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Random FAQs
- Your barometer is undoubtedly scaled in inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter).
- Force is a measure of the interaction between bodies, 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) and load usually means the force exerted on a surface or body.
- 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.
- 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.
- To the best of our knowledge, the NPL on-line barograph is unique inasmuch as it provides a fully traceable pressure measurement, with a properly calculated uncertainty, and also allows historical data to be displayed both graphically and digitally.
- Check plugs and parametric measurements are not mutually exclusive. It is NPL's opinion that for checking master gauges (i.e. not products) both check plugs and parametric measurements have their place.
- The length of time that it will take for the step gauge to reach ambient temperature in the laboratory will depend on both its mass and the temperature differential between the car boot and the laboratory.
- Fundamentally, temperature is the potential for heat transfer. Thus an object at a higher temperature will lose heat to an object at a lower temperature when they are placed in contact.
- There is no clear boundary between pressure and vacuum and the word vacuum simply refers to part of the pressure scale. Its definition is not precise but it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure.
All FAQs
- 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.
- Mercury barometers must be firmly supported, made vertical and given time to adopt ambient temperature before attempting to use them.
- 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 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.
- Pressure is not a base quantity but a derived quantity, with dimensions of length (L), mass (M) and time (T).
- Radiation thermometers measure the thermal energy emitted by a source and relate this to its temperature by means of the Planck law of radiation. They consist of optics (generally lenses) to collect and focus the emitted energy onto a detector.
- NPL gets the traceability of its 300 mm diameter Master Flat from the surface of a pool of silicone oil, which conforms to the local radius of curvature of the earth and from which the departure from flatness can easily be calculated.
- Mass is therefore a base quantity in the SI and its unit is the kilogram, which is abbreviated to kg.
- The time displayed on a well designed sundial will vary from the mean time during the year, up to 16 minutes ahead (November 3), and up to 14 minutes behind (February 11), only agreeing around April 15, June 13, September 1 and December 25th. Subject to these corrections a good sundial can be read to within a few minutes accuracy.
- 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.