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# FAQs

## Random FAQs

• ### How do I correct lengths to 20ºC? (FAQ - Length)

To correct a length you need to calculate the difference in temperature multiplied by the product of the length and the expansion coefficient.
• ### Is it acceptable to apply rounding to tolerances? (FAQ - Length)

In short the answer is no. What you do need to take into account is the uncertainty of measurement.
• ### What do ‘high vacuum’ and ‘low vacuum’ mean? (FAQ - Pressure)

The terms high- and low- (and also medium-) vacuum are not very intuitive; they are used to describe the various pressure ranges known under the general banner of vacuum but are neither defined nor used consistently.
• ### Is a calibration the same as a specification? (FAQ - Force)

No, definitely not.
• ### How accurate are pressure unit conversion values? (FAQ - Pressure)

When converting between pressure units consideration should be given to the number of significant figures to use, bearing in mind that many of the underlying conversion factors are not themselves exact and cannot be made so.
• ### What does MOY/SCMI stand for? (FAQ - Length)

Metrology/Specification Certification Measuring Instruments.
• ### How should I clean, handle and store my weights? (FAQ - Mass & Density)

Refer to our Good Practice Guide.
• ### What are the main characteristics of force measurement systems? (FAQ - Force)

Force measurement systems can involve a number of different physical principles but their performance can be described by a number of common characteristics and terms.
• ### What is emissivity and why is it important? (FAQ - Thermal)

All objects at temperatures above absolute zero emit thermal radiation. However, for any particular wavelength and temperature the amount of thermal radiation emitted depends on the emissivity of the object's surface.

## 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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