Laboratory Balances, Analytical Balances, Semi-Micro Balances, Micro Balances, Why?
Almost all of these balances are based upon electro-magnetic force restoration load cells. These load cells produce a counterbalancing force that essentially picks up the unknown weight and keeps the weigh pan from moving down under the gravitation effect upon the unknown weight. The current required to produce the necessary magnetic field to balance the unknown weight is proportional to the weight and is what is measured and computer compensated for magnetic non-linearity and temperature effects. Please note that what is measured is current, which is then “scaled” to equal weight. These devices could be called scales, or they could be called balances as the current produces a counterbalancing force
With all of these high precision devices it is essential to remember that the parameter we call “weight” is a direct result of the gravitational pull upon the mass of the unknown weight. This means that all of these balances must be calibrated at the site where they will be used (because the gravitational field varies from location to location and especially with altitude) with a known precision mass.
The Laboratory Balance is the workhorse of the group and covers weight ranges from 50 grams to 10 kilograms. In the weight ranges from 50 grams to 500 grams they have a resolution of 1 milligram and a linearity (usually) of +/- 2 milligrams. Broadly speaking we generally think of them as 1 part in 100,000 parts (0.001%) for accuracy. In the 1,000 gram to 5,000 gram ranges they have a resolution of 10 milligrams and a linearity of +/- 20 milligrams. In the 10,000gram and up ranges they have a resolution of 100 milligrams and a linearity of +/- 200 milligrams. Always 0.001% type devices.
The Analytical Balance is a specialized device and is an order of magnitude higher in resolution and accuracy but is limited in weight ranges to 50 grams, 100 grams, and 200 grams. With a resolution of 0.1 milligrams and a linearity of +/- 0.2 or 0.3 milligrams it is a fine weighing device, and requires a wind screen to keep air turbulence from causing unstable readings. We think of these as 1 part in 1 million parts (0.0001%) for accuracy. They deserve to be treated with care and are the backbone of a great deal of research work.
The Semi-micro Balance can be thought of as the extension of the Analytical Balance into a one decade lower weight range of 10 grams. The resolution becomes 10 micrograms and the linearity becomes fuzzy, but usually around +/- 30 micrograms. Many balances in this class are dual range and try to be a combination of Analytical and Semi-micro all in one. We are now at a weight level where not only do we need a wind screen, but we have to compensate for the buoyancy effect of air.
Next come the Micro Balances and, you guessed it, we go down another decade in weight ranges, usually from 1. to 5. grams. The resolution becomes 1 microgram and the linearity gets even fuzzier at around +/- 4 micrograms. Once again we see combination dual range balances that are combination Semi-micro and Micro Balances. Dust can be painful at this level. Yes, there are Ultra-micro Balances that go a decade farther to 0.1 microgram in resolution, but these are very specialized balances.
Lots of weight ranges, resolutions, and linearity’s, but each has its place in the lab or QC or inspection station and there is a demand for every type.