NBS Josephson Array Voltage Standard, C. A. Hamilton, R. L. Kautz, F. L. Lloyd, R. L. Steiner, and B. F. Field, IEEE Trans. Instrum. Meas. IM-36, 258 (June 1987). A Sub-PPM Automated One-to-Ten Volt Measuring System, B. F. Field, IEEE Trans. Instrum. Meas. IM-34, 327 (1985). Volt Transfer Program Instructions, NBS Internal Document, Unpublished, Revised (1983). A High-Resolution Prototype System for Automatic Measurement of Standard Cell Voltages, D. W. Braudaway and R. E. Kleimann, IEEE Trans. Instrum. Meas. IM-23, 282 (1974). Volt Maintenance at NBS via 2e/h: A New Definition of the NBS Volt, B. F. Field, T. F. Finnegan, and J. Toots, Metrologia 9, 155 (1973). Designs for Surveillance of the Volt Maintained by a Small Group of Saturated Standard Cells, W. G. Eicke and J. M. Cameron, Natl. Bur. Stand. (U.S.), Tech. Note 430 (Oct. 1967). Standard Cells Their Construction, Maintenance, and Characteristics, W. J. Hamer, Natl. Bur. Stand. (U.S.), Monogr. 84 (Jan. 1965). Complete Characterization of Zener Standards at 10 V for Measurement Assurance program (MAP), Y. Tang and J. Sims, IEEE Trans. Instrum. Meas. Vol. 50, pp 263-266, (Apr. 2001). back to top of page | back to index of electromagnetic measurements AC Voltage MeasurementsTechnical Contact: Denise D. Prather Please contact the administration and logistics staff before shipping instruments or standards to the address listed below. Mailing Address:
back to top of page | back to index of electromagnetic measurements Digital Multimeters (DMMs) and Multifunction Calibrators (53200S)Voltage measurements are performed at dc at amplitudes between 1 mV and 1 kV. Relative expanded uncertainties as low as 1 x 10-6 are possible in the mid-voltage range. Low-frequency (0.1 Hz to 100 Hz) measurements of ac voltage are made between 1 mV and 7 V using a NIST-developed calculable voltage standard in which waveforms are digitally synthesized using a lookup table and a digital-to-analog converter. Relative expanded uncertainties as low as 5 x 10-6 are possible around 7 V. Wideband ac voltage measurements between 10 Hz and 30 MHz are made between 1 mV to 1 kV using a thermal voltage converter standard in an automatic calibration system. Relative expanded uncertainties range from 10 x 10-6 to 0.2%. AC current measurements are performed on the same automatic calibration system using a thermal current converter. Current sources can be measured from 10 Hz to 100 kHz at current levels between 1 mA and 2 A. Digital multimeters (DMM) tests are normally limited to an upper frequency of 5 kHz; however, special arrangements may be made for tests at higher frequencies and currents. Relative expanded uncertainties are typically less than 100 x 10-6. Direct current resistance measurements are performed between 1 Ω to 100 M Ω, for both DMMs and calibrators. Relative expanded uncertainties of 2 x 10-6 are possible for certain resistance values. back to top of page | back to index of electromagnetic measurements Low-Voltage AC-DC Transfer Standards (53201S)Measurements of the ac-dc difference of low-voltage (1 mV to 200 mV) thermal transfer standards, micropotentiometers, and voltage dividers are also offered as a Special Test in the dc to 1 MHz frequency range. Relative expanded uncertainties of 15 x 10-6 are possible in the audio-frequency range at 100 mV. back to top of page | back to index of electromagnetic measurements Special 25-Point Test of Digital Multimeters (DMMs), by Prearrangement (53202S-53203S)This is a special reduced cost, 25-point test covering all five functions (ac and dc voltage and current, and dc resistance) of most precision DMMs. DMMs submitted for test must have an IEEE-488 interface bus, and a list of DMM bus commands for the instrument may be required. The 25 test points available are shown in Table 9.6 below, together with the best possible expanded uncertainties. Additional test points are available over a wide range of amplitudes and frequencies. Table 9.6 . 25-Point Standard DMM Test
back to top of page | back to index of electromagnetic measurements References-AC Voltmeters and SourcesNIST Multifunction Calibration System, N. M. Oldham and M. E. Parker, NIST Spec. Publ. 250-46 (Feb. 1998). Low-Voltage Standards in the 10 Hz to 1 MHz Range, N. M. Oldham, S. R. Auramov, M. E. Parker, and B. Waltrip, IEEE Trans. Instrum. Meas. 46 (2) 395-398 (April. 1997). A Calculable, Transportable Audio-Frequency AC Reference Standard, N. M. Oldham, P. S. Hetrick, and X. Zeng, IEEE Trans. Instrum. Meas. 38 (2), 368-371 (April 1989). A High-Accuracy, 10 Hz-1 MHz Automatic AC Voltage Calibration System, N. M. Oldham, M. E. Parker, A.Young, and A. G. Smith, IEEE Trans. Instrum. Meas. 36, 883-887 (Dec. 1987). back to top of page | back to index of electromagnetic measurements AC-DC Thermal Voltage and Current Converters (to 1 MHz)Technical Contact: Denise D. Prather Please contact the administration and logistics staff before shipping instruments or standards to the address listed below. Mailing Address:
back to top of page | back to index of electromagnetic measurements General Information-Thermal Voltage and Current Converters (10 Hz to 1 GHz)Alternating voltage and current are most accurately measured by comparing the heating effect of the alternating quantity to the heating effect of the average of both polarities of the direct quantity using thermal transfer standards according to the relationship
δ is the ac-dc difference in proportional parts, Note that δ is given in µV/V or µA/A for measurements at frequencies of 1 MHz and below, and in % for measurements at frequencies greater than 1 MHz. Thermal converters are constructed according to their intended frequency and voltage range. They may consist of a simple thermal sensor (a thermoelement or thin-film device) for use at voltages up to a few volts, or a thermal sensor in series with a resistor for voltage ranges up to 1000 V. For ac current measurements, the thermal sensor is generally used with a high-precision shunt to measure currents up to 100 A. Metrology-grade thermal converters generally have small ac-dc differences that are independent of changes in frequency or input signal amplitude at voltages from about 150 mV to 100 V, or, for current converters, from 1 mA to 1 A, at mid to high audio frequencies. The ac-dc differences of thermal converters generally increased (in some cases significantly) as the applied voltage or current is increased, or as the frequency departs from the audio region. Various methods of construction are used to reduce the ac-dc differences of thermal converters at the extremes of input signal amplitude and frequency. See the references for more information regarding thermal converter construction and use. back to top of page | back to index of electromagnetic measurements Special AC-DC Measurement Services, by Prearrangement (53310S)This service provides for the measurement or evaluation of prototype ac voltage or current standards, sources, or measurement instrument, and for other measurements of alternating voltage, current, or ac-dc difference not provided for in the calibration service described below, at the discretion of NIST technical experts. Components used to ac-dc conversions will generally not be tested unless they show promise of metrology-standard behavior, and only in limited numbers for prototyping purposes. Special ac-dc difference calibration of appropriate thermal converters are now offered with an expanded uncertainty of 0.8 x 10-6. This calibration service is the result of an extensive study of a group of multijunction thermal converters that make up the NIST primary standards. Thermal converters will be accepted for this uncertainty provided that their performance, included stability and square-law response, is compatible with the NIST standards and comparator systems. In general, uncertainties below 10-6 is available for voltage from 0.5 V to 10 V, and currents from 5 mA to 20 mA, at frequencies from 40 Hz to 10 kHz. As in the case of other special ac-dc difference calibration services, an additional cost and an extended measurement time at NIST are required. Prospective clients should contact T. E. Lipe to discuss the requirements and arrangements related to this service. back to top of page | back to index of electromagnetic measurements AC-DC Difference Calibration of a Standard or Standards Set (Voltage or Current) (53350C-53352C)This service covers the calibration of thermal transfer standards covering the parameter space shown in Tables 9.7a through 9.7d . These transfer standards include coaxial standards, active transfer standards using solid-state sensors, passive multirange standards, thermal current converters, RF micropotentiometers, and peak-to-peak detectors. Measurements are recommended at all voltages or currents and frequencies where the transfer standard is normally used by the customer. In addition, if 1000 V or 1200 V ranges are measured, tests at 600 V are recommended to evaluate the input level dependence of the resistor. Since some thermal transfer standards show large ac-dc differences at frequencies below about 40 Hz, additional measurements may be required to determine the low-frequency performance of the instrument. Unless an instrument has a previous calibration history, the user may wish to discuss the calibration parameters with the NIST staff. The plane of reference for thermal voltage converter and voltage calibrations of thermal transfer standards below 100 MHz is generally at the center of a GR Type 874 tee. If the transfer standard has an input connector other than this type, the transfer standard will be connected to the plane of reference using an adaptor. In special cases, the plane of reference may be at the center of a Type N tee. In either case, a description of the plane of reference for the measurements will be provided in the calibration report, as well as the adapters, if any, used for the calibration. At frequencies of 1 MHz and below, these adaptors will make a negligible contribution to the ac-dc difference, compared to the uncertainty. At frequencies exceeding 1 MHz, corrections for the adapter will be included in the ac-dc difference provided to the customer. Calibration of thermal current converters are performed with the converters connected in series with the current source. In this case, the low side of the unit under test is connected to signal ground. although this arrangement floats the NIST standard above ground potential, the NIST standard thermoelements provide sufficient isolation to reduce stray currents in the potential leads to a negligible level. Uncertainties for low frequency voltage and current calibrations are provided in Tables 9.7a through 9.7d. Uncertainties for calibrations of thermal converters up to 100 MHz are shown in Table 9.7e. Calibrations at frequencies above 100 MHz are performed only on TVCs with an integral tee connector. For these devices the reference plane of the measurements is at the front face of the Type N output connector of the TVCs. Uncertainties for the calibration parameter space for these devices are shown in Table 9.7f. Measurements on peak-to-peak detectors are performed from 100 kHz to 500 kHz and are referenced to the center of a GR Type 874 tee. The RF-dc difference of these devices is measured, where a 50 kHz ac reference signal is applied to the instrument instead of a dc reference, and the RF-dc difference is defined as the difference between the signals required to produce a -zero- output signal. Uncertainties for the calibration parameter space for these devices are shown in Table 9.7g. NOTE: Work is underway to evaluate peak-to-peak detectors using the NIST Sampling Waveform Analyzer. This method has the potential to significantly reduce uncertainties for these devices. Until this evaluation is complete, peak-to-peak detectors will be calibrated as a Special Test (53310S) RF micropotentiometers are usually calibrated at their nominal rated output voltages. Frequency suggested for a normal calibration are 5 MHz, 100 MHz, 300 MHz, 400 MHz, 500 MHz, 700 MHz, and 900 MHz. Special arrangements may be made for calibrations at other frequencies. RF micropotentiometers having resistive elements greater than 10 m Ω in combination with thermoelements with ratings between 5 mA and 100 mA usually have RF-dc differences less than 1 % at 5 MHz. Since the RF-dc difference approaches zero below 5 MHz, calibrations at 50 kHz and 5 MHz are sufficient to establish the RF-dc difference dependence on frequency between these two points, and intermediate frequencies may be interpolated from this relationship with no appreciable loss of accuracy. RF micropotentiometers having resistive elements greater than 1 m Ω in combination with thermoelements with ratings between 5 mA and 100 mA may have RF-dc differences of about 5 % at 1 MHz. Interpolation below 1 MHz is not recommended for these devices. The RF-dc difference (in percent) is defined as the difference between the RF and dc output voltages required to produce the same thermocouple output, with the resistor terminated in 50 Ω; that is:
As a special service RF micropotentiometers with rated output voltages greater than 200 µV and may be calibrated from 50 kHz to 1 GHz with increased uncertainty. Uncertainties for the calibration parameter space for these devices are shown in Table 9.7h. NOTE: Since the RF micropotentiometer calibration service was recently moved to Gaithersburg, this calibration service is not yet in operation. Please contact the NIST staff for more information about this service. Some of the uncertainties offered and the parameter space covered in Tables 9.7 for this calibration service are presently being reevaluated. Significant reductions in the uncertainties and expansion of the parameter space are expected. To obtain the most recent information, customers are requested to contact the NIST staff, or visit the AC-DC Difference Project website at http://www.acdc.nist.gov. Routine calibrations of thermal voltage and current converters are generally performed on an on-demand basis. However, occasional extensive calibration requests may create scheduling problems; therefore, to facilitate rapid turnaround, please contact T. E. Lipe prior to sending the equipment. Ongoing research at NIST will help to improve the Nation's capability to provide accurate measurements of alternating voltage and current and ac-dc difference to NIST customers. To see the latest research in the AC-DC Difference Project, please visit http://www.nist.gov/eeel/quantum/fundamental_electrical/acdc.cfm and follow the "Research Programs" link. back to top of page | back to index of electromagnetic measurements
back to top of page | back to index of electromagnetic measurements
back to top of page | back to index of electromagnetic measurements
back to top of page | back to index of electromagnetic measurements
back to top of page | back to index of electromagnetic measurements
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back to top of page | back to index of electromagnetic measurements References-AC-DC and RF-DC Voltage and Current Converters (10 Hz to 1 MHz)Extension of the NIST AC-DC Difference Calibration Service for Current to 100 kHz, J. R. Kinard, T. E. Lipe, and C. B. Childers, J. Res. Natl. Inst. Stand. Technol. 102 (1), 75 (1997). A Reevaluation of the NIST Low-Frequency Standards for AC-DC Difference in the Voltage Range 0.6-100 V, T. E. Lipe, IEEE Trans. Instrum. Meas. IM-45 (6), 913 (Dec. 1996). Performance of Multilayer Thin-Film Multijunction Thermal Converters, J. R. Kinard, D. X. Huang, and D. B. Novotny, IEEE Trans. Instrum. Meas. IM-44 (2), 383 (April 1995). AC-DC Difference Characteristics of High-Voltage Thermal Converters, D. X. Huang, T. E. Lipe, J. R. Kinard, and C. B. Childers, IEEE Trans. Instrum. Meas. IM-44 (2), 387 (April 1995). NIST Measurement Services: AC-DC Difference Calibrations, J. R. Kinard, J. R. Hastings, T. E. Lipe, and C. B. Childers, Natl. Inst. Stand. Technol., Spec. Publ. 250-27 (May 1989). Determination of AC-DC Difference in the 0.1-100 MHz Frequency Range, J. R. Kinard and T. X. Cai, IEEE Trans. Instrum. Meas. IM-38 (2), 360 (April 1989). Recharacterization of Thermal Voltage Converters after Thermoelement Replacement, J. R. Kinard and T. E. Lipe, IEEE Trans. Instrum. Meas. IM-38 (2), 351 (April 1989). RF-DC Differences of Thermal Voltage Converters Arising from Input Connectors, D. X. Huang, J. R. Kinard, and G. Rebuldela, IEEE Trans. Instrum. Meas. 40 (2) (April. 1991). NBS RF Voltage Comparator, L. D. Driver, F. X. Ries, G. Rebuldela, Natl. Bur. Stand. (U.S.), NBSIR 78-871 (Dec. 1978). High-Frequency Microvolt Measurements, F. X. Ries and G. Rebuldela, ISA Proc., 18,1, 37.2.63, Instrum. Soc. of Amer. Res. Triangle Park, NC (Sept. 1963). Thermal Voltage Converters for Accurate Voltage Measurements to 30 Megacycles Per Second, F. L. Hermach and E. S. Williams, Trans. AIEE, Pt. 1, Commun. Elect. 72, 200 (July 1960). Accurate Radio-Frequency Microvoltages, M. C. Selby, Trans. AIEE, Pt. 1, Commun. Elect. 72, 158 (May 1953). Thermal Converters as AC-DC Transfer Standards for Current and Voltage Measurements at Audio Frequencies, F. L. Hermach, J. Res. Natl. Bur. Stand. (U.S.), 48 (2), 121 (1952). A Wideband Sampling Voltmeter, T. M. Souders, B. C. Waltrip, O. B. Laug, and J. P. Deyst, IEEE Trans Instrum. Meas. IM-46 (4) 947 (August 1997). back to top of page | back to index of electromagnetic measurements Program questions: Calibrations Phone: 301-975-2200, Fax: 301-975-2950 NIST, 100 Bureau Drive, Stop 8363, Gaithersburg, MD 20899-8363 |
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