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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

• ### Should I have my instrument calibrated and, if so, how often? (FAQ - Force)

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.
• ### What 'uncertainty in the measurement' will be quoted on my certificate of calibration? (FAQ - Force)

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.
• ### What are 'g-forces' and are they caused by gravity? (FAQ - Force)

The term g-force, or Gs is not well defined but is sometimes used to convey something like a value of force, expressed as a proportion of the nominal gravitational force experienced when standing on the earth's surface or a value of acceleration, expressed as a proportion of the nominal gravitational acceleration experienced when in free-fall just above the earth's surface.
• ### What are multi-axis and dynamic force calibrations? (FAQ - Force)

Multi-axis calibration is similar to single axis calibration but done once for each axis. There are very few sources of traceable dynamic force calibration – and certainly none in the force and frequency ranges measured by the majority of commercially available dynamic force transducers.
• ### What are the differences between force, mass, weight and load? (FAQ - Force)

Force is a measure of the interaction between bodies, 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) and load usually means the force exerted on a surface or body.

## Mass & Density

• ### Are any problems caused by having the kilogram defined in terms of a physical artefact? (FAQ - Mass & Density)

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.
• ### Can any weight be calibrated? (FAQ - Mass & Density)

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.
• ### Can any weight be calibrated? (FAQ - Mass & Density)

Give the weight a general inspection to check its construction, surface finish and the suitability of its magnetic properties.
• ### Can magnetic fields affect weighing? (FAQ - Mass & Density)

Yes, magnetic fields - and indeed magnetically permeable materials close to a balance - can effect a weighing result.
• ### Conversion Factors for Mass and Density (FAQ - Mass & Density)

Historically there have been a variety of units of mass and density, and approximate conversion factors to some of these are given below.

## 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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