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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

• ### Is there a difference between 'accuracy' and 'uncertainty'? (FAQ - Mass & Density)

Yes there is a difference. The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
• ### What are the differences between mass, true mass and conventional mass values? (FAQ - Mass & Density)

The mass of a body relates to the amount of material it contains and there is no difference between mass and true mass. When a weight is calibrated the mass value quoted on its certificate of calibration is normally a conventional mass value - appropriate where the value is determined by weighing the item in air.
• ### What are the differences between mass, weight, force and load? (FAQ - Mass & Density)

Mass is a measure of the amount of material in an object, weight is the gravitational force acting on a body (although for trading purposes it is taken to mean the same as mass), force is a measure of the interaction between bodies and load usually means the force exerted on a surface or body.
• ### What are the environmental requirements for balances? (FAQ - Mass & Density)

Many factors will have an influence on how well a mass balance or comparator will perform. One key factor is the location and environment in which the balance is located.
• ### What are the thermal effects on balances and weight? (FAQ - Mass & Density)

See our Good Practice Guidance Note.

## 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

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