Computerized Digital Data Acquisition System for Thermogravimetry and Similar Applications Glauco Romeo, Eric Lifshin, Michael F. Ciccarelli, and Donald B. Sorensen Corporaie Research and Development, General Eleciric Company, Schenectady, N. Y. 12301
Continuous thermogravimetric analysis is widely employed in several fields of study, such a s oxidation of metals, degradation of resins, adsorption on catalysts, magnetic properties, etc. Precision electrobalances available on the market provide a n extremely sensitive means of measuring weight changes. The balance output voltage is recorded as a function of time, usually by a pen-recorder which can he coupled with an automatic range expander. However, this data-recording method often does not match the sensitivity of the balance, and its reliability may he impaired by failures such as nonconstancy of the chart speed, faulty inking, and jamming of the pen in the chart. These shortcomings may he particularly inconvenient on overnight runs and long-term experiments. Moreover, frequently the chart has to he replotted into an ordinary size diagram, and this operation can be tedious and time consuming. As a n alternative to the use of pen-recorders, we designed a package consisting of a digital voltmeter (DVM), a n interval timer, an elapsed time clock, and a teletype equipped with digital data controller. Figure 1 shows the block diagram of the system, a n actual picture of which is shown in Figure 2. The operation sequence is as follows. After the data controller (Figure 2, i) has been activated, the analog output of the electrohalance is fed into the DVM. The time interval for data recording is preselected by means of thumb-wheel switches (Figure 2, a ) . The first data set is recorded with the teletype switch in the ?TY NORMAL position (Figure 2, b ) when the start button (Figure 2, c) is depressed. The time is then recorded as zero (000000) seconds and the thermobalance output is recorded in millivolts as shown by the DVM display (Figure 2, h ) . The interval timer is started from the preset selection and counts down toward zero while the elapsed timer counts up simultaneously. The teletype switch may then ,be moved to the CLOCKED position (Figure 2, d ) as soon a s the first data set is printed. This action will cause the teletype motor to be turned off. Ten seconds before the interval timer reaches zero, the teletype turns on automatically. At zero on the interval timer, the DVM will be triggered to take a measurement; concurrently the elapsed time will be sampled and a second data set will be recorded. The teletype will turn on and operate only when printing and tape punchihg is required. This greatly reduces its mechanical wear and the unpleasant noise of the idling motor. The data format consists of two six-digit numbers separated by a comma, the first of which corresponds to the elapsed time in seconds (or tenths of a second as preset by switch e in Figure 2), while the second one corresponds to the DVM output with the sign. The time interval may be changed during a run a t any moment, except during printing. Upon completion of the run, the STOP button (Figure 2,)'j is depressed. The data are buffered by operating the DVM in the READ and HOLD mode (Figure 2, g). The elapsed timer
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DIAGRAMWQIGI~ DATA ACQUlflTlOW SYSTEM
Figure 1. Block
diagram of digital data acquisition system
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Figure 2.
Front panel
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digital data acquisition system
data are also buffered so that time is continuously logged during printing and no time error is made. The STOP button (Figure 2, f ) affects only the interval time and not the elapsed time so that if the experiment is to he restarted, the time data are not missed. Data sets presented in serial form to the teletype are printed out and punched at the same time on paper tape. Upon completion of a run, the data recorded on paper tape can he transmitted into a time shared data file for subsequent processing. Manipulation of the data such as curve fitting, linear regression analysis, etc., can he easily done by computer using comparatively simple software. In particular, if the computer is interfaced to a plotter, the data can he displayed terminally in graphic form. For example, to obtain weight gain us. time plots of thermogravimetric data, the stored "nosort" data file (GE 625 computer) is processed by a time-sharing program written in FORTRAN. A flow chart of the program is shown in Figure 3. The output of the main program and its associated CALCOMP subroutines (I) command a "slave" com(1) J. M. Miller, "A Fortran Programmer's Guide to CALCOMP PLOTTING," General Electric Company. Telecommunications and Inlormation Processing Dept.. Rept NO.66TIP3 (July 1966)
CHEMISTRY. VOL. 45. NO. 14, DECEMBER 1973
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Example of computer generated thermogravirnetric plot. While retaining the pristine printout of the curve, the axes have been relettered for better clarity
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puter (GE 4020) which, in turn, drives the pen of the CALCOMP plotter. The resulting plot displays weight gained or lost per unit area (mg/cm2) us. time (minutes). An example of such a plot is shown in Figure 4. The computer program provides the flexibility necessary to adjust the data to a predetermined plot size and t o label the axes appropriately.
In addition to the application to thermogravimetry, the digital data acquisition system can be used for monitoring and recording a variety of phenomena which involve either time-dependent parameters or, in general, up to three independent variables. Simple modifications of the basic hardware would, of course, be required for handling some specific tasks. All system components are commercially available as building blocks, as listed in Figure 1. Assembling of the system using commercial modules should be done with minimal design by any competent electronic staff.
ACKNOWLEDGMENT The assistance of S. Ludke and R. J. Herbs with the design and troubleshooting of the apparatus is gratefully acknowledged. Received for review March 29, 1973. Accepted June 25, 1973.
New Stirring Method for Microtitration J. R. Walsby Marine Research Laboratory, University of Auckland, R. D. Leigh, New Zealand A method of indirect stirring of a small volume of liquid during its potentiometric microtitration, is described. It has been used in the study of a marine worm's coelomic fluids chloride levels. The chloride levels were up to sea water concentration (35Oho) and therefore heavy silver chloride precipitate was involved during titration with silver nitrate. Precipitate accumulation about the microburet tip inhibits accurate titration, and none of the various methods of dealing with this problem, summarized by Ramsey, Brown, and Croghan ( I ) , were satisfactory. With small titration volumes, there is no room about the microburet tip for any moving mechanical or magnetic stirrer in the titrate and with potentiometric titration, care must be taken not t o disturb the indifferent electrode (anode). The (1) J . A. Ramsay. R . H. J. Brown, and P. C. Croghan, J. Exp. Bioi., 32, 822 (1 955).
method of Sanderson ( 2 ) of stirring by bubbling an air stream through the titrate, was unsatisfactory, because as the bubbles burst a t the surface, liquid containing chloride was lost from the titrate as minute droplets. With titration on a hydrofuge surface ( 3 ) ,the air stream directed a t the titrate body to effect stirring, did not efficiently disperse the precipitated silver chloride about the microburet opening and the indifferent electrode. Walsby ( 4 ) describes a flask for a reciprocating shaker in which the internal contours cause the contents to be swirled around as a body. The technique was developed for algal culture work and is also used for long term con(2) P. H. Sanderson, Biochern. J . , 52, 502 (1952) (3) J. Shaw, J. Exp. B i d , 32,321 (1955). (4) A. E. Walsby, Eiotechno/. Eioeng., 9,443 (1967).
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