- This is an arrangement amongst the majority of the world's National Metrology Institutes whereby each institute agrees to recognise measurement standards and calibration and measurement certificates issued by the other.
- Mass is therefore a base quantity in the SI and its unit is the kilogram, which is abbreviated to kg.
- Many old pressure units have obvious practical and historical origins; for example, inches of water was the unit used where pressures were measured with a water column whose top surface was sighted against an inch scale.
- Refer to our Good Practice Guide.
- Values of atmospheric pressure in Teddington, measured at about 10 metres above sea level, going back to 1 January 1998 and shown both graphically and digitally, can be found on the historical pressure page of the NPL on-line barograph.
- If you look at several atomic clocks all set to the same time you'll find that they still agree within ten millionths of a second after a week.
- The gigasecond is the only scientifically-preferred time interval that is passed in adult life.
- 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.
- 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.
- 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.
- Yes there are - some are listed here.
- The time at which summer time begins and ends is given in the relevant EU Directive and UK Statutory Instrument as 1 a.m. Greenwich Mean Time (GMT).
- 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.
- A number of factors need to be taken into account when considering sources for calibrating radiation thermometers.
- Yes - Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosed space.