• 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 do I use a Fortin or Kew Pattern mercury barometer? (FAQ - Pressure)

Mercury barometers must be firmly supported, made vertical and given time to adopt ambient temperature before attempting to use them.
• ### How does 'pressure' fit into the International System of units (SI)? (FAQ - Pressure)

Pressure is not a base quantity but a derived quantity, with dimensions of length (L), mass (M) and time (T).
• ### How many types of barometer are there? (FAQ - Pressure)

Some barometer measurement principles are listed below; there are bound to be others.
• ### Is a certificate of calibration best mounted on the wall or put in a drawer? (FAQ - Pressure)

Instrument calibration is often thought to be expensive but the information contained in the resultant certificate is usually worth considerably more. This is not always appreciated and, on receipt, many certificates are wastefully consigned to drawers, shelves or even wall plaques.
• ### Is a measurement uncertainty of, say, 0.095% meaningfully better than 0.10%? (FAQ - Pressure)

No, the difference is not very meaningful at all.

## Registration

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