Definitions Handout

What is METROLOGY?

Metrology is the science of measurement, embracing both experimental and theoretical 

determinations at any level of uncertainty in any field of science and technology.

What is a MEASUREMENT?

Process of experimentally obtaining one or more quantity values that can reasonably be 

attributed to a quantity.

What is a MEASURAND?

Quantity intended to be measured

What is a CALIBRATION?

Operation that, under specified conditions, in a first step, establishes a relation between the 

quantity values with measurement uncertainties provided by measurement standards and 

corresponding indications with associated measurement uncertainties and, in a second step, uses this 

information to establish a relation for obtaining a measurement result from an indication

Measurement Result + Uncertainty

*Do not confuse calibration with ‘adjustment’*

What is TRACEABILITY? (Answer is in 3 parts)

Metrological traceability: Property of a measurement result whereby the result can be related 

to a reference through a documented unbroken chain of calibrations, each contributing to the 

measurement uncertainty

Metrological traceability chain: traceability chain sequence of measurement standards and 

calibrations that is used to relate a measurement result to a reference

Metrological traceability to a measurement unit: metrological traceability where the reference

is the definition of a measurement unit through its practice realization

Note: The expression “traceability to the SI” means ‘metrological traceability to a measurement unit of 

the International System of Units.’
What is OBJECTIV EVIDENCE?

Proof in the form of records

What is a DOCUMENT?

To maintain. A collection of instructions. How to. Procedure

What is a RECORD?

To retain. Your completed document. History. Evidence

What is a lab’s SCOPE?

Measuring capabilities. 
Metrology Acronyms

Acronym Name

NIST National Institute of Weights & Measures

AB Accrediting Body

CMC Calibration and Measurement Capability (scope)

GLP Good Laboratory Practices

GMP Good Measurement Practices

GUM Guide to the Expression of Uncertainty in Measurement

RMO Regional Metrology Organization

SDO Standards Development Organization

SI International System of Units

SOP Standard Operating Procedure

SAP Standard Administrative Procedure

VIM International Vocabulary of Measurement

RMAP Regional Measurement Assurance Program

SWAP Southwestern Measurement Assurance Program (our region of RMAP)

BIPM International Bureau of Weights and Measures

ILAC International Laboratory Accreditation Cooperation

OIML International Organization for Legal Metrology

ASTM American Society for Test and Materials

NCSLI National Conference of Standards Laboratories, International

ISO International Organization for Standardization
Symbol Description 

S Standard reference weight

X Weight to be calibrated (unknown)

Sc Check standard

t Small calibrated tare weight, A subscript to s or x is used to indicate the larger 

weight with which it is associated 

sw Small calibrated weight used to evaluate the sensitivity of the balance

M The mass (true mass) of a specific weight. Subscripts s, x, t, sw are used to 

identify the weight (equals Nominal plus Correction)

N The nominal value of a specific weight. Subscripts s, x are used to identify the 

weight

C The correction for a specific weight. Subscripts s, x are used to identify the 

weight.

CM The conventional mass of a specific weight. Subscripts s, x, t, sw are used to 

identify the weight.

ρa Density of air at time of calibration

ρn Density of normal air (1.2 kg/m3)

ρ Density of masses; subscripts s, x, ts, tx

, sw are used to identify the weight

dof Degrees of freedom

k k value; coverage factor

O Observation; subscripts 1 - ∞ are used to identify the observation

d bias

u uncertainty

us Uncertainty of the Standard mass(es)

sp Accepted standard deviation of the process

udrift Uncertainty of the allowable drift

usw Uncertainty of the allowable sensitivity error

ub Uncertainty of the maximum magnitude of the buoyancy correction

ud Uncertainty associated with bias

uo Uncertainty due to other factors

U Expanded uncertainty (u * k)

MABC Magnitude of the Air Buoyancy Correction

En Normalized Error Test

Pn Normalize Precision Test
Symbol Description 

Vx60 Volume of the unknown vessel at 60°F

Vs60 Volume of the standard vessel at 60°F

NSCV Neck Scale Correction Value

srf Scale reading, final

sri Scale reading, initial

ρ1, ρ2,... ρN

Density of the water in the standard prover where ρ1 is the density of the water 

for the first delivery, ρ2 is the density of the water for the second delivery, and 

so on until all N deliveries are completed

Δ1, Δ2,...,

ΔN

Volume difference between water level and the reference mark on the 

standard where the subscripts 1,2,...N, represent each delivery as above. If the 

water level is below the reference line, Δ is negative. If the water level is above 

the reference line, Δ is positive. If the water level is at the reference line, Δ is 

zero for slicker-plate type standards.

t1, t2,...tN Temperature of water for each delivery with the subscripts as above

αα Coefficient of cubical expansion for the standard in units /°F

β Coefficient of cubical expansion for the prover in units /°F

tx Temperature of the water in the filled unknown vessel in units °F

ρx Density of the water in the unknown vessel in g/cm3

um Ability to read the meniscus in both S and X
What are the differences between mass, true mass and 

conventional mass values? 

Mass and True Mass

The mass of a body relates to the amount of material it contains (see full definition) and there is 

no difference between mass and true mass. The prefix true is sometimes added to the word mass

where it is important to make it clear that a particular value of mass being considered is not a 

conventional mass value and it is particularly important to avoid this potential ambiguity when, 

for example, specifying the value of weights (see below).

The international prototype kilogram, on which the mass scale throughout the world is realized, 

is defined as a true mass of exactly 1 kilogram. Most high accuracy comparisons are performed 

on a true mass basis but the values are usually converted to conventional mass values when 

quoted on a certificate.

Conventional Mass

When a weight is calibrated the mass value quoted on its certificate of calibration is normally a 

conventional mass value - appropriate where the value is determined by weighing the item in air 

in accordance with International Recommendation OIML R 33. This Recommendation says 

formally: For a weight at 20 °C, the conventional mass is the mass of a reference weight of a 

density of 8 000 kg/m3 that it balances in air of density 1.2 kg/m3.

Some background to conventional mass

Air is a fluid and, as such, exerts a buoyancy force on all objects weighed in it. The value of 

buoyancy will depend on the volume of the object being weighed and the density of the air 

surrounding it - which itself changes with temperature, humidity, composition and pressure (see 

air density change with altitude). The buoyancy effect cancels out if a weight is calibrated on a 

two-pan balance by comparison with a reference weight of identical density. In practice though, 

the weight being calibrated will undoubtedly have a density value that is different to that of the 

reference weight (even if only slightly) and a correction for the differential buoyancy effect will 

need to be applied.

Every day hundreds of thousands of weighings are performed where the accuracies required 

make buoyancy effects very significant. It is not realistic to perform such measurements in 

vacuum - where there is no buoyancy effect - but, even if this were possible, most weights are 

used in air and the mass values attributed to them must reflect the conditions under which they 

will be used.

International Recommendation OIML R 33 essentially provides an in-air 'datum', or set of 

reference conditions, which defines the conventional mass of a weight, at 20 °C, as the (true) 

mass of a reference weight of a density of 8 000 kg/m3 that just 'balances' it when in air of 
density 1.2 kg/m3 (see above for exact wording). In practice these conditions will not be realized 

exactly and small corrections for temperature, material density and air density have to be applied 

to allow for this.

Completely unofficial text for conveying the spirit of what is meant by conventional mass might 

therefore be a value, slightly different from the true mass value, which an object appears to have 

when it is used in normal-ish ambient conditions - but please don't quote us on this one!

Last Updated: 25 Mar 2010

Created: 9 Aug 2007

Source:

http://www.npl.co.uk/

http://www.npl.co.uk/reference/faqs/what-are-the-differences-between-mass,-true-mass-andconventional-mass-values-(faq-mass-and-density)
Mass calibration (metrology) terms explained 

in plain English.

Air Buoyancy

The buoyant effect of air on an object, acting against the attraction of gravity.

The air is always exerting a lifting effect on an object – on all objects. The magnitude of that 

lifting effect is a direct function of the density of the air relative to the volume of the object 

under consideration.

“Normal” air under “perfect” conditions, that is, 20.00 degrees Celsius, atmospheric pressure of 

760.5 mm of mercury, and humidity of 50%, has a density of 1.200 mg/cm3.

From a practical standpoint, everything we weigh is surrounded by air, so the convention for 

expressing the weight, or the mass, of an object, has evolved to express that perceived weight as 

“mass in air”, or Conventional Mass.

Conventional Mass (Mass in air versus reference density of 8.0 g/cm3)

A hypothetical weighing of a mass in “perfect” air, that is, air of typical composition, at exactly 

20.00 degrees C, atmospheric pressure of 760.5 mm of mercury, and a relative humidity of 50%.

Under these “perfect” conditions, the density of the air would be 1.2000 mg/cm3.

Since realization of these parameters is a practical impossibility, in a calibration lab the ambient 

conditions are stabilized to the greatest extent possible, and the temperature, pressure and 

humidity are carefully measured. From these values the density of the air is calculated, and the 

results of the calibration are adjusted mathematically to present the result as though the 

calibration had been conducted in “perfect” conditions.

No discussion of Conventional Mass would be complete without a mention of the 8.0 g/cm3

standard. Stainless steel is the “standard” material from which most high quality laboratory 

weights are fabricated. Stainless steel has a density of approximately 8 g/cm3, hence the 

reference. Years ago, brass was the most common material for laboratory weights, and 8.4 g/cm3

was the reference density.

In the real world, the ideal density of 8.0 g/cm3 is seldom realized. Most stainless steel weights 

have densities of from 7.84 to 7.95 g/cm3, although some of the new weights on the market come 

very, very close to the ideal density of 8.0 g/cm3.

True Mass (Mass in vacuum)
Contrary to what the name implies, “Mass in Vacuum” is NOT what a weight would weigh in a 

perfect vacuum, with no air buoyancy effect. The term ‘Mass in Vacuum’ is a misnomer, and 

causes confusion.

To illustrate the concept of True Mass, consider the following hypothetical example: We have 

two perfect kilogram weights, each having a Conventional Mass value of exactly 1000.000000 g. 

The first mass is a calibrated reference mass, made from stainless steel, with an ideal density of 

8.0 g/cm3. The second weight is a calibrated mass is also made from stainless steel, but has a 

commonly seen density of 7.95 g/cm3.

Now let’s consider effect of the air buoyancy on each of the masses:

From basic physics we know that Volume = Mass / Density, so:

The first stainless steel mass has a claimed density (from the manufacturer) of 8.0 g/cm3

, 

accordingly, the volume of this mass is:

1000.000000 divided by 8 = 125.00000000 cm3

The second stainless steel mass has a density from the manufacturer of 7.95 g/cm3, so its volume 

is:

1000.000000 divided by 7.95 = 125.78616352 cm3

Now – if we could compare these two masses inside a perfect vacuum, there would be NO air 

buoyancy effect, so we can calculate how much MORE each mass would weigh in a vacuum by 

multiplying the volume of the mass by the density of “perfect” air:

The 8.0 g/cm3 mass: 125 x 1.200 = 150.00000000 mg

The 7.95 g/cm3 mass: 125.78616352 x 1.200= 150.943396224 mg

We can see that the 7.95 mass weighs more (in a vacuum) than the 8.0 mass by .943396 mg:

 150.943396224

-150.000000000

————————-

 0.943396224

Accordingly, the True Mass of the 7.95 mass is 1000.000944 (rounding to 6 decimal places).

True Mass, therefore, is not what a weight would weigh in a vacuum, but rather, what it would 

weigh compared against a reference standard mass (with a known value) on a “perfect” equal 

arm balance inside a “perfect” vacuum chamber.’
The magnitude of the difference between the Conventional Mass and True Mass is the 

effect of the air buoyancy on the difference between the volumes of the two masses.

Now we can turn it around and convert back to Conventional Mass:

The formula (from NIST mass calibration procedures) to convert True Mass to Conventional 

Mass is as follows:

Where:

CMx = Conventional Mass of weight X

Mx = True Mass of weight X

Pn = ‘Normal Air’ density of 1.2 mg/cm3

Px = Density of weight X (7.95)

Work the calculation (1000.000944(1-0.0012/7.95))/(1-0.0012/8) and the result is 1000.000000 

(rounded to 6 decimal places).

A well equipped calibration laboratory seeking to undertake high precision measurements of this 

type, approaches the calibration of mass by creating an environment where the ambient variables 

are stabilized to the greatest extent possible; then precisely measuring temperature, atmospheric 

pressure and humidity, calculating the air density from these parameters, and then conducting a 

matrix of intercomparisons (weighings) between a standard, the weight being tested, and a check 

standard (a second calibrated mass), using approved procedures and mathematical equations. In 

the course of the calibration the buoyant effects of the air on the standard, the test weight and the 

check standard are calculated and compensated for. The laboratory then reports Conventional 

Mass and True Mass with an appropriate measurement uncertainty attributable to each result.

Think about it: 

• If the density of the mass is less than 8.0 g/cm3, its True Mass will be greater than its 

Conventional Mass.

• The inverse is also true – a mass with a density greater than 8.0 g/cm3 will have a True 

Mass value smaller than its Conventional Mass value.

• A mass with a density of 8.0 g/cm3 will have exactly the same value for its True Mass 

and Conventional Mass.
Bottom line: use the Conventional Mass values for standard weight uses – i.e., balance 

calibration and verification. The True Mass values are typically used only by calibration 

laboratories, or for reference information.

Source:

https://icllabs.com/

https://icllabs.com/mass-calibration-metrology-terms-explained-in-plain-english/