• 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

• ### 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

• ### How can I identify the type of barometer I have? (FAQ - Pressure)

There are many different types of barometer but they fit into two broad categories - those containing mercury and those that do not.
• ### How do I adjust my barometer? (FAQ - Pressure)

The answer to this question depends on the type of barometer in question, particularly whether its pressure sensing mechanism is influenced by gravity or not, and what you are trying to get it to measure.
• ### How do I calculate pressure height corrections? (FAQ - Pressure)

Pressure in a fluid, whether it be gas or liquid, varies with height. It doesn't matter whether the fluid is in pipework or more loosely confined such the atmosphere or the sea - just so long as there is gravitational attraction and something stopping free-fall (or a centripetal force - for those who are pedantic, in orbit or who twirl open cups of tea on horizontal cords).
• ### How do I convert pressure values from one unit to another? (FAQ - Pressure)

To convert one non-pascal pressure unit to an alternative non-pascal unit, multiply the 'starting' pressure value by the 'number of pascals' shown against its unit and then divide the product by the number of pascals shown against the second unit.
• ### How do I transport my mercury barometer? (FAQ - Pressure)

Mercury barometers must be transported with extreme care - the prime concerns are to prevent air from entering the vacuum space or the tube from being broken and leaking mercury.

## Registration

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