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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

• ### 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

• ### Is an instrument's accuracy likely to be around the figure given by the manufacturer? (FAQ - Pressure)

Probably not. The figures should be interpreted with caution, especially if an instrument has not been independently calibrated for some time.
• ### Is it more accurate to measure the depth of a liquid in units like 'metres of water'? (FAQ - Pressure)

No, ultimately it is less accurate. Liquid depth can be determined from pressure measurements and the reason can be seen by looking at the fundamental (and simplified) equation for calculating the pressure at a particular depth in a liquid.
• ### Is there a the difference between 'accuracy' and 'uncertainty'? (FAQ - Pressure)

The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
• ### My barometer has a scale marked '28' to '31' - what does this mean? (FAQ - Pressure)

Your barometer is undoubtedly scaled in inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter).
• ### My pressure gauge is scaled in 'inches' - what does this mean? (FAQ - Pressure)

There are no pressure units called just inches but there are inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter) and inches of water.

## Registration

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