09 May

Balance/Scale

A lab balance is used to precisely measure reagents, standards, and samples.  The word scale generally refers to instruments for weighing higher masses than a balance.  A balance uses an arm or beam to balance between known calibrated weights and the sample weight.  A scale depends on gravity to compress a spring; but differences in local gravity can vary by as much as 0.5%.  A balance compares two masses and so is independent of any gravity variations.  The analytical balances used in the laboratory today are a misnomer; they use a spring to determine the final weight.  A coil of wire is suspended in a magnetic field by a spring.  A sample placed on the weighing pan depresses the coil against the spring.  When current is applied to the coil it also has a magnetic field.  The amount of current through the coil needed to exactly counter the downward pressure of the sample on the weighing pan and return the coil back to it’s original position determines the digital reading.  Strictly speaking an electronic lab balance is a scale.  Like all scales, laboratory balances must be calibrated periodically.

Robin Prymula