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National Measurement Institute
      

Mass and Related Quantities Capabilities

NMI maintains specialised calibration facilities for mass and related quantities and is well equipped to perform many types of measurement of mass, density, flow, force, pressure, viscometry and volume. Click here:

Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact calibration.coordinator@measurement.gov.au.

Mass

The primary standard of mass is the International Prototype Kilogram, a platinum-iridium cylinder held at the International Bureau of Weights and Measures. Copy No. 44 of the International Prototype Kilogram is held by NMI as the Australian standard of mass, and its value is known to 5 ppb. NMI uses the Australian standard to calibrate a set of 1 kg stainless steel standards which, in turn, are used to calibrate sets of standards comprising masses from 0.5 mg to 20 kg. These standards form the basis for all calibration work on mass as well as for the measurement of such quantities as force, pressure and density.

Sets of Weights

NMI's calibration service is intended primarily for weights used for reference purposes in science and industry and should be requested only when it is essential to know the masses of the weights with an accuracy approaching 1 ppm. All weights submitted for calibration are expected to be of appropriate construction (see below), in good condition, and to be sufficiently close to nominal that they warrant the accuracy available at NMI.

Weight sets for which it is adequate to know the uncertainty to 2 ppm or greater can be calibrated by laboratories registered by the NATA.

All calibrations are done at laboratory conditions close to 20 °C.

Basis of Calibration

Weights made from material having a density within the range 7500 to 8500 kg/m³ are calibrated on a weight-in-air basis in which the weights are compared in air with standards of known mass and of appropriate density without applying buoyancy corrections. The mass assigned to each weight is that of a hypothetical weight of density 8000 kg/m³ which, in air of density of 1.2 kg/m³, would exactly balance the weight under calibration. When calibrated in this way, the weights can be used as a set of standards of mass of uniform density 8000 kg/m³.

In use such weights should be assumed to have a uniform density of 8000 kg/m³ and air buoyancy corrections if required should be based on this figure and on the actual density of the air in the balance case. If the actual air density falls within the range 1.14 to 1.26 kg/m³, the residual error in air buoyancy due to the use of this procedure will not exceed:

  • 0.5 ppm for weights of stainless steel, nickel-chromium alloy or other material of density not less than 7500 nor greater than 8500 kg/m³
  • 5 microgram for weights of platinum or gold up to a total value of 1 g
  • 1.5 microgram for aluminium weights up to a total of 0.1 g

General Requirements for Weights

The material, construction and finish of weights submitted for calibration must be such as to ensure that their values will be substantially unaffected by changes in ambient atmospheric conditions.

Weights should be made from austenitic stainless steel, nickel-chromium alloy, or other non-magnetic and stable materials. They should also be integral, i.e. consisting of a single piece of material. Weights of plated or lacquered brass or bronze are not accepted for calibration. All sharp corners must be removed from weights and the surface should be smoothly finished and free from porosity. A high polish is not necessary.

Uncertainty of Measurements

The uncertainty of the calibrated weights will be given according to the weights of classes defined in OIML R 111.

Weights can, if required, be calibrated on a true mass basis or to a higher order of accuracy than that quoted. Arrangements for such special calibrations should be made with NMI before the weights are submitted.

Calibration to a special order of accuracy is offered for weights of nominal mass 10 mg and less which are required for use in microanalysis. The results and the associated uncertainty of these tests are rounded to the nearest 0.1 microgram.

Balances

NMI calibrates balances only under special circumstances. In general, tests on nearly all types of balances can be carried out by laboratories accredited by NATA.

Density

Density measurements are directly traceable to the Australian standards of mass and length. Suitable liquids are chosen as density standards and their density determined by the hydrostatic weighing of volume standards of precisely known dimensions. Hydrometers to be used as secondary standards are then calibrated in these liquids.

In general all results are referred to a standard temperature of 20 °C. If required results may be given with reference to a standard temperature other than 20 °C but if this involves additional work there will be a corresponding increase in the fee.

Density of Solids

Solid specimens that present no special difficulties, such as arise from porosity or liability to chemical attack, are weighed in air and in a suitable liquid of known density the density of the solid being computed using the Principle of Archimedes.

Specimens should have a volume of at least 5 cm³, be compact with smooth surfaces and weigh not more than 250 g. The uncertainty for the density determination of a specimen conforming to these general requirements is 1 in 103.

Where necessary and where the size and nature of the specimen permit, the density of a solid specimen may be measured with an uncertainty of a few parts in 105.

Density of Liquids

The densities of liquids that present no special difficulties, such as arise from high volatility at room temperature, corrosive properties, high viscosity or toxicity, are measured by means of a pycnometer or vibrating element densitometer. The sample volume required is dependent on the method used. The specimens should be supplied in sealed containers clearly labelled to indicate their contents.

The uncertainty for the density determination of a liquid sample that presents no special difficulties is 5 in 105.

Densities of liquids may be determined to a lower uncertainty by hydrostatic weighing. This normally requires a large sample volume.

Hydrometers

NMI calibrates glass hydrometers having a sensitivity greater than type M100, described in AS 2026 Density Hydrometer, and graduated in density or specific gravity units or having recognised scales such as Brix, Baume, Twaddle, API or percentage alcohol by volume. Pressure hydrometers for use in liquefied petroleum gas are also accepted for calibration. No hydrometer will be accepted unless it carries a serial number on the scale.

Hydrometers are calibrated at five points on the scale and the results are given with an uncertainty which is usually around one-fifth of a scale division. More sensitive hydrometers can be calibrated to a lesser uncertainty.

Since the indication of a hydrometer is affected by the surface tension of the liquid in which it is floating the value of surface tension applicable to the liquid in which it is floating should be marked on the hydrometer. When a hydrometer received for calibration is not so marked, a value of surface tension that is considered appropriate to the instrument will be selected from AS 2026-1994 and stated in the report on the instrument.

Glass hydrometers having scales other than those above or for use at elevated temperatures will only be accepted for calibration after consultation with NMI staff.

Metal hydrometers are not usually accepted for calibration.

Flow

Liquid Flow — Volumetric Prover Calibration

The traceability of liquid flow is usually done through volume provers. These discharge a known volume of the liquid at different rates through the meter to be proved. Volume provers are calibrated by NMI either on-site or, by suitable arrangement, on other sites. The particular calibration requirements, and applicable fees, should be discussed with NMI staff.

Liquid Flow – Petroleum Meter Calibration and CNG Meters

NMI's petroleum meter calibration service is carried out at our Flow Measurement Facility.

Petroleum meters are calibrated using test fluids known as NORPAR and D130 (which have viscosities and densities similar to petrol and diesel fuel respectively) at flow rates up to 4 000 L/min and pressures up to 850 kPa. The calibration system uses a 60 L piston prover and can provide uncertainties under 0.05%.

Liquid petroleum gas (LPG) meters are calibrated using liquid propane and butane at flow rates up to 1 700 L/min and pressures up to 850 kPa above product equilibrium vapour pressure. The calibration system uses a 40 L piston prover and can provide uncertainties under 0.10%.

Compressed natural gas (CNG) meters are calibrated at up to 100 kg/min and pressures up to 300 bar. The calibration system uses sonic nozzles to control flow rate and the dispensed gas is weighed on a scale.

We also offer pattern approval testing of metering systems.

Gas Flow

NMI offers a range of gas flow calibration services using various types of gases for many instruments such as:

  • Ultra-sonic meters
  • Variable area flowmeters
  • Orifice plates
  • Mass flow meters
  • Laminar flow elements
  • Rotary meters
  • Turbine meters
  • Wet gas meters
  • Coriolis meters
  • Critical flow Venturi (or sonic) nozzles
  • Soap film meters.

Currently, the services available at NMI cover a wide range of flows and pressures as follows:

  • Calibration of critical flow Venturi nozzles for volume flowrate (normalised to atmospheric conditions and room temperatures) up to 250 m³ h-1 and upstream pressures up to 7 bars. NMI also provides services for calibration of nozzles at various pressures and relative humidities.
  • Calibration of flow measurement devices for flowrate ranging from 0.1 cc min-1 up to 7000 m³ h-1 (normalised to atmospheric conditions and room temperatures) using various gases
  • Calibration of flow measurement devices at pressures up to 7 bars and flowrates up to 500 kg h-1

The gas flow facility at NMI is a NATA accredited laboratory.

Force

NMI maintains force standardising machines which allows all force measurements done by NMI to be traceable to the Australian standards for mass, length and time. NMI maintains deadweight force standardising machines and a hydraulic force machine.

The deadweight machines, absolute machines by definition, operate in the range of 2 N to 550 kN in both tension and compression modes. The hydraulic machine covers the range from 0.2 to 4.5 MN in the compression mode. Although the latter machine is not an absolute force standardising machine, it is traceable to the deadweight machine using reference load cells. These machines are used to characterise or calibrate force measuring instruments such as proving rings, load cells and aircraft weighing kits used as transfer standards in industry. The table below lists the principal capabilities of these machines.

Capabilities of force standardising machines
Force range Minimum force increment Uncertainty of applied force
2 to 100 N (compression and tension) 0.01 N 0.002%
50 to 5600 N (compression and tension) 1 N 0.002%
5 to 550 kN (compression and tension) 200 N 0.002%
0.2 to 1.5 MN (compression) 0.02%
1.5 to 4.5 MN (compression) 0.04%

 

NMI calibrates high-quality force measuring devices based on AS 2193-2005 Methods for the Calibration and Grading of Force-measuring Systems of Testing Machines, clause 4.4 Elastic Calibrating Devices. Forces up to 550 kN will, in general, be applied only in multiples of the minimum force increment given the in table above. Values of the applied forces and the deflections of the test instrument for each direction of loading will be supplied with the report, together with the equations relating force to deflection and vice versa. The force range for which the instrument meets the classification requirements of AS 2193-2005 is also given in the report.

Arrangements may also be made for the calibration of instruments which are not subject to AS 2193-2005, such as aircraft weighing kits. Calibrations based on other national or international standards may be performed after consultation with NMI staff.

Under certain circumstances, we will also undertake the testing of hydraulic jacks used in the construction industry.

The routine testing of materials is not undertaken and the calibration of testing equipment is undertaken only when facilities are not available elsewhere. Contact NATA for more information.

Pressure

Pressure is defined as force per unit area so that the normal unit of pressure in the SI system is the newton per square metre which is given the name Pascal (Pa). Pressure metrology with gas and hydraulic fluids in gauge, absolute and differential modes, using liquid columns, high-precision pressure devices and high-quality piston gauges are performed at NMI.

NMI's primary standard of low pressure is a mercury column manometer. This interferometric manometer is used to calibrate pressure balances that are used for the working standards and in build-up processes to reach higher pressures.

NMI's primary standard of high pressure is a large diameter piston gauge and a controlled clearance piston gauge. These piston gauges are used to calibrate pressure balances that are used for the working standards and in build-up processes to reach higher pressures. NMI has established an independent national pressure scale to 500 MPa (5000 bar) at appropriate levels of uncertainty.

Generally pressure devices in the range up to 130 kPa absolute and gauge are calibrated at the Sydney laboratory and devices above this range are calibrated at the Melbourne laboratory, however some barometers and pressure transducers are calibrated at both laboratories. In special cases, e.g. where facilities for a particular calibration are not available elsewhere, the calibration of pressure gauges will be undertaken up to a maximum pressure of 500 MPa.

In general, all results are referred to a standard temperature of 20 °C. If required, results may be given with reference to a standard temperature other than 20 °C.

Mercury Barometers (Fortin)

Due to the problems of transportation and safety concerns of mercury, Fortin barometers are no longer accepted for calibration. Calibration is traceable by comparison with either an NMI travelling standard or via the client's own aneroid/digital barometer.

When NMI performs the calibration the Fortin barometer is checked at one point in situ at the ambient pressure operating at the time of the calibration using a travelling standard barometer. This barometer is compared against NMI standards pre- and post-calibration. In Fortin barometers, the mercury surface in the cistern must appear clean and bright and the tip of the pointer must be sharp and clean.

Barometers or manometers which have an accessible pressure port which may be connected in parallel with the travelling standard may be calibrated over their full range of operation, taking into account the stability of the ambient temperature and the ability to apply appropriate corrections for gravity and temperature.

The thermometer attached to a barometer is not given a detailed examination unless this is specially requested. Ordinarily the thermometer is checked for gross error only at the temperature of test.

In situ Fortin barometer calibrations are usually only carried out in the greater metropolitan areas of Sydney and Melbourne. Other types of mercury barometers (Kew etc.) may be calibrated in situ.

Aneroid/Digital Barometers

Depending on the uncertainty of measurement required, aneroid/digital barometers are calibrated by comparison with either a standard pressure balance, reference standard barometer or secondary standard pressure transducer over the calibration range required by the client. Electronic barometers with in-built software processing including temperature compensation are treated as direct reading devices if they have an associated display module. Otherwise arrangements must be made to provide the associated software tools to interface and access the device memory and display.

The corrections and hysteresis effects are determined concurrently by subjecting the instrument to a cycle of pressure changes during which the corrections to be applied to the reading are determined for decreasing and increasing pressures. Tests on the temperature compensation of the barometer are not made unless specially requested.

Manometers

Manometers are generally calibrated at our Sydney laboratory where several standards for calibration in the range 0 to 130 kPa (abs and gauge) are maintained.

High-grade digital and liquid manometers of varying types are accepted for calibration.

Liquid manometers are accepted provided that the manometric fluid is stable, non-volatile, forms a good stable meniscus and has a low viscosity, and the tube is of sufficient diameter to provide an accuracy commensurate with the scale graduation.

The density of the manometer fluid is not measured unless it is specifically requested. Sufficient liquid must be provided at the time of submission of the instrument.

High Vacuum Gauges

NMI calibrates vacuum measuring instruments in the pressure range 1 x 10-3 to 1.3 x 103 Pa absolute against a primary standard, which is a static expansion system, or by comparing with reference standards traceable to the static expansion system. Typical instruments accepted for calibration include capacitance diaphragm gauges, thermal conductivity gauges and silicon resonance gauges. Dry high purity nitrogen is the normal calibration gas. Vacuum instruments above this range can be calibrated but are done against other absolute pressure standards maintained at NMI.

The uncertainty assigned to a client's instrument is dependent on its characteristics. For a high-quality instrument, it is anticipated that the uncertainty is:

  • 0.004 Pa in the range 0.1 Pa to 1 Pa
  • 0.35% in the range 1 Pa to 10 Pa
  • 0.2% in the range 10 Pa to 1300 Pa

Hydraulic Pressure Balances/High Range

Hydraulically operated pressure balances, deadweight testers and piston gauges, with their set of weights are calibrated by cross-float comparison against reference piston gauges.

When submitted for test the instrument must be emptied of fluid and clean. For many common instruments it is possible for the client to send the piston cylinder assembly by itself, which will then be calibrated in a test base. This may add a small further component of uncertainty for those instruments where the variable torque applied to the piston cylinder unit when re-assembling it on the base column may affect to a small degree the stress distribution on the cylinder (and hence the effective area) when pressurised. This is normally a small effect, and in any case is also relevant if the client removes and re-assembles the piston cylinder at any time after they are calibrated together.

The pressure range extends from 100 kPa to 500 MPa, and calibration can be in terms of mass of weights, parameters A0 (effective area at zero pressure and 20 °C), λ (elastic pressure distortion coefficient) and Me (effective taring mass); or pressure equivalent of the weights at 20 °C and defined value of the gravitational constant g (either standard gravity 9.806 65 m/s² or value specified by client).

Two classes of test are offered, a high accuracy test and a standard test.

The high accuracy test can, depending on the quality of the test instrument, achieve an uncertainty on the value A0 as low as around 30 ppm. The test reports the values and uncertainties of A0, λ and Me of the piston cylinder assembly, the mass values and uncertainties of the weights, and if requested the pressure equivalent applicable to each weight together with a table showing the uncertainty as a function of pressure, when the pressure equivalents table is used.

The standard test is suitable for those who require an industrial quality pressure balance capable of calibrating test gauges. The calibration procedure involves fewer test points, and is performed to a lower resolution. A simplified equation, a table showing the pressure generated by each weight and the pressure range of the instrument in which the uncertainty is less than or equal to 0.05% of indicated pressure are presented in the report.

Gas-operated Pressure Balances

There are two classes of tests offered and they are designed to cater for users who require the highest precision and those who require an instrument capable of calibrating test gauges. At the time of submission, the client should specify which test is required.

The high accuracy test involves the calibration of the weights and the piston cylinder unit. Weights are weighed in air against mass standards.Calibration of the piston cylinder unit involves the experimental determination of its instrumental constants A0, λ and M e. This is achieved by cross-floating with a pressure standard of known instrumental constants over a range of pressure from 2 kPa to 7 MPa. A formula incorporating the instrumental constants and all pertinent input quantities is supplied, enabling the client to calculate the pressures generated by the instrument to the highest accuracy. The uncertainty associated with the pressure generated by the instrument under test is also estimated and presented in the report. The uncertainty is typically in the range of 15 to 35 ppm but depends on the quality and condition of the instrument under test.

The standard test is suitable for those who require an industrial quality pressure balance capable of calibrating test gauges. The calibration procedure involves fewer calibration pressures than the high accuracy test to determine the two instrumental constants: effective area over the pressure range of the instrument and Me. A simplified equation, a table showing the pressure generated by each weight and the pressure range of the instrument in which the uncertainty is less than, or equal to, 0.05% of indicated pressure are presented in the report.

Industrial Pressure and Vacuum Gauges

NMI does not normally calibrate industrial pressure and vacuum gauges except where suitable facilities are not available elsewhere.

Note: If a gauge is to be used for the measurement of oxygen pressure, it is the responsibility of the client to advise NMI accordingly and to mark the gauge with a warning in red lettering.

In the examination of such gauges, water is used as the pressure fluid whereas, for other gauges, oil is generally used. The presence of even slight traces of oil in an oxygen pressure gauge, e.g. a contaminated pressure seal, may cause a serious explosion in the plant in which it is subsequently used. Attention is drawn to AS 1349-1986, clauses 3.8.2 and 3.8.3 Pressure Gauges for use with Oxygen and Other Oxidants and Gauges for use with Acetylene.

High-precision Pressure Transducers

NMI calibrates high-precision pressure transducers, for example those with a frequency output and diaphragm types.

Viscometry

NMI maintains a set of master viscometers but, due to limited demand for viscosity standards, does not offer the routine distribution of viscosity-certified materials. Information on alternative sources of viscosity-certified materials is available from NMI.

Volume

In general, all results of volume calibrations are referred to a standard temperature of 20 °C. If required, results may be given with reference to a standard temperature other than 20 °C, but if this involves additional work there will be a corresponding increase in the fee.

Volumetric Measures

NMI usually only accepts high-grade standards of volume of capacity above 2 L. Items with a capacity below 2 L, especially volumetric glassware, can be calibrated by laboratories appointed by NMI as verifying authorities, or NATA-accredited laboratories.

Proving Measures for Industrial Use

Proving measures and mechanical displacement provers can be calibrated to an uncertainty of 0.02%. Since these calibrations must frequently be made in situ and there is considerable diversity in the type of equipment, calibration requirements should be discussed with NMI staff.