A double beam balance, commonly called double pan balance, is used to to compare the weight of two objects. Specifically, a double beam balance is used to compare an unknown mass with a known mass. It is used to quickly and accurately weigh up to 2 kms when used with supplementary standard masses, which usually range from 10 g to 500 g, or riders on a beam.
The parts of a double beam balance consist of two weighing pans, a pointer, rider, also known as the sliding mass, and rider beam. The weighing pans are suspended on each end of the rider beam, which is a pivoted horizontal lever of equal lengths marked off in tenths of a gram up to 10 gs. The left pan is where you place the object you want to determine the mass of and the right pan is for the standard masses. You can determine the mass of one object by moving the rider from left to right until the pointer rests in the middle of the scale.
The triple beam balance is a typical mechanical balance used to determine the mass of one object without having to compare it to another. It is named for the three beams which carry weights and is used to quickly and accurately weigh up to 600 gs without extra masses.
A triple beam balance only has one weighing pan. However, opposite the pan are three parallel beams -- each used to support one weight. The middle beam reads in 100 g increments, the far beam reads in 10 g increments and the front beam reads continuously up to 10 gs. This balance also has a pointer, riders on each beam and an adjustment knob, which is used if the pointer doesn't stop in the middle of the scale. Unlike adding weights in a double beam balance, each of the three riders are slid along the beam to measure the weight of an object. The mass of an object is determined by adding the readings on all the beams.