• Science + Technology
• Commercial Services
• Educate + Explore
• Joint Ventures
• Publications
FAQs

Measurements 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.

## Force

• ### Are there any general 'do's and don'ts' in force metrology? (FAQ - Force)

Yes there are - some are listed here.
• ### How can I determine my local values of gravitational acceleration and altitude? (FAQ - Force)

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.
• ### How do I convert between different force units? (FAQ - Force)

Equivalent force values are given here.
• ### How does 'force' fit into the SI system? (FAQ - Force)

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.
• ### How many different types of force transducer are there? (FAQ - Force)

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.

## Mass & Density

• ### Does gravity or altitude affect weights? (FAQ - Mass & Density)

Yes it does - provide that you mean the 'scientific' definition of weight where the weight of a body is the gravitational force acting on it and hence it should be measured in newtons; this force depends on the local acceleration due to gravity.
• ### Does gravity or altitude effect weighing? (FAQ - Mass & Density)

Yes, but unless you are making weighings at the very highest level of accuracy, the consequences are probably too small to worry about.
• ### How can I determine my local values of gravitational acceleration and altitude? (FAQ - Mass & Density)

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.
• ### How do I assess the performance of my balance? (FAQ - Mass & Density)

There are commercial organisations that are accredited for this purpose.
• ### How do I calculate and apply air buoyancy corrections? (FAQ - Mass & Density)

An air buoyancy correction has to be made, where the measurement uncertainty required of a weighing warrants it, when the volumes (and hence the amount of displaced air) of two masses being compared are different.

## Pressure

• ### Are all pressure units equally valid? (FAQ - Pressure)

No, they are not. The internationally recognised SI unit for pressure is the pascal, abbreviated to Pa, and this is the unit realised by the primary measurement standards in the world's national metrology institutes to provide traceability for pressure measurements.
• ### Are mercury barometers more accurate than non-mercury ones? (FAQ - Pressure)

The most accurate barometers are indeed the mercury primary barometers used at national measurement institutes. Most barometers, though, are secondary instruments rather than primary ones and when considering these it is not correct to say that those based on a mercury column are invariably more accurate than those that are based on an alternative principle.
• ### Does atmospheric pressure affect pressure balances? (FAQ - Pressure)

Yes it can in some circumstances but, where it does, it is fairly easy to ensure that its effect on the measurement uncertainties obtained is negligible.
• ### How accurate are pressure unit conversion values? (FAQ - Pressure)

When converting between pressure units consideration should be given to the number of significant figures to use, bearing in mind that many of the underlying conversion factors are not themselves exact and cannot be made so.
• ### How can I determine my local values of gravitational acceleration and altitude? (FAQ - Pressure)

To calculate a pressure value using a liquid column - for example a mercury barometer - or a pressure balance it is necessary to know the gravitational acceleration at the location of the instrument. It can be determined by measurement on site, calculation or interpolation of measured values.

## Registration

Please note that the information will not be divulged to third parties, or used without your permission