A Look at Analytical Balances: A Matter of Balance
An Introduction to Analytical Balances
An analytical balance is used in modern laboratories as a precision instrument to determine the weight of an object up to 100g or down to 10ug.
The word 'balance' arose from the Greek word 'bilanx' meaning 'two-pans.' Ancient balances are dated as far back as 5,000 B.C. according to the National Institute of Health (NIH).
From these ancient times until the 1950's laboratories used two-pan balances to determine weight. The single-pan balance was invented in the 1950's and saw widespread use. Currently modern laboratories use electronic balances today.
Weight is force exerted upon an object by gravity while mass is the quantity of matter in an object. Weight is different at different locations while mass always stays constant.
So while the mass of an object in grams is what is being measured, the term weight and weighing is used.
Two-pan and single-pan balances use a reference mass, or substitute weight, to determine the unknown mass of the object being weighed. Electronic balances also use reference weight to set a calibrated weight prior to an accurate measurement.
Reference weights above one gram are made of brass and bronze and plated with either chromium or lacquer coating. These weights range from 1 gram to 100 grams.
Smaller reference weights, or fractionals, are made of aluminum or platinum. These weights range from 500 mg to 5 mg.
The National Institute of Standards and Technology has two classifications for analytical weights. Class M is used for high precision and Class S is used primarily for calibration.
Two-Pan or Equal-Arm
Two-pan or equal-arm balances are first class levers where a fulcrum lies between two arms of equal length (I1 = I2). A well used example of a first class level is a 'simple seesaw.'
Pans are suspended from the arms. The object being weighed or M1 is placed on the left pan while a known mass or M2 is placed on the right pan. Both M1 and M2 are attracted to the earth due to gravity.
The operator adjusts M2 until a pointer is facing the fulcrum. At this point M1 = M2.
The precision and accuracy of two-pan balances reached maximum efficiency when the Scottish Chemist Joseph Black (1728-1799) introduced three prism formed "knife-edges" where the fulcrum and two arms were placed. Each of the prism formed "knife-edges" is made of hard yet brittle agate.
Two-Pan Source of Errors:
- I1 and I2 have to be equal in length. If one arm is 1/100,000 longer the measurement will be 1/100,000 off.
- An increasing load can bend the beam slightly over the knife edges leading to slight errors in measurement.
Single-Pan or Unequal-Arm
Also called a constant-load balance the single-arm balance has two instead of three knife edges with two arms of unequal length.
The smaller arm is the balance pan and has a full complement of weights suspended. The longer arm holds a constant counter weight with a damping device built into the beam.
When the object is placed on the pan, the suspended weights are removed from the shorter arm. This type of weighing is called weighing by substitution and leaves a constant load on the beam.
The beam is released when the sum of the weights removed is 0.1 g the weight of the object, the beam is released.
A reticle, or a scale etched on glass, displays its reading onto a reading display and the weight of the object is taken.
Single-pan balances use a tare device where the weight of the container can be removed from the weight of the object being weighed by subtracting the weight of container from total weight.
When a current is passed through a wire that is placed between the two poles of a permanent magnet, a force is generated. This system is called an electromagnetic servo system.
In an electronic balance this force is used to move a wire outside the magnetic air gap and generate a reading used to formulate weight.
When the force of gravity of an object is coupled mechanically to the servo motor an opposing magnetic force is generated.
A null-indicator checks the position of the wire in a magnetic field. This indicator could be optical, a vane attached to the beam, a small lamp, or a photo detector.
When the force of the object is at equilibrium to the opposing magnetic force an "error" indicator moves to a reference position.
When the beam is displaced the rapid change in current through a coil the "error" signal generates a correction current. This correction current is measured and is equal to the mass of the object.
Due to the sensitivity of the measurement air pressure and humidity can alter results. To prevent this from happening most electronic balances are covered in glass.
Electronic balances need to be calibrated with known masses prior to use.
Due to changes in design balances today have accuracies of less than 0.001 milligrams or they could detect differences of less than 1 part per 10,000,000.
Some of these changes included:
- pan brakes
- magnetic damping of beam oscillation
- built-in weight sets operated by dial knobs
- microscopic or microprojection reading of the angle of beam inclination
Has been dated as far back as 5000 B.C.
Has two equal length arms.
Was created in the 1950's.
Has one long and one short arm.
Used in modern Labs today.
Errors in Weighing
Three ways to avoid error in your measurements are:
- All samples that can take up water should be covered during measurement.
- All glass vessels need to be extremely dry prior to measurement.
- Make sure that the object being measured is the same temperature as the balance.
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© 2013 Jamie Lee Hamann