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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

• ### What are weight classes and tolerances? (FAQ - Mass & Density)

Classification systems are used to specify most aspects of modern weights and weight sets.
• ### What is a 'comparative' weighing as opposed to a 'direct' weighing? (FAQ - Mass & Density)

Two-pan balances make comparative weighings by comparing the weight of an artefact with reference weights - by balancing moments about a beam. Direct weighing is where mass values are read directly from a scale.
• ### What is buoyancy and how does it affect weighing? (FAQ - Mass & Density)

Buoyancy is the upward force exerted on an object when it is immersed, partially or fully, in a fluid - and strictly where the fluid is subjected to a gravitational force but is not in free fall.
• ### What is meant by traceability and is it important? (FAQ - Mass & Density)

The international definition of 'traceability' is the... property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.
• ### What is OIML Recommendation R 111? (FAQ - Mass & Density)

In Europe the weight classification system adopted is that produced by the International Organization of Legal Metrology (OIML), known as document OIML Recommendation R 111.

## 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.

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