FAQs
Mass & Density - FAQs
The answers to some frequently asked questions about mass and density metrology are grouped below.
Mass, Weight, Buoyancy & Conventions
- An air buoyancy correction has to be made, where the measurement uncertainty required of a weighing warrants it, when the volumes (and hence the amount of displaced air) of two masses being compared are different.
- The conventional mass value of an artefact can be calculated from its true mass value using the following equation.
- When comparing weights of dissimilar materials the effect of air buoyancy becomes more significant and must be applied even for routine calibrations when true mass values are being determined.
- The mass of a body relates to the amount of material it contains and there is no difference between mass and true mass. When a weight is calibrated the mass value quoted on its certificate of calibration is normally a conventional mass value - appropriate where the value is determined by weighing the item in air.
- 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), force is a measure of the interaction between bodies and load usually means the force exerted on a surface or body.
- 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.
Gravity & Altitude
- Yes it does - provide that you mean the 'scientific' definition of weight where the weight of a body is the gravitational force acting on it and hence it should be measured in newtons; this force depends on the local acceleration due to gravity.
- Yes, but unless you are making weighings at the very highest level of accuracy, the consequences are probably too small to worry about.
- 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.
Density
- The methods used to determine values of air density fall into two broad categories – indirect (that is calculation from measurement of other parameters and known as the parametric method) and direct measurement. The parametric method is used more commonly.
- The density of a solid or liquid may be fundamentally determined by hydrostatic weighing using Archimedes' principle.
- Density is defined as mass per unit volume; it has the SI unit kg·m-3 and is an absolute quantity. Specific gravity is the ratio of a material's density with that of water at 4 °C and is therefore a relative quantity with no units.
- For a given composition and temperature, air density varies in direct proportional to air pressure and this reduces with altitude for two reasons - both related to gravity.
Weights
- 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.
- Give the weight a general inspection to check its construction, surface finish and the suitability of its magnetic properties.
- The frequency with which calibrations should be carried out is an important, if sometimes difficult, question; there is no hard and fast rule but there are two main considerations.
- As infrequently as possible.
- Refer to our Good Practice Guide.
- Classification systems are used to specify most aspects of modern weights and weight sets.
- 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.
Balances, Comparators & Other Instrumentation
- Yes, magnetic fields - and indeed magnetically permeable materials close to a balance - can effect a weighing result.
- There are commercial organisations that are accredited for this purpose.
- Many factors will have an influence on how well a mass balance or comparator will perform. One key factor is the location and environment in which the balance is located.
- See our Good Practice Guidance Note.
- 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.
- In scientific circles the word balance has many meanings but in the context of weighing it originates from the abbreviation of beam-balance - where the weight of a sample or artefact contributes to the balance of moments of a beam about a central fulcrum.
Units, Traceability, the SI & Weighing History
- 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.
- Historically there have been a variety of units of mass and density, and approximate conversion factors to some of these are given below.
- The relationship between the kilogram and the pound is defined exactly as: 1 lb = 0.453 592 37 kg.
- Mass is therefore a base quantity in the SI and its unit is the kilogram, which is abbreviated to kg.
- For the last 20 years there has been a considerable amount of work undertaken looking for an alternative, more fundamental, definition for the SI unit of mass - the kilogram.
- 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.
- In 1790 the French National Assembly obtained Louis XVI's assent to commission the country's leading scientists to recommend a consistent system for weights and measures.
- In 1799 it was agreed that the unit should be the mass of one cubic decimetre of water at a temperature of 4 °C, which would be called a kilogram (kg). The mass of one cubic centimetre of water would be called a gram (g).
- The UK National Standard Kilogram is kept in a basement vault at NPL. It is stored in an air-tight enclosure, only exposed to the air via two micropore filters.
- Platinum-iridium was chosen as the material for the kilogram for a number of reasons.
Accuracy & Uncertainty
- No, the difference is not very meaningful at all.
- Probably not. The figures should be interpreted with caution, especially if an instrument has not been independently calibrated for some time.
- Yes there is a difference. The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
Miscellaneous
- Yes but not enough to notice.
- Calibration is often thought to be expensive but the information contained in the resultant certificate is usually worth considerably more.