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FAQs

Random FAQs

• How would we know if the speed of light was varying with time? (FAQ - Quantum)

The very term fundamental physical constants invites two questions: are they fundamental and are they constant.
• My barometer has a scale marked '28' to '31' - what does this mean? (FAQ - Pressure)

Your barometer is undoubtedly scaled in inches of mercury - properly abbreviated to inHg (with only the 'H' a capital letter).
• Is there a difference between 'accuracy' and 'uncertainty'? (FAQ - Length)

The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.

• 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.
• Why does my radio-controlled clock receive the MSF signal less well at night? (FAQ - Time)

Users of the MSF service receive predominantly a ‘ground wave’ signal. However, there is also a residual ‘sky wave’ which is reflected off the ionosphere and is much stronger at night, this can result in a total received signal that is either stronger or weaker.
• Is it more accurate to measure the depth of a liquid in units like 'metres of water'? (FAQ - Pressure)

No, ultimately it is less accurate. Liquid depth can be determined from pressure measurements and the reason can be seen by looking at the fundamental (and simplified) equation for calculating the pressure at a particular depth in a liquid.
• Is an instrument's accuracy likely to be around the figure given by the manufacturer? (FAQ - Pressure)

Probably not. The figures should be interpreted with caution, especially if an instrument has not been independently calibrated for some time.
• How do I select gauge blocks to pick a specific size combination? (FAQ - Length)

The micrometre (0.001 mm) gauge should be taken first, followed by the hundredth, tenth and millimetre gauges.

All FAQs

• Are all pressure units equally valid? (FAQ - Pressure)

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.
• 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.
• Are inches still legal units? (FAQ - Length)

In some situations.
• Are mercury barometers more accurate than non-mercury ones? (FAQ - Pressure)

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.
• Are there any general 'do's and don'ts' in force metrology? (FAQ - Force)

Yes there are - some are listed here.
• At what time should clocks go forward or back for summer time? (FAQ - Time)

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).
• 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 I use an anodised aluminium plate or a tungsten ribbon lamp, rather than a blackbody source, to calibrate my radiation thermometer? (FAQ - Thermal)

A number of factors need to be taken into account when considering sources for calibrating radiation thermometers.
• Can I use salt solutions to generate values of relative humidity? ... How? (FAQ - Thermal)

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.

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