Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Micro-Analytical Balance, Single-Pan, Substitution-Mettler M5 S/A
Ser. No. 105736
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 for objects placed on the pan.
In the M5 micro-analytical balance the final 20 mg is read off to 0.001 mg from an illuminated/projected optical scale attached to the long end of the beam and adjusted with a dial. Because the total mass on the balance remains constant in all weighings, weighing precision also remains constant over the range of the instrument. Because of the high sensitivity of this balance, the temperature of the balance components can affect weighings. In the S/A version which is on display, a thermal shield is provided to protect the balance form the operators body heat, saving the time normally required by such balances to allow thermal equilibrium between operator and balance to be achieved.
The great sensitivity of the M5 also required a very stable balance table, so as part of the Science A building design a solid concrete pier, extending about ten feet into the ground and isolated from the building, was installed in the north-east corner of the analytical chemistry balance room to accommodate it.3 The scanned catalog description is from the 1961 Braun-Knecht-Heiman-Co. (Division of Van Waters & Rogers, Inc.) Catalog No 63, Laboratory Instruments Apparatus and Supplies. San Francisco.
This balance was purchased by the chemistry department in 1962 along with two other specialty Mettler balances, a H15 precision-analytical balance and a H16 semi-micro analytical balance. On the 1988 inventory, the H15 was in the qualitative analysis prep room (SB 122), while the M5 and H16 were in the analytical chemistry lab balance room (SA 369A), where they remained until 2008.
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 20 g., a range of 20 mg on the optical scale, an accuracy of ±0.002 mg and a precision of ±0.001 mg, and weights adjusted to within Class M tolerances. Note the thermal shield, consisting of a plate of sheet aluminum in front of the balance, and double glass window with round, double pane door. The balance is also equipped with a device to present the sample pan top the operator through the door. Compare the two balance views, from the right with the pan presented, and from the left with double-door closed.
Two views of the mechanism are also available. The first image is of the weighing mechanisms with the top of the balance removed and viewed from the right. In the second image the the view is from above, (note the chamois and brush stored within the balance for cleaning weights and knife-edges).A photocopy of the instruction manual for the Mettler M5 and an M5 setting up brochure accompanies the balance. The chemistry department also had posters showing the features of Mettler balances as exemplified by the Mettler H5 and H10 balances. 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 1962.
2 Kupper, Walter E. "Balance" in McGraw-Hill Encyclopedia of Science and Technology, 8th ed. v2, McGraw-Hill, New York (1997) p 486.
3 The remaining balances in the analytical balance room were also provided with special custom poured concrete balance tables, in this case attached directly to the floor slab or wall, topped with 3/8" black glass for a clean surface. The center table was demolished to make room for a 300 MHz NMR in 1990. The column now serves as a table and attachment site for gas tanks necessary to maintain the cryogens in the NMR magnet.