A Matter of Balance: A Look at Analytical Balances.
An Introduction to Analytical Balances
An analytical balance is used in the laboratory as a precision instrument to determine weight of an object of 100g to within +/- 0.0001g.
Until the 1950s laboratories used two-pan balances to determine weight. The single-pan balance was used after the 1950s and modern laboratories use electronic balances today.
Weight is the force exerted upon an object by gravity while mass is the quantity of matter the object is composed of. Weight is different at different locations while mass stays invariant.
So while the mass of the object in grams is what is being measured, the term weight and weighing has become the terminology used.
Analytic balances use reference masses to be the substitute weights in two-pan and single-pan balances, and is used for calibration purposes with electronic and pan balances.
Analytical 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 analytical weights are called fractionals and are made of aluminum or platinum. These weights range from 500 mg to 5 mg.
The National Institute of Standards and Technology have 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 frist class levers where a fulcrum lies between two arms of equal length. (I1 = I2) 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 Scottich Chemist Joseph Black (1728-1799) introduced three prism formed "knife-edges" where the fulcrum and two arms are placed.
Each of the prism formed "knife-edges" is made of hard yet brittle agate.
Two-Pan Source of Errors:
1.) 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.
2.) 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.
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 would be 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 the container from the 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 the 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 of that generated by the opposing magnetic force an "error" indicator moves to the reference position.
When the beam is displaced the rapid change in current through the coil the "error" signal generates a correction current. This correction current is measured and is equal to the mass of the object.
Electronic balances need to be calibrated with known masses prior to use.
Errors in Weighing
Three ways to avoid error in your measurements are:
1. All samples that can take up water should be covered for the measurement.
2. All glass vessells need to be extremely dry prior to measurement.
3. Make sure that the object being measured is the same temperature as the balance.
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