INSTRUMENTATION by Ralph H. Müller
N e w electromagnetic weighing balance developed
M technology
ODERN techniques in science and
often require unusual and unconventional approaches to t h e classical problems of precise weighing. Electromagnetic weighing is an ancient art but its possibilities become increas ingly attractive largely as a consequence of the unremitting advances in elec trical measurement. The very earliest studies of Emich in quantitative inor ganic microprocedures made use of Nernst's electromagnetic balance [Nernst, W., Gottinger Nachrichten, pp. 75-82 (1903), Nernst, W., Riesenfeld, Ε. Η., Ber. 36, 2086 (1903)]. Related techniques were employed by Lamb in very precise determination of density. The earlier methods, as a rule, required
Figure 1. General view of Electrobalance
empirical calibration b u t there is no sound reason why this should be abso lutely necessary and indeed slight nonlinearities can be accommodated in properly designed circuitry. Some years ago, a British develop ment was described in this column in which the pointer and coil of a D'Arsonval movement was used as a torquemeasuring element. The principle was extended to weighing in the milligram and microgram range by the simple expedient of applying the load to the pointer and then passing measured cur rents through the coil to counterbalance the load. Completely automatic re balancing was achieved electronically by sensing the needle position capacita-
MAGNETIC FIELD
Figure 2. Schematic drawing of Electrobalance VOL. 29, NO. 4, APRIL 1957 ·
49 A
INSTRUMENTATION
Cary
instrument abstracts
Applied Physics Corporation/Pasadena/California
For Ultraviolet-Visible S p e c t r o p h o t o m e t r y
The Cary Model 11 provides performance comparable to the finest; cost comparable to the cheapest. Figure 3. partment
Details o f weighing com
tively and providing a feed-back loop to supply the load-balancing current. Ob viously, this system could provide a continuous or recorded record of changes in weight as a function of time or en vironment. New Electromagnetic Weighing Balance
Considering the purchase of a recording spectrophotometer? The f o l l o w i n g comments m a y help y o u get much m o r e f o r your m o n e y .
Most spectrophotometer users regard the Cary Model 14, with its 1860 Â to 2.6 m u wavelength as the finest r e c o r d i n g s p e c t r o p h o t o m e t e r available. We have been surprised to discover t h a t quite a few p e o p l e d o n o t r e a l i z e t h a t t h e M o d e l 14 has a c o m p a n i o n i n s t r u m e n t — t h e C a r y M o d e l 1 1 — w h i c h gives t h e same high quality of the Model 14 at a cost comparable to the lowestprice recording spectrophotometer. T h e difference between the Model 11 and the Model 14 is in wavelength range. Of course, where the wider wavelength range is required, the Model 14 is t h e finest i n s t r u m e n t available. However, for applications in the ultraviolet and visible ranges
(2100 Â to 8000 Â) the Cary Model 11 provides the same high degree of accuracy, ruggedness, and dependability as the Model 14, as well as its convenience and flexibility, including linear wavelength recording, speed of scanning, accessories, etc. Important performance data on the Cary Model 11 are outlined below. Stray Light: Less than 0.0001% over most of the range. Scanning Speeds) 1.0 A. per second (ultraviolet region) to 125 A per second ( visible region ). Resolution: 1.0 A or better throughout most of the range. Wavelength Accuracy: Better than 5.0 A in the Ultraviolet region and better than 10.0 A in the visible region. Reproducibility: Better dian 0.5 A in the ultraviolet and 3.0 A in the visible region. Photometric Reproducibility: Reproducibility better than .004 in absorbance can be achieved with the Model 11.
20,000 HOUftS OF SERVICE WITHOUT MAJOR REPAIRS
The first Cary Model 11 was produced in service other than routine maintenance. 1Θ47, and since then nearly every lead This instrument has now begun a second stint of reliable service which will ing analytical laboratory in the United undoubtedly run into additional thou States—and many abroad—has acquired one or more Model l l ' s . The perform sands of hours. ance, flexibility and reliability of the FREE BULLETIN Model 11 have been proved in all kinds If your spectrophotometer applications of research and control applications. are in the visible or ultraviolet range, One of the first instruments to be man investigate the many advantages of the ufactured—Serial No. 2—was recently Cary Model 11. For complete informa overhauled at the factory after having tion write to Applied Physics Corpora tion, 362 West Colorado Street, Pasadena been in use twelve hours per day for 1, California, for Bulletin AC-24. over six years without requiring any For furtter information, circle number 50 A en Readers' Service Card, page 77 A 50 A
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ANALYTICAL CHEMISTRY
An instrument employing some of these principles has been developed by the Cahn Instrument Co., 7712 Danvers St., Downey, Calif. I t is manually operated, although a recording version is contemplated. A general view of the Electrobalance is shown in Figure 1. I t is rugged, unaffected by shock, vibra tion, or temperature change and is port able. The dimensions are 10 X 10 X 7 inches and it weighs less than 12 pounds. The weighing compartment is in the upper right and the only essential con nection between it and the rest of the instrument is electrical. The beam, stirrup, and pan are made extremely light and these protect the jewel bear ings in the electromagnetic torque ele ment from severe acceleration forces. The circuit has been designed for speed and convenience as well as ac curacy. In use, the pan, calibrating mass, and sample weights are succes sively balanced by controlling current through the precision torque motor. The zero control "remembers" the cur rent for zero weight, and the calibrate control "remembers" the resistance needed to make the dial read directly in micrograms. I n making a weighing, the beam is balanced with the balance control. The galvanometer (upper left) is brought to null position with the mass dial, from which the mass is then read directly. These operations require 20 to 30 seconds. A schematic of the Electrobalance is shown in Figure 2. The pointer scale, behind which the pointer moves, is illuminated from the rear. Weight ranges are selected by means of a panel control. A larger view of the weighing compartment is shown in Figure 3.
INSTRUMENTATION
BIDDLE Instrument News A P I E Z O N O I L DISTILLATES for H I G H VACUUM W O R K T h e s e A p i e z o n p r o d u c t s h a v e t h e recognition of leading scien t i s t s w h o a c c o r d t h e m t o p p r e f e r e n c e for h i g h v a c u u m w o r k . B e c a u s e of t h e e x c e e d i n g l y l o w v a p o r p r e s s u r e of a l l t h e s e oil d i s t i l l a t e s , a t u s u a l r o o m t e m p e r a t u r e s , t h e y a r e i n g r e a t d e m a n d for a l l s o r t s of h i g h v a c u u m w o r k . T h e i r v a p o r pressures are as low as 10-6 m m . H g , a n d e v e n unmeasurable at room temperature. As t h e exclusive distribu tor in t h e United States, we offer immediate delivery from stock on t h e complete r a n g e o f t h e s e oils a n d compounds. For applications, working c h a r a c t e r i s t i c s a n d list prices, write for Bulletin 43-AC.
LEYBOLD MERCURY EJECTORS a n d DIFFUSION PUMPS T h e M o d e l H g 3 m e r c u r y diffusion p u m p reaches i t s full p u m p i n g speed a t pressures below 10- 2 m m . H g a n d r e m a i n s c o n s t a n t down t o 10- r ' m m . H g . T h e m e r c u r y ejectors models H g 12 a n d H g 45 'reach m a x i m u m p u m p i n g speed a t pressures b e t w e e n 10-! a n d 10- 2 m m . H g . Specially designed j e t s p e r m i t c o n s t a n t p u m p ing speed a t pressures b e t w e e n 10- :! a n d 10- 5 mm. Hg. T h e p u m p s begin t o o p e r a t e a t considerablyhigher pressures. T h e u l t i m a t e v a c u u m is b e t t e r t h a n 10- 6 m m . H g . W e welcome y o u r inquiries o n t h i s e q u i p m e n t a n d t h e accessories for it. Write for Bulletin 44-AC
Figure
4.
Separate
JAMES G. BIDDLE CO. •
ELECTRICAL
• S P E E D •
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M E A S U R I N G
LABORATORY
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I N S T R U M E N T S I N S T R U M E N T S
SCIENTIFIC
1316 ARCH STREET PHILADELPHIA
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EQUIPMENT
For further information, circle numbers 52A-1, 52 A 2 on Readers' Service Card, page 77 A 52
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ANALYTICAL
CHEMISTRY
de
Remote Weighings Possible
The Eleetrobalance is particularly suited for remote operation in hot radia tion cells, dry boxes, or other inacces sible environments because, aside from adding or removing samples, the weigh ing operation is performed electrically and can be done remotely. Figure 4 illustrates the ease with which the bal ance compartment can be removed and connected by a four-terminal lead to the measuring system. I t is also possible to monitor a number of weighing assemblies from one measuring and con trol box by suitable switching means. The conventional microbalance, of course, covers a much wider range and with high precision. According to Niederl, such a balance, suitable for quantitative organic elementary analy sis, will be reproducible to ± 2 7 a t a 1-gram load, ± 5 7 for 1- to 10-gram loads, and no more than ± 1 0 γ for 10to 20-gram loads. Nevertheless the Eleetrobalance in a limited range can be very useful for rapid, small-sample weighings. An in dication of the range and precision is given by the manufacturer as follows: Range of Sample Weight, Mg. 0-5 0-10 0-20 0-50
Β 52 6
component
sign f a c i l i t a t e s r e m o t e w e i g h i n g
Sensitivity, γ 1 2 5 8
Accuracy, y
2.5 5 12 30
a precision of about 0.02% of full scale and an accuracy of about 0.05%. The ease and convenience for small sample weighing would seem to be very useful in the direct weighing of small samples for the KBr pellet technique in infrared work, for d.c. arc hollow elec trode technique in emission spectros copy, and in the direct preparation of solutions for ultraviolet spectrophotom etry.
