It depends ...
Rachel said:
Hi everyone,
The term "primary standard" - what exactly does that imply? I know it's a standard reference material that is used for calibration, but what are the requirements for traceability? I googled the term and got a definition from the SixSigma website stating that it had to be traceable to NIST...but I don't know if this is just because the SixSigma website is based out of the US. Is NIST the be-all and end-all, or are there other acceptable sources? What are some of them, out of curiosity? (NIST is the only one that comes to mind for me...)
Rachel,
You actually used two different terms in your question:
primary standard and
standard reference material.
A
primary standard is one "that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity." (item 6,4
International Vocabulary of basic and General Terms in Metrology, ISO, 1993. [Commonly referred to as the "VIM", from the French title.])
The property of being a primary standard is not necessarily an inherent property of the device. A good example is the standard of mass. The primary standards for the International System of Units (SI) are the kilogram artifacts at BIPM near Paris. Many countries also have kilogram artifacts which serve as primary standards within their own systems, but they are all secondary standards with respect to the ones at BIPM. As another example, in one lab the primary DC Volt standard is a 10 V zener reference. A lab with more money may have a Josephson junction array as their primary standard, and the same 10 V zener reference would be a secondary standard for them. Some judgement should be used, though. While it may be appropriate to refer to a zener voltage reference as a primary standard, it would never be appropriate to use an adjustable DC power supply that way.
A
standard reference material is "a material or artifact that has had one or more of its property values certified by a technically valid procedure and is accompanied by ... a certificate or other documentation which is issued by [
a national metrology institute]." (NIST web site, paraphrased.)
There are many types of standard reference materials (SRM). They include but are not limited to chemicals, building materials, biological samples, colour reference samples, x-ray density step tablets, radiological standards, particle size standards and so on.
For a particular application, a SRM might be used as a primary standard; however a primary standard is not necessarily a SRM. For instance, as a general rule you will not find a measuring instrument (micrometer, voltmeter) or a source (gage block, voltage standard) referred to as a SRM.
SRMs are often provided by a national metrology institute, in which case they are "traceable" by definition. They may also be produced by other organizations - such as an association of a particular industry - in which case they are consensus standards. But the values assigned by an NMI may also be the result of a consensus, so ...
Primary standards are often confused with intrinsic standards! An intrinsic standard is virtually always an primary standard, but the reverse is not true. An
intrinsic standard is "a standard recognized as having or realizing, under its prescribed conditins of use and intended application, an assigned value the basis of which is an inherent physical constant or an inherent and sufficiently stable physical property." (
NCSL Glossary of Metrology-related Terms, NCSL International, 1999.) An intrinsic standard includes by reference the methods and conditions for its use. The value and uncertainty is assigned by consensus and does not need to be established by comparison or calibration. However, intercomparison or some other form of verification is highly recommended.
Probably the most commonly used intrinsic standard is an ice point bath. When properly made and used an ice point bath is an intrinsic standard at 0.0 degrees C with a small uncertainty that I would have to look up. Other commonly used intrinsic standards are a caesium beam oscillator, which is a standard for frequency and time interval when properly operated; and a Josephson junction array, which is a standard for voltage when properly operated.
As Hershal said, traceability is always to the relevant SI base or derived units, because the purpose of traceability is to establish the relationship between the measurement result you have just obtained, and the SI value. Note that traceability applies only to
the result of a measurement: an instrument, certificate or laboratory is never "traceable". Traceability is usually documented by proof of calibration in a chain leading to a national metrology institute (NMI). Most NMI's are members of regional cooperatives and intercompare their measurements with each other and BIPM. Results are published in the Key Comparison Database and can be accesed through the BIPM web site (
https://www.bipm.fr/)
(To clarify, the meter is defined as the legth of the path traveled by light in vacuum in the interval 1/299 792 458 second. As the duration of the second is fixed by the oscillation of the caesium atom, a side result is that the speed of light in vacuum is now a fixed constant.)
Summary: calling something a primary standard depends on its location and use. A primary standard for one organization may be a secondary standard for another. The value of a primary standard must be traceable to the SI through an NMI except if the primary standard is also an intrinsic or consensus standard. Common sense dictates that things which are not of the higher grades of metrological quality should not be used as primary standards.
Are you confused yet?
