Dimensional Measurement - FAQs
The answers to some frequently asked questions about length metrology are grouped below.
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FAQs
General
- In some situations.
- There are many good books and in addition NPL produces a range of Good Practice Guides relating to dimensional metrology.
- No. Since 1983 the metre has been re-defined in terms of the speed of light and is realised using laser wavelengths.
- No, but there are at least two companies that do.
- Not for a long time, and we are unsure of any companies that do?
- It depends on the exact item. Generally we try to offer calibrations in imperial units, where we have the facilities available.
- Formulae are given in Elastic Compression of Spheres and Cylinders at Point and Line Contact by M J Puttock and E G Thwaite.
- There is no fixed period for the validity of a calibration certificate - it is not like your cars MOT certificate. However, measurement results stated on certificates are usually 'on the day' values and no allowance is made for subsequent drift.
- The words accuracy and uncertainty are sometimes interchanged but the difference between them is significant and, in many applications it is vital.
- The (international) inch has been exactly 25.4 mm since July 1959. At this point in time the (international) yard was redefined as 0.9144 metre.
- 1 inch = 25.4 mm.
- An internet search should lead you to companies which sell such items.
- The only UK laboratory that is UKAS accredited for the calibration of theodolites is Falcon Scientific Instruments.
Accreditation
- Some services at NPL have been accredited by UKAS and for calibrations made using these accredited services, a UKAS certificate is issued.
- UKAS is the United Kingdom Accreditation Service.
International
- The services of NPL are equivalent to those offered by NIST (and other National Labs), as covered by the Mutual Recognition Arrangement.
- 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.
- Traceability is the property of a measurement or the value of a standard whereby it can be related to stated references.
Standards
- MOY/SCMI specifications are available on the NPL Length web-site.
- Metrology/Specification Certification Measuring Instruments.
Temperature
- To correct a length you need to calculate the difference in temperature multiplied by the product of the length and the expansion coefficient.
- 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.
- The internationally agreed temperature is 20°C.
Tolerancing
- In short the answer is no. What you do need to take into account is the uncertainty of measurement.
- The 10% rule says that if a component tolerance is given as x then the instrument used to demonstrate compliance should have an uncertainty of better than x/10.
CMMs
A manufacturer will calibrate a CMM and produce an error map that corrects for the various geometric errors in the machine. When delivered an acceptance test will be carried out to verify that it meets its specification. This verification should be repeated periodically, in addition interim checks should be carried out.
End Standards (gauge blocks, length bars)
- The micrometre (0.001 mm) gauge should be taken first, followed by the hundredth, tenth and millimetre gauges.
- Following these steps ...
- There are several factors which contribute to the discussion on re-calibration intervals for gauge blocks.
Screw Threads
- 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 complete inspection by direct measurement of parallel screw threads necessitates the measurement of the effective diameter which, in the case of external threads, is obtained by measurement across cylinders placed in the threads on either side of the screw.
- This publication has been updated by NPL and an electronic version has been put on the website.
Surface Texture
- It depends on the job at hand. However, you should have at least ...
- Yes, the Measurement of Surface Texture using Stylus Instruments, Guide No 37.
- The following is a list of manufacturers and distributors ...
- Stylus (mechanical contact) and optical (non-contact) instruments operate on different physical principles and will always tend to differ in the results they give for a surface.
Optical
- To some degree yes. Ideally, the reflectivity of both the reference and test surface should be equal to give maximum contrast fringes.
- 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.
- We would recommend that reference flats in interferometers be recalibrated every three to five years depending upon the precision of measurement required.
- The HSE/NPL Test Slide was developed by the Health and Safety Laboratory (HSL) on behalf of the Health and Safety Executive (HSE) in collaboration with PTR Optics and the National Physical Laboratory.