INSTRUMENTATION The Gram-atic balance represents a notable advance in the science of precise, rapid weighing that may be the beginning of a series of advances in the field by Ralph TT^OR many years it has been uncomfortably apparent that -*- progress in technical and industrial weighing has exceeded the attention devoted to the ordinary analytical balance. The analyst has been inclined to regard his balance as a highly developed precision instrument, susceptible to only minor improvements in the interest of convenience of operation. There have been, distinct improvements, including better damping schemes, weight adding mechanisms, chain loading, and improved illumination. As a rule, these attachments have enhanced balance performance with no impairment of precision; otherwise they would receive little consideration. Gram-atic Balance There have been relatively few radical approaches to the problem of weighing in the analytical range. One of these is
Figure 1. Top View of Balance
Müller
the quartz fiber microbalanee of Emich. Another interesting case is the beautifully executed example illustrated herewith. I t is designated as the Gram-atic balance, made by E. Mettler of Switzerland and available in this country through Fisher Scientific Co. and Eimer & Amend. A general view of the balance is shown in Figure 2. The balance has but one pan and all weighings are made under constant load because weights are removed, not added, as required by the sample. The load on the beam is always brought t o 200 grams and this is accomplished by removing weights equal to that of the sample. These weights are indicated on the direct-reading scale as they are removed. The uniform loading not only provides constant sensitivity, independent of sample weight, but enables the final beam deflection to be indicated as milligrams. The sequence of operations relating to the case illustrated by Figure 2 shows that the upper lefthand knob controlling the 100-gram weight has been set at zero. The lower left-hand knob controlling tens of grams has been set a t 10 and a figure denoting unity in the tens place appears on the scale. Similarly, whole gram values have been set in with the upper right-hand knob and tenths of a gram with the lower right-hand knob. For these settings the numbers 2 and 3 appear on the scale in the units and tenths places, respectively. In the next operation, the pan release lever has been turned by the operator's left hand, and figures for the second, third, and fourth decimal places automatically appear, by optical projection, on the scale. The entire sequence of operations, including the placing of the object on the balance pan, requires less than 21 seconds. This is the average time required by a technician with limited experience in handling the new balance.
1. Weights, 1.0 to 199.0 grams 2. Cams and fingers for removing weights of 1.0 to 199.0 grams 3. Knife edge from which the pan and weights are suspended 4. Sturdy reverse cantilever type of balance beam 5. Center knife edge and plane of sapphire 6. Weight which compensates for pan and weights on opposite end of balance, and air-damping plunger 7. Series of plates which eliminate errors due to atmospheric changes 8. Engraved glass scale of 100 equal divisions representing 100 mg. 9. Light eource for optical system 10. Housings for two right-angle prisms which focus light beam 11. Plane prism for adjusting zero point up to 4 mg. 12. Compensating weight for initial zero adjustment up to 5 mg. 13. Sensitivity compensation adjustment weight 14. Weights 0.1 to 0.9 gram 15. Cams and fingers for removing weights of 0.1 to 0.9 gram 16. Prism which directs projected scale onto ground-glass screen 17. Ground-glass screen with etched-in vernier onto which weight of 100 mg or less is projected 18. -Dials which directly indicate total weight removed from beam, 0.1 gram and greater
29 A
H.
A top view of the balance is shown in Figure 1, in which the left side is the front of the balance. Additional features are an aluminum case finished in green, capacity 200 grams, optical adjustment of zero point, built-in weights adjusted to ± 0 . 0 3 mg., and accuracy of balance ± 0 . 0 5 mg. independent of load.
ANALYTICAL
30 A
CHEMISTRY
INSTRUMENTATION
"A.B.C."has developed t h e s e exclusive f e a t u r e s . . · . . 1.
Aluminum case, smooth finished
2.
Superior beam construction
3.
Stirrup construction (Patent pending)
4.
Stirrup arrestment (Patent pending)
5.
Ball bearing arrestment (Patent pending)
6.
Rider pickup device (Patent pending)
7.
Inverted arrestment contact points
Figure 2.
TYPE A N - 2 0 1 2
$228
and these other features found for the first time in this low price range 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
A r c movement arrestment Capacity 2 0 0 grams Sensitivity: 1/20 mg. Rapid w e i g h i n g Black glass base plate Bows and pans of stainless steel H a r d e n e d point on indicator V a r i a b l e magnetic damper Independent pan arrest Equilibrium adjustor A . B . C . Balances ate available from your dealer. If he cannot supply you — w r i t e direct t o
AMERICAN
BALANCE CORP. H. C. Becker, Pres.
148 POHER AVENUE
NEW ROCHELLE, Ν. Υ.
Grain-atic B a l a n c e
This is a notable advance in the science of precise, rapid weighing. We cannot help feeling that it is but the beginning of a series of advances in the field. If we recall that there are photoelectric and other electronic means for detecting microinch displacements and simple servos for restoration and compensation, it will be seen that the possibilities are un limited. They must be studied, because it will soon happen that weighing operations will become the bottleneck in our modern methods of analysis. Perhaps it will be recognized that there are several classes of precision weighing: sample weighing, changes in weight over very short or very long periods, differential weight changes, etc. Some of those may be important enough to warrant a special instrument for the restricted problem. Let us not expect the balance manufac turer to investigate these possibilities. If he attempted to write off the research and development costs, we would promptly call his prices outrageous. Balance
Repair
Our recent notice concerning the scarcity of balance repair men and experts elicited specific replies to the effect that Raymond E. Saatman, 1002 Van Kirk St., Philadelphia 24, Pa., and the Meyer Scientific Supply Co., Inc., 221 Atlantic Ave., Brooklyn 2, Ν. Υ., are engaged in that business. We shall continue to list other names as they are brought to the attention of this column.
