INSTRUMENTATION by Ralph H. Müller
N e w automatic recording balance suggests possible uses besides weighing directly in milligrams, so sample weight \ SIGNIFICANT and important conis the duodial reading plus chart deflec ·**- tribution to the art of automatic weighing is to be found in the new tion, visible at all times. In order to visualize the entire course H M automatic Electrobalance which is of a weight change as a function of manufactured by the Cahn Instrument time, the total weight change can be Co. of 14511 Paramount Blvd., Pararestricted to the chart width. Coarse mount, California. We take personal recorder changes may be selected by delight in this fine development, having means of a panel switch. For more some years ago repeated the British precise examination the curve can be trick of hanging weights on a microamdrawn at high sensitivity and periodi meter needle and then feeding currents cally dropped to zero in order to draw through the meter to supply a comthe next segment of the process. The pensating torque. The new Cahn balance is an extension and an improvement on their original M10 Electrobalance. As indicated in the schematic drawing, beam deflection due to weight change is sensed by a phototube. The amplified photosignal is fed back to the torque motor to rebalance the beam. The loop gain of this servo system is in excess of 1000, so the actual beam deflection under load is very small, and balancing torque is essentially equal to sample torque. Whatever difference occurs is roughly proportional to load, so that most of this minute residue is calibrated out. The torque motors used in these balances are as linear as precise weights and precision potentiometers can determine. Thus the balancing current is a direct measure of the sample weight to an accuracy of better than 0.1% and a precision of better than 0.01 % of full scale sample weight. Xo recorders are available which can accommodate this sensitivity, consequently additional circuits are required. Zero suppression plus scale expansion are used in this instrument. As little as 1 7r> of full scale voltage can deflect the recorder full scale, and 100% of full scale voltage can be suppressed. Thus, for example, a variation from 99 to ΊϋΟ mg. could be displayed full scale across a recorder chart, with each 1% division equal to 0.01 mg. The sup RM automatic Electrobalance and control pression dial is a duodial calibrated on assembly panel.
recorder range selector is accurately calibrated and is independent of the suppression control. The output signal of the RM auto matic Electrobalance is such that it can be used with any present day re corders or indicators, from the economi cal 5-in. Varian to the 10-in. larger strip chart recorders. In conjunction with a two-pen or X-Y recorder, weight can be recorded as a function of tempera ture. It can also be used with voltmetc
unit.
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Weighing mechanism mounted
(Insert)
VOL. 32, NO. 3, MARCH 1960 · 9 7 A
MAINTAIN
INSTRUMENTATION
-50 το -140 F INDEFINITELY... in high vacuum work, specimen freezing, laboratory cold baths Far superior to conven tional dry ice—acetone dewar flasks, the Serfass automatic cold trap is me chanically refrigerated. Set any temperature from - 5 0 to - 1 4 0 F and the com pletely closed refrigeration s y s t e m will maintain t h e temperature around-theclock, around-the-year without attention. The noxious fumes, flammable liquids, and service re quirements of old-fashioned cold traps are completely eliminated. Complete information is provided in Bulletin 1259. Write Milton Roy Company, 1300 Mermaid Lane, Philadelphia 18, Pennsylvania.
milton
Controlled Volume Pumps · Quantichem Analyzers Chemical Feed Systems · pH Instruments Circle No. 169 on Readers' Service Card ·
sophisticated electronic data-handling equipment. The servo system is extremely fast having a 22 cycle per second natural frequency. Although such fast re sponse time is probably not required in chemical analysis, Cahn reports that some interest has been shown in using the instrument for measuring muscle forces. In such applications, the re corder response would have to be equally fast. Also, there have been some reports in the literature that a slow or sluggish recording balance can introduce errors in thermogravimetry. Almost every investigator wishing to use a recording balance has his indi vidual requirements—vacuum, high temperature, dry box, etc. The manu facturer has tried to please everyone— mostly in the direction of making the unit very small. Without change, the weighing chamber can be used in a small confined space from —20e C to 50° C. and from dryness to saturation. From 50°-100° C , only the amplifier unit need be placed outside and cabled in. As shown in the inset photograph there are two loops on the balance beam. The left-hand loop is counterweighted for 15 mg. of container and stirrup, and can weigh 0-20 mg. of sample. The right-hand loop is counterweighted for 75 mg. of container and stirrup, and can weigh 0-100 mg. of sample or a maxi mum capacity of 175 mg. This ap parent limit in sample range actually has several advantages, especially in thermogravimetry because with a suit able choice in sample, furnaces can be smaller, and adsorption and desorption effects can occur more quickly.
Roy
CHEMICAL INSTRUMENTATION SYSTEMS
98 A
Schematic of RM automatic Electrobalance
ANALYTICAL CHEMISTRY
The use of a phototube to detect beam position ensures that the detector will not exert any stray force on the beam. It also provides a strong d.c. signal so that moderate amplifier gain will suffice. The amplifier drift is equivalent to a small fraction of a mi
crogram. Only one stage of voltage amplification is required and a bal anced bridge output stage follows this. Because the servo loop gain is so high, tube nonlinearities and drift have no effect. Line voltage fluctuation also has no effect. To minimize a-c pickup, the filaments and lamp are supplied with d.c. The lamp is also regulated with a zener diode. The suppression voltage must be stable to 0.01% also. It is regulated by two cascaded zener diodes. Although line variations of 5-10 volts permit rated performance, a Sola con stant-voltage transformer is used in the instrument which permits satisfactory operation between 90-130 volts. All rectifiers and regulators are solidstate devices. The two tubes in the amplifier and the phototube are the only tubes in the system. In our opinion this instrument is a fine example of modern design. It is compact, has fast response, and the electronics represents first class con temporary practice. Most analysts are getting used to recording balances and for good reason associate them with the art of weighing. It would seem that this is but one of many possible uses. It is interesting to note that when this torque principle was first described in a British journal, it was not used for weighing at all—it was developed for torque measurements of a very delicate nature and the obvious extension to weighing soon followed. The abovementioned use of the Cahn instrument in measuring muscular forces recalls an interesting application some years ago by Davidson who used a recording bal ance (strain-gage type) to follow the contraction or relaxation of the gas trocnemius muscle of the frog. This was used as an extremely sensitive means for the analysis of narcotics or stimulants which were perfused around the muscle. The buret and the beaker are still indispensable but there are other ways of doing important things.
