CRITERIA FOR LABORATORY ACCREDITATION IN THE FIELD OF TIME AND FREQUENCY METROLOGY

Transcription

1 CRITERIA FOR LABORATORY ACCREDITATION IN THE FIELD OF TIME AND FREQUENCY METROLOGY Approved By: Chief Executive Officer: Ron Josias Senior Manager: Mpho Phaloane Revised By: Specialist Technical Committee Members Date of Approval: Date of Implementation: SANAS Page 1 of 9

2 CONTENTS: 1. Purpose and Scope References Definitions and Abbreviations Environmental Requirements Technical Requirements General Requirements Accredited Parameters... 5 APPENDIX 1: Example of Schedule of Accreditation... 7 ADDENDUM 1: Amendment Record... 9 SANAS Page 2 of 9

3 1. Purpose and Scope The purpose of this document is to define the specific environmental, technical and general requirements, as agreed by the Specialist Technical Committee (STC), to be met by accredited laboratories in the field of Time and Frequency metrology. This document is applicable to South African National Accreditation System (SANAS) Accredited Laboratories in these fields. 2. References [1] SANAS A01 References, Acronyms and Definitions [2] EURAMET cg-7 Calibration of Oscilloscopes [3] SANAS R48 Proficiency Testing Requirements for Calibration Laboratories [4] SANAS TR45 Criteria for Laboratories Accredited to Calibrate Tachometers and Measure Rotational Speed [5] GUM Guide to Uncertainty in Measurement, ISO/IEC Guide 98-1 and its various parts. Also JCGM 100:2008 (GUM 1995 with minor corrections) Evaluation of measurement data Guide to the expression of uncertainty in measurement. [6] OHS Act, 1993 Environmental Regulations Lighting [7] CCTF-K001.UTC BIPM key comparison in the field of time metrology, through which participating laboratories can obtain traceability to the SI for the various related quantities. 3. Definitions and Abbreviations BIPM CCTF CGGTTS CMC GNSS ILC ISO JCGM NMI NMISA PT UTC UTC (ZA) International Bureau of Weights and Measures Consultative Committee on Time and Frequency CCTF Group for GNSS Time Transfer Systems Calibration and Measurement Capability Global Navigation Satellite System, for example the American system (GPS), the Russian system (GLONASS) and the future European system (Galileo) Inter-Laboratory Comparison International Standards Organisation Joint Committee for Guides in Metrology National Metrology Institute National Metrology Institute of South Africa Proficiency Testing Co-ordinated Universal Time UTC as implemented in South Africa by NMISA 4. Environmental Requirements An accredited laboratory working in the field of time and frequency shall operate under the following environmental conditions: 4.1 The laboratory shall be maintained at a temperature of 23 C ± 5 C. Rates of change of the ambient temperature shall be kept below 2 C per hour. 4.2 The relative humidity in the laboratory should be maintained to be 50 % rh ± 15 % rh. Changes in humidity may affect free running oscillators and shall be monitored continuously if used in the laboratory. Lower humidity values may lead to static discharge which may result in damage to sensitive equipment. 4.3 Adequate lighting shall be provided as detailed in the environmental regulations for workplaces (1987) of the OHS Act, Act 6 of Where measurements are performed outside the requirements specified above, for example when work is conducted on-site, the laboratory must maintain documented evidence that the results have not been invalidated. Should the results be adversely SANAS Page 3 of 9

4 affected, the evidence must be produced to indicate that these influences are considered in the uncertainty budgets. 5. Technical Requirements 5.1 The laboratory should be equipped with both generating and measurement capability throughout the full accredited range in order to demonstrate that CMCs can be attained. If a laboratory has only generating or only measurement capability, they may still be accredited provided that this is indicated on the accreditation schedule and the laboratory can provide suitable evidence, for example, ILC/PT reports and equipment history records which demonstrate that they are capable of maintaining their CMC s. 5.2 Laboratories may use a GNSS system to obtain traceability of their own local frequency standard without the need to send the standard for calibration. For this to be accepted, the laboratory must show how they take the differences between UTC (ZA) and the specific GNSS system into account. The laboratory may not use the GNSS time scale as their source of traceability, since it is not a time scale controlled by an international NMI. Laboratories must still comply with the verification and validation requirements of ISO/IEC The following points are applicable when using a GNSS system: The laboratory may use a common-view technique, as long as they are following the common-view schedule as published by the BIPM. Laboratories may also use the all-in-view technique, as long as the track start and stop times agree with the common-view schedule. The data captured must be reduced using the CGGTTS method. The laboratory must make an arrangement with either NMISA, or a participant of the BIPM key comparison CCTF-K001.UTC, to obtain their CGGTTS compliant data and must have evidence of the analysis of results depicting the offset to the NMISA and the signature of the system The laboratory may use two (or more) collocated GNSS receivers. The laboratory must track the differences between the two receivers and regularly analyse the data to show that the receivers are operating within acceptable limits required to prove the CMC of the laboratory. The laboratory must log at least one data point per hour. The values published by either NMISA, or a participant of the BIPM key comparison CCTF-K001.UTC, must be used to obtain the relative frequency error and stability of the receivers The laboratory may use a single GNSS receiver and oscillator method, if they record the time difference between the GNSS receiver and the oscillator. (This must be a second oscillator and not the oscillator inside the GNSS receiver.) This system may either be a modular system consisting of a GNSS receiver, an oscillator and a frequency counter, or a standalone unit, as long as the time difference data is available. The laboratory must log at least one data point per hour and reduce the data points to a single point representing 00:00 UTC time. This single data point per day must be compared to the values published by either NMISA, or a participant of the BIPM key comparison CCTF-K001.UTC, to obtain the relative frequency error and stability of the receiver. The laboratory must regularly analyse the data to show that the local frequency standard is operating within acceptable limits required to prove the CMC of the laboratory The laboratory may use a single standalone GNSS receiver, if the laboratory has a way of verifying the performance of the GNSS receiver. (One such a way would be to continuously monitor the frequency output of the GNSS receiver using a calibrated frequency counter.) The output of the receiver must also be calibrated regularly by an external party with traceability to the national measurement standard for frequency. The laboratory must have data for the stability of the oscillator for periods shorter than the verification and calibration cycle. 5.3 Laboratories may use a calibrated reference oscillator as their source of traceability. This may be a standalone oscillator or the internal time base of a frequency counter or generator. The laboratory must have evidence of regular traceable calibrations as well as SANAS Page 4 of 9

5 verification data for the standard between calibrations. The uncertainty calculations for this method must take the retrace error and drift between calibrations into account. 6. General Requirements 6.1 Measurement uncertainties shall be addressed in accordance with the requirements of ISO/IEC as detailed in the GUM. [5] 6.2 Measurements in this field contain time correlated measurement data that cannot be analysed using normal statistics. The laboratory must show proof that they correctly analyse the random variations in the observations of an input quantity. (See paragraph of JCGM 100:2008 or GUM). [5] 7. Accredited Parameters A laboratory may ask for accreditation for one or more of the parameters listed below. These parameters assume a repetitive waveform. 7.1 Frequency: The CMC of a laboratory will be based on a sinusoidal wave Standard frequency source: This CMC will be expressed as a relative frequency uncertainty. The calculations of this CMC must take the method, the uncertainty of the local frequency standard (stability and accuracy uncertainty) and the best standard frequency source calibrated by the laboratory into account. The uncertainty will typically be expressed as specific values, e.g. 1 MHz, 5 MHz or 10 MHz General frequency source: This would be any frequency source not at a standard frequency value and includes all adjustable frequency equipment. Typically, this accreditation would be for a direct frequency measurement using a frequency counter, or the direct generation of a frequency using a waveform generator Optical frequency standard: For the measurement of the frequency of laser light sources. 7.2 Time interval: General time interval measurements Rise/fall time measurements: These measurements must be performed using a calibrated pulse generator, or an oscilloscope calibrated for rise/fall-time. 7.3 Oscilloscopes: Separate uncertainty budgets must be available for the following parameters and each of the following parameters must be listed on the accreditation schedule Vertical deflection (amplitude): Horizontal deflection (timebase): Bandwidth: Frequency where the measured amplitude of the signal drops 3 db below the reference point for the first time Rise time: The rise-time measurement may only be obtained from the formula 0,35/BW for analogue oscilloscopes operating from DC up to a maximum frequency of 1 GHz [2]. For digital sampling oscilloscopes, analogue oscilloscopes operating at frequencies above 1 GHz or analogue oscilloscope with a starting frequency above DC, a measurement must be performed using a pulse generator calibrated for rise/fall-time. SANAS Page 5 of 9

6 7.3.5 Cal output: Frequency and amplitude of the cal output of the oscilloscope. 7.4 Rotational speed: The calibration of tachometers is covered by an extension of accreditation and it must be clearly indicated how the requirements of SANAS R45 have been addressed by the laboratory. 7.5 Phase angle: 7.6 Onsite calibrations shall be listed separately An example accreditation schedule is detailed in Appendix 1. SANAS Page 6 of 9

7 APPENDIX 1: Example of Schedule of Accreditation ANNEXURE A SCHEDULE OF ACCREDITATION TIME AND FREQUENCY METROLOGY Facility Number: 500 Permanent Address of Laboratory: Technical Signatories: Mr T Interval Jittery Measurements cc 29 Caesium Avenue Lynnwood Ridge 0184 Postal Address: PO Box 267 Lynnwood Ridge 0040 Nominated Representative: Mr J Smit Tel: (012) Issue No.: 09 Fax: (012) Date of Issue: 04 February info@jittery.co.za Expiry Date: 03 February 2019 ITEM MEASURED QUANTITY OR TYPE OF GAUGE OR INSTRUMENT RANGE OF MEASURED QUANTITY CALIBRATION AND MEASUREMENT CAPABILITY EXPRESSED AS AN UNCERTAINTY (±) 1 Time 1.1 Time Scale Difference 2 Frequency 2.1 Standard Frequency Source 1 MHz, 5 MHz, 10 MHz f 2.2 General Frequency Source 1 mhz to 1 GHz 1 GHz to 50 GHz f + 1 mhz 3 Hz 2.3 Optical Frequency Source 300 THz to 600 THz f 3 Time Interval General Time Interval Measurements Rise- and Fall-Time Measurements 1 ns to s s to s t + 1 ns t 100 ps to 1 μs t ps Original Date of Accreditation: February 2000 Page 1 of 2 The CMC, expressed as an expanded uncertainty of measurement, is stated as the standard uncertainty of measurement multiplied by a coverage factor k = 2, corresponding to a confidence level of approximately 95% Field Manager ANNEXURE A SANAS Page 7 of 9

8 Facility No.: 500 Date of Issue: 04 February 2014 Expiry Date: 03 February 2019 ITEM MEASURED QUANTITY OR TYPE OF GAUGE OR INSTRUMENT RANGE OF MEASURED QUANTITY CALIBRATION AND MEASUREMENT CAPABILITY EXPRESSED AS AN UNCERTAINTY (±) 4 Oscilloscopes 4.1 Vertical deflection Horizontal deflection Bandwidth Rise-Time Cal output 5 Rotational Speed < 250 MHz > 1,4 ns Frequency accuracy Amplitude accuracy 1% 1% 3% 4% 1 mhz 1% 5.1 Medical, Laboratory and Industrial Centrifuges 5.2 Contact Tachometers Non-Contact Tachometers (optical) Non-Contact Tachometers (inductive/acoustic) 60 rpm to rpm t + 1 rpm 5 rpm to rpm rpm to rpm 5 rpm to 100 rpm 100 rpm to rpm rpm to rpm t + 0,1 rpm t + 1 rpm t + 0,01 rpm t + 0,1 rpm t + 1 rpm 5 rpm to rpm 1 rpm 5.5 Stroboscope 5 rpm to rpm 1 rpm 6 Phase angle 6.1 Phase angle 0 to 360, 5 V equal amplitude: 1 Hz to 10 khz 10 khz to 1 khz 1 khz to 100 khz 0 to 360, 50 mv to 100 V: 1 Hz to 100 khz 0,005 0,01 0,02 0,05 7 On site calibration for items Original date of accreditation: February 2000 Page 2 of 2 The CMC, expressed as an expanded uncertainty of measurement, is stated as the standard uncertainty of measurement multiplied by a coverage factor k = 2, corresponding to a confidence level of approximately 95% ISSUED BY THE SOUTH AFRICAN NATIONAL ACCREDITATION SYSTEM Field Manager SANAS Page 8 of 9

9 ADDENDUM 1: Amendment Record Proposed By: Section Change STC Complete Total revision of all requirements STC Appendix 1 Add Example of T & F accreditation schedule SANAS Page 9 of 9