1 Standard Weights and Classes of Balances Nihal Gunasekara Sri Lanka Bangladesh BEST Programme.

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Standard Weights and Classes of Balances

Nihal Gunasekara

Sri Lanka

Bangladesh BEST Programme

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Primary Measurement Standard

The international prototype kilogram made from platinum iridium

alloy and maintained at the BIPM - France

The kilogram remains to be the only artifact standard of the SI system of units

Definition:Measurement standard established using a primary reference measurement procedure, or created an artifact, chosen by convention


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Primary Measurement Standard

Redefining the Kilogram through the Avogadro Constant

It is proposed to redefine the kilogram in terms of the Avogadro constant NA By definition an Avogadro number of carbon-12 atoms weight exactly 12g


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Secondary Measurement Standards

Definition:

Measurement standard established through calibration

with respect to a primary measurement standard for a

quantity of the same kind

Prototype 1kg Secondary Standard


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Reference Measurement Standards

Definition:

Measurement standard designated for the calibration of

other measurement standards for quantities of a given

kind in a given organization or at a given location

E2 Set of Weights


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Working Measurement Standards

Definition:

Measurement standard that is used routinely to calibrate

or verify measuring instruments or measuring systems

M1 Set of Weights


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Travelling Measurement Standards

Definition:

Measurement standard some times of special

construction, intended for transport between different

locations

Portable battery operated caesium-133 frequency measurement standard


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Mass and Weight Mass- Amount of matter in an object.

F = m x a

(Force) = (mass) x (acceleration)

Weight- Force experienced by it due to the earth’s

gravity

W = m x g

(weight) = (mass) x (acceleration due to gravity)M

W=mg where g is the acceleration due to gravity


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Mass Standards- Types of Masses

Masses are classified into categories depending on : Material and Construction

Main Types :

Integral Masses – non magnetic stainless steel

Non-integral or two piece masses - non magnetic stainless steel (can be

adjusted)

Masses made from brass – plated or un plated ( integral or non-integral )

Cast iron masses – usually painted


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Mass Standards- Classes of Masses

Classification of mass standards based on :

Conventional value and nominal value :

Deviation = Conventional value – Nominal value

OIML classification system is widely used

OIML R111 - 1

Masses are classified as : E1 E2 F1 F2 M1 M2 M3


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ACCURACY CLASS

0.200.060.0200.0061 mg

0.200.060.0200.0060.0035 mg

0.250.080.0250.0080.00310 mg

0.40.120.040.0120.00450 mg

1.60.50.160.050.0160.005100 mg

103.01.00.30.100.030.0101 g

124.01.20.40.120.040.0122 g

165.01.60.50.160.050.0165 g

206.02.00.60.200.060.02010 g

30103.01.00.30.100.0350 g

50165.01.60.50.160.05100 g

10030103.01.00.30.10200 g

50016050165.01.60.51 kg

100030010030103.01.02 kg

250080025080258.02.55 kg

5000160050016050165.010 kg

Class M3Class M2Class M1Class F2Class F1Class E2Class E1

Tolerance in mgNominal value

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0.003


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Different Classes of Masses


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Types and Classes of Balances

Weighing balances are classified into different types

The main categories are :

Triple beam balances

Two pan, three knife – edge balances

Single pan, two knife – edge balances

Electromagnetic force – compensation balances


Triple Beam Balance

A triple beam balance is a type of balance commonly used in the laboratory to determine the mass (by weight comparison) of samples. It is called a triple beam because of the three beams on the scale that are used for determining the weight of the item. The first beam measures 0 to 10 grams, the middle beam weighs in 10 gram increments and the far beam weighs in 100 g increments.


Two pan three knife edge

Bangladesh BEST ProgrammeA schematic of a two pan three knife-edge balance is shown below. The balance consist of a main beam carrying two pans at its ends. The beam has a central knife edge resting on a bearing pad when the balance is in operation. The two pans are supported by knife –edges at the extremities of the beam. All there knife edges lie in a plane. The central knife edge is normally equi-distant from the from the pan knife edges and due to this reason these balances are also known as equal arm balances. A schematic of a two pan three knife-edge balance is shown below. The balance consist of a main beam carrying two pans at its ends. The beam has a central knife edge resting on a bearing pad when the balance is in operation. The two pans are supported by knife –edges at the extremities of the beam. All there knife edges lie in a plane. The central knife edge is normally equi-distant from the from the pan knife edges and due to this reason these balances are also known as equal arm balances. A schematic of a two pan three knife-edge balance is shown below. The balance consist of a main beam carrying two pans at its ends. The beam has a central knife edge resting on a bearing pad when the balance is in operation. The two pans are supported by knife –edges at the extremities of the beam. All there knife edges lie in a plane. The central knife edge is normally equi-distant from the from the pan knife edges and due to this reason these balances are also known as equal arm balances.

SSingle pan three knife balance consist of main beam carrying two pans at its ends. The beam has a central knife edge resting on a bearing pad when the balance is in operation The two pans are supported by knife-edges at the extremities of the beam. All three knife edges lie in a plane. The central knife edge is normally equi-distant from the pan knife edges and due to this reason these balance are also known as equal arm balances.This type of balance is mostly used in high precision metrology laboratories for calibration of secondary level masses

A schematic of a two pan three knife-edge balance is shown below. The balance consist of a main beam carrying two pans at its ends. The beam has a central knife edge resting on a bearing pad when the balance is in operation. The two pans are supported by knife –edges at the extremities of the beam. All there knife edges lie in a plane. The central knife edge is normally equi-distant from the from the pan knife edges and due to this reason these balances are also known as equal arm balances.

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Single-Pan, two knife edge

These instruments fall into two categories usually termed top loading and analytical balances. In the analytical balance the load is suspended below the balance arm and the beam is arrested during loading and unloading of the pan. In the top loading balance the pan is supported above the beam by a parallelogram linkage and there is usually no arresting mechanism. Both types of balance are usually critically damped. Most balances have built in masses attached to the pan assembly so that whenever a load is placed on the pan an equivalent mass is lifted from the pan. This means that the load to be supported by the knife edges is fairly constant and balances of this type are often referred to as constant load balances.


Single Pan Two Knife Edge


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An electronic force is generated, via a current carrying coil in a magnetic field, to oppose the out of balance force on a knife edge balance and return the beam to a reference horizontal position. The force is usually measured by reference to the current in the coil. The balance is usually single pan or top loading.

Electromagnetic force compensated balance


Electromagnetic Force Compensated Balance


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Mass comparators are used for comparison of precision masses. Generally the construction of these balances is similar to electromagnetic force compensation type but they are built with more precision and stability. Mass Comparators are available in capacities in the range 0.1 µg to 20 kg with excellent repeatability and linearity.

Mass Comparator


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Thank You

1 Standard Weights and Classes of Balances Nihal Gunasekara Sri Lanka Bangladesh BEST Programme.

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