Instruments for weighing materials are called balances, and most laboratories have more than one type of balance, depending on the amount of material being measured and the degree of accuracy required.
Mechanical balances weigh an object on a pan hanging from a beam that has a counterbalanced weight.
The simplest of these is a double pan balance, which has two pans: you can measure a specified mass in one pan by counterbalancing it with that calibrated weight placed in the other pan. When the two pans are evenly balanced, you have measured the correct amount.
In a single pan balance, you can measure a specified mass in one pan against calibrated weight that slides along a calibrated scale on the beam. This works like the balances used in most doctors’ offices; since there is an adjustable scale, it is much more convenient to use than a double pan balance.
Analytical mechanical balances are similar to single pan balances, but are calibrated to measure extremely small weights with a high degree of accuracy, often as small as 0.1 milligrams.
Electronic balances have replaced most mechanical balances due to their greater accuracy and ease of operation. They are easier to use because they usually have a digital readout, and weighing dishes can be tared to read zero mass before using. Most balances used for preparation of solutions have a sensitivity of +/- 0.01 g, but electronic analytical balances can be sensitive to +/- 0.1 mg or less. Electronic balances require routine maintenance and recalibration.