FAQsMeasurements of mass, force, pressure and density are some of the most commonly made in the UK. NPL ensures that these measurements can be made traceable to internationally agreed standards.
- Yes there are - some are listed here.
- The variation in the value of g across the earth's surface is about 0.5 % due to latitude, plus a change of approximately 0.003 % per 100 m altitude. Local topography and tidal forces also can have small effects.
- Equivalent force values are given here.
- The Système International d'unités - the SI system - is the coherent system of units adopted and recommended by the General Conference on Weights and Measures (CGPM). It is based on seven base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.
- There are many types of force transducer and they are used with instrumentation of varying complexity. In designing or specifying a force measurement system for an application, it is useful to understand the basic operation of the transducer to be used and also their broad operating characteristics.
- The International Prototype Kilogram is not perfectly stable (its mass changes with time), the amount it changes cannot be known perfectly (there is no 'perfect' reference against which to judge it) and the values of the national copies cannot be monitored at the highest level of accuracy without being compared directly with it.
- Up to a point yes, but unless a weight is of suitable design and material and in appropriate condition it will not be possible to give it a meaningful calibration and it would certainly be a waste of money.
- Give the weight a general inspection to check its construction, surface finish and the suitability of its magnetic properties.
- Yes, magnetic fields - and indeed magnetically permeable materials close to a balance - can effect a weighing result.
- Historically there have been a variety of units of mass and density, and approximate conversion factors to some of these are given below.
- The purpose of this guidance note is to explain why it is not possible to claim formal traceability for the pressure measurements made by a pressure balance, at anywhere approaching state-of-the-art levels of measurement uncertainty, when such traceability is based on locally obtained dimensional measurements of individual pistons and cylinders.
- The diagram below shows the approximate operating ranges of some commonly available pressure and vacuum measuring instruments.
- Calibration is the process of comparing a measuring instrument with a measurement standard to establish the relationship between the values indicated by the instrument and those of the standard.
- The SI unit of pressure is the pascal, symbol Pa, the special name given to a pressure of one newton per square metre (N/m2).
- When requesting a calibration it is reasonable to try and establish beforehand something about the measurement uncertainties that are likely to be provided on the ensuing certificate; unless they are going to be adequate there is not much point in asking for the calibration.