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

Primary Measurement Standard Bangladesh BEST Programme Primary Measurement Standard Definition: Measurement standard established using a primary reference measurement procedure, or created an artifact, chosen by convention 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

Primary Measurement Standard Bangladesh BEST Programme 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

Secondary Measurement Standards Bangladesh BEST Programme 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

Reference Measurement Standards Bangladesh BEST Programme 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

Working Measurement Standards Bangladesh BEST Programme Working Measurement Standards Definition: Measurement standard that is used routinely to calibrate or verify measuring instruments or measuring systems M1 Set of Weights

Travelling Measurement Standards Bangladesh BEST Programme 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

W=mg where g is the acceleration due to gravity Bangladesh BEST Programme 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

Mass Standards- Types of Masses Bangladesh BEST Programme 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

Mass Standards- Classes of Masses Bangladesh BEST Programme 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

Bangladesh BEST Programme ACCURACY CLASS 0.20 0.06 0.020 0.006 1 mg 0.003 5 mg 0.25 0.08 0.025 0.008 10 mg 0.4 0.12 0.04 0.012 0.004 50 mg 1.6 0.5 0.16 0.05 0.016 0.005 100 mg 10 3.0 1.0 0.3 0.10 0.03 0.010 1 g 12 4.0 1.2 2 g 16 5.0 5 g 20 6.0 2.0 0.6 10 g 30 50 g 50 100 g 100 200 g 500 160 1 kg 1000 300 2 kg 2500 800 250 80 25 8.0 2.5 5 kg 5000 1600 10 kg Class M3 Class M2 Class M1 Class F2 Class F1 Class E2 Class E1 Tolerance in mg Nominal value 3 0.003

Different Classes of Masses Bangladesh BEST Programme Different Classes of Masses

Types and Classes of Balances Bangladesh BEST Programme 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 Bangladesh BEST Programme 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 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. 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. Bangladesh BEST Programme Two pan three knife edge S Single 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

Single-Pan, two knife edge Bangladesh BEST Programme 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 Bangladesh BEST Programme Single Pan Two Knife Edge

Electromagnetic force compensated balance Bangladesh BEST Programme Electromagnetic force compensated balance 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 Bangladesh BEST Programme Electromagnetic Force Compensated Balance

Mass Comparator Bangladesh BEST Programme 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.

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