Humboldt State University ® Department of Chemistry
Robert A. Paselk Scientific Instrument Museum
The single-pan, substitution balance is much faster and more convenient then the traditional two-pan, equal-arm balance. It thus rapidly replaced the older type after its commercial introduction by Erhard Mettler in 1946.2 In this type of balance the pan and weights are counterbalanced by a single fixed weight on the long end of the beam. Weighing is accomplished by removing the built-in weights to compensate (within 1 g) for objects placed on the pan. The final 1 g is read off to 0.1 mg from an illuminated/projected optical scale attached to the long end of the beam and adjusted with a rotary counter. Because the total mass on the balance remains essentially constant in all weighings, weighing precision also remains constant over the range of the instrument. This is in contrast to the double-pan balance, where precision decreases with load. The Mettler H10 balances include a taring system, allowing the balance to be zeroed with a weighing container in place, and a filling guide (100 mg range) showing approximate weight while adding material.
The Mettler H10Tw, which cost a third more than the H10, differs in having the readout and weight knobs mounted on the base of the balance below the weighing chamber, an arrangement more convenient for some users, as opposed to the more common arrangement with the knobs and readout window above the weighing chamber as seen on the Mettler H5. The scanned catalog description is from the 1968 Van Waters and Rogers catalog, Catalog 69 Scientific Apparatus instruments and supplies for: Industrial, Educational, Clinical and Research Laboratories.
This balance was purchased by the chemistry department along with six Mettler H10 balances in 1971. On the 1988 inventory, one of the H10's was in the student research lab (SA 460), the remainder, along with the H10Tw, were all in the analytical chemistry lab balance room (SA 369A).
The single-pan mechanical substitution balance is rapidly being replaced by the electronic balance, introduced in the 1980's, which is even more convenient, as well as more precise and accurate.2
The balance case, of formed aluminum sheets with glass doors, is 16 1/2" high, 9 1/2" wide and 20" deep. The balance has a capacity 160 g., a range of 1,000 mg on the optical scale, an accuracy of ±0.1 mg, and weights adjusted to within Class S tolerances. For more information see the scanned table of H10T specifications.
The chemistry department had a number of posters showing the features of the Mettler balance. A scan of the remaining 24" x 36" poster featuring the Mettler H10 is displayed.
1 According to the HSU Inventory this balance was purchased in 1971.
2 Kupper, Walter E. "Balance" in McGraw-Hill Eincyclopedia of Science and Technology, 8th ed. v2, McGraw-Hill, New York (1997) p 486.