Humboldt State University ® Department of Chemistry
Robert A. Paselk Scientific Instrument Museum
Analytical Balance, Single-Pan, Substitution-Christian Becker/Torbal EA-1
Ser. No. 117823
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. In the original Mettler designs 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. 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.
Christian Becker/Torbal followed Mettler in using the substitution principle, but used an electronic null-system to determine the last gram, and substituted a torsion suspension for the traditional knife-edges. In essence an electronic circuit is used to drive an electromagnet to add the last gram. The current is adjusted until the beam reaches a "null" position as indicated on a meter. This balance is thus a transitional stage in the development of the fully electronic balances of today. Single-pan substitution analytical balances are rapidly being replaced by the fully electronic balance, introduced in the 1980's, which is even more convenient, as well as more precise and accurate.2
The scanned catalog description, of an earlier version of this instrument, is from the 1968 Van Waters and Rogers catalog, Catalog 69 Scientific Apparatus instruments and supplies for: Industrial, Educational, Clinical and Research Laboratories. Some of the features of this balance are described on the front page and back page of a c. 1980 Torbal flyer.
The balance case, of formed metal sheets attached to a cast aluminum base and with curved glass doors, is 20" high, 12 1/2" wide and 17 12" 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. Additional characteristics of this balance are detailed in scanned pages from a Torbal brochure.
1 According to the HSU Inventory this balance was purchased in 1982.
2 Jenemann, Hans R. Die Waage des Chemikers / The Chemists Balance. DECHEMA, Frankfurt am Main (1997).