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 terms high- and low- (and also medium-) vacuum are not very intuitive; they are used to describe the various pressure ranges known under the general banner of vacuum but are neither defined nor used consistently.
- A hard vacuum is not well defined but it sometimes seems to be used erroneously to describe a pressure that is so low that extreme forces are involved.
- The measurement uncertainties achievable with pressure gauges, particularly traditional circular 'dial' gauges, are often expressed in one of two ways - as a percentage of reading or as a percentage of full-scale reading and the differences can be very significant, particularly when working at pressures much lower than an instrument's full-scale.
- Transducers are voltage-output devices that can be used with simple signal conditioning but are more sensitive to electromagnetic interference. Transmitters are current-output devices and may have two or three wires.
- Sometimes, the word calibration is misused to describe the process of altering the performance of an instrument to ensure that the values it indicates are correct within specified limits, strictly this is adjustment.