spacer spacer Go to Kaye and Laby Home spacer
spacer
spacer spacer spacer
spacer
spacer
spacer
spacer spacer

You are here:

spacer

Chapter: 1 Units and fundamental constants
    Section: 1.1 Units
        SubSection: 1.1.5 Uncertainty of primary standards

spacer
spacer

spacer

« Previous Subsection

Next Section »

Unless otherwise stated this page contains Version 1.0 content (Read more about versions)

1.1.5 Uncertainty of primary standards

Setting up and evaluating a primary standard usually entails a long and painstaking programme of experiments spread over several years before the necessary low level of uncertainty is achieved. A great effort is made to eliminate, or at least to identify and quantify, all possible contributions to the uncertainty of the standard. Statistical (random) contributions may be reduced arbitrarily, e.g. by increasing the number of measurements made, but there always remains the possibility that some unsuspected, non-statistical (systematic) source of uncertainty exists. The application of rules for the statement of uncertainty, as in section 6, is thus less straightforward in the case of primary standards work than it is in routine calibration work.

When the results of measurements are compared by those whose measurements are traceable to the same primary standard, they will be using identical units and any uncertainty in the primary standard may be neglected. Where traceability is to different primary standards, information about any difference in the sizes of the units may be obtained from the BIPM or from one of the national laboratories concerned.

In some scientific work it may be essential to consider the uncertainty in the primary standard when assessing the significance of the measurements. Much relevant information is published by the BIPM, particularly in reports on international comparisons of the units realized and maintained by the various national laboratories. It is always advisable to approach the appropriate expert at the NPL or other national laboratory in order to confirm the interpretation of such material, and to ascertain whether any more recent information is available.

For illustrative purposes only, the table below lists the 1-sigma uncertainties assigned to a number of NPL standards in 1991. Note that standards set up to provide calibrations of very large or very small values of quantities such as pressure, for example, will have much greater uncertainties than those of the base units.


Table of uncertainties

Quantiity

Unit

Uncertainty
(one sigma)

Notes

 

 

 

 

Time interval

  second

  4 × 10−14

 

Length

  metre

  2.5 × 10−11

 

Mass

  kilogram

  2.3 μg

 

Potential difference

  volt

  10−8

 

Electric resistance

  ohm

  10−8

 

Temperature

  kelvin

  0.000 1 K

  At triple point of water

Luminous intensity

  candela

  3 × 10−4

  For monochromatic radiation


Angle


  arc second


   10−2

 

Density

  kgm−3

   10−5

  For solids

Force

  newton

   10−5

  Up to 1.2 MN

Pressure:

  pascal

 

 

   atmospheric

 

   3 × 10−6

  At 100 kPa

   high pressure

 

   2 × 10−5

  At 100 MPa

   vacuum

 

   10−2

  At 1 μPa

Sound pressure level

  dB re 10 μPa

   0.04 dB

  At 5 KHz in air

Acoustic pressure

  pascal

   1.5 × 10−2

  At 5 MHz in water


Thermal conductivity


  Wm−1 K−1


   10−2 to 2 × 10−2


  Material-dependent


Electric power:


  watt

 

 

   supply frequency

 

   5 × 10−5

  At 50 Hz

   microwave

 

   10−3

  At 10 GHz

Laser pulse energy

  joule

   1.5 × 10−2

  0.4 μm to 1.06 μm


Optical power


  watt


   4 × 10−5

 

Colour

  CIE L*A*B

   0.25

  For colour surface


Neutron emission rate


  s−1


   2 × 10−3


  For radionuclide neutron sources

Activity

  becquerel

   1.5 × 10−3 to 10−2

  Depends on radionuclide

Absorbed dose

  gray

   3.5 × 10−3

  In graphite

Amount of substance

  mole

   3 × 10−4


  For gases, usually as gas mixtures,
     mole fraction

The BIPM suggests the use of ‘category A’ and ‘category B’ instead of ‘random’ and ‘systematic’.

O.C. Jones

spacer


spacer
spacer
spacer spacer spacer

Home | About | Table of Contents | Advanced Search | Copyright | Feedback | Privacy | ^ Top of Page ^

spacer

This site is hosted and maintained by the National Physical Laboratory © 2017.

spacer