INSTRUMENTATION


INSTRUMENTATIONpubs.acs.org/doi/pdf/10.1021/ac60029a725make no unusual demands upon the balance. Con- versely, its limit...

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INSTRUMENTATION The analytical laboratory needs a high-speed precision automatic recording balance, application for which may be found in every field of physical science by Ralph X ^ 7 E HAVE stated repeatedly that progress in indus» * trial weighing and proportioning is much greater than its counterpart in the laboratory. When our good friends remind us that an analytical balance is one of our most important instruments, we concede the truth of the statement but feel that progressive thinking about its possible improvement is completely lacking. Lest balance manufacturers accuse us of ingratitude along with ignorance, we hasten to add that a lot of constructive thinking about the general art of weighing is needed. Instinctively we associate a good balance with the problems of weighing a sample, the components of a standard solution, or a carefully ignited precipitate. Associated problems, such as density determinations, make no unusual demands upon the balance. Conversely, its limitations impose severe restrictions on the type of problem that can be handled. Precision

Sample

Balance

H.

Müller

10, 436 (1938) ] and have repeatedly regretted the necessity of dismantling it for badly needed components. There are so many problems for which a precise, continuous record of weight is required that, in effect, such a device becomes almost a new tool. This includes kinetic studies, evaporation rates, sedimentation thermal transition points, and, as we suspect, many original analytical procedures. As we have pointed out before, there is no scarcity of devices for attacking every phase of the problem. Photoelectric, inductive, and capacitative detectors are available for sensing displacements of the order of microinches. Mechanical and electrical restoring mechanisms of comparable precision are at hand and high speed recorders are now a standard feature of at least a dozen familiar analytical instruments. In all probability, such combinations will become commercially available as soon as anyone can prove to the instrument designer that widespread use of the device is certain. An interim solution might lie in the design of a skillfully packaged unit containing the detecting and restoring elements easily adapted to a standard balance and capable of passing the information directly to a standard recorder. ENG. CHEM., ANAL. E D . ,

Needed

We still think there is need for a high speed precision sample balance and believe that its design calls for a bright young engineer with no preconceived notions or experience in balance design. If his product is really good and fast, we suspect it will bear no recognizable resemblance to existing instruments. Any citizen can step on an automatic scale and promptly receive a card on which is printed the date, his weight, and a gratuitous horoscope. If he guesses his weight to within 1% or less, he even gets his penny back. Aside from the good-will features, there is much here which the laboratory man might reasonably expect. There is ample proof in industrial practice .that the inclusion of many automatic features in a weighing machine has immediately suggested applications which are not customarily considered as weighing, such as counting or checking dimensional tolerances. The need becomes more apparent when we consider what could be accomplished with good recording balances. The applications could cover every field of physical science. One does not have to look far for cases in which weighings have been recorded automatically, but invariably a standard balance has been rebuilt and modified. More than ten years ago we designed and built a recording analytical balance [IND.

Automatic

Recording

Balance

According to P. M. Hackett of the Niagara Electron Laboratories, Andover, Ν. Υ., this problem has been revived and the company is now prepared to quote on automatic recording analytical balances. The heart of this equipment is the Thermocap relay which was de­ scribed some time ago in this column. The Thermocap picks up any motion of the balance beam without direct contact and controls a servo system in magnitude and direction to restore the balance to equilibrium. Com­ pensation is effected by chain control of the balance and the chain position is recorded mechanically or by an equivalent electrical signal. The equipment is intended for studies of para- and diamagnetic properties, curie points, preferential evaporation rates, drying, and cor­ rosion. The device is being used by the Navy Group at Alfred University to determine oxidation rates of metals and alloys at high temperatures. In this appli­ cation, metal samples are hung from the balance pen in a 25A

26 A

ANALYTICAL

INSTRUMENTATION

Klett.... Conductivity Bridge

furnace, in a choice of atmospheres relative to oxygen content. Automatic records can be taken over 10-hour periods and afford a continuous indication of the oxida­ tion process. In this installation and similar to other previous methods, the range of automatic compensation is ordinarily limited to the range of the chain block. When this range is exceeded at either the upper or lower limit, it is necessary to add or subtract 100-mg. weights. This can also be done automatically by standard weightlifting mechanisms which are actuated by limit switches. T h e alternative choice of using a heavier chain involves some difficulties in retaining adequate precision. I t is interesting t o note the present status of our means for measuring the three most fundamental quantities—mass, length, and time. While the ac­ curacy and precision in all three cases are comparable, there is ample evidence to show t h a t length and time and their derivable consequences such as displacement and frequency can be measured in numerous ways and with high speed. Of the three, the one which is of greatest interest to the chemist has received the least attention. Monograph on Vacuum Tube

The bridge, earthing circuit, oscillator, and amplifier are all contained in one m e t a l cabinet.

The balance point is indicated by

a "magic-eye" and by phones w h i c h give a sensitivity of 0.01% or better.

The coils

of t h e decades are adjusted to ± 0.1% of their n o m i n a l resistance.

The range of

resistances is from 0 to 11,111 o h m s , a s h u n t i n g resistor is used when higher re­ sistances are measured.

Conductivity Cells are available.

Κlett

CHEMISTRY

Manufacturing

Co.

179 EAST 87TH S T R E E T , NEW YORK, Ν. Υ.

Amplifiers

Comments and recommendations in this column for the M . I . T . Radiation Laboratory series of monographs will not subside until the series is completed. Once more it seems compelling to refer analysts to Volume 18 on vacuum tube amplifiers, edited by G. E. Valley, Jr., and H. W'allman. Chapter I I on direct-coupled ampli­ fiers is written b y J. W. G r a y and contains information available in no other source. This is particularly true in the compilation of vacuum t u b e characteristics a t low currents, some of which are extremely useful and are not available in ordinary tabulations of tube be­ havior. We single out this chapter particularly for its importance for the instruments of the analyst, wherein the direct-coupled amplifier is often the only reasonable approach. T h e other excellent chapters cover almost every other aspect of modern amplifier theory and practice. One difficult}' attending the use of these monographs has been their great degree of interdependence, so t h a t the discussion of any one topic is never complete unless one has related volumes at hand. Perhaps more serious has been the inadequate literature references and at times it becomes difficult to decide whether a circuit or technique is brand-ηβλν or 15 years old. Although the editor-in-chief has explained the difficulties and in some cases impossibilities of giving adequate references and acknowledgments, it seems t h a t one must accept this situation and regard the series as a priceless and indis­ pensable collection of modern techniques.