A Comparison of Mechanical and Electronic Balances David F. Rohrbach and Miles Pickeringl Princeton University, Princeton, NJ 08544 The microelectronics revolution is touching every phase of life, and analytical chemistry is no exception. In this note we report a comparison of the use of digital readout electronic balances with the conventional mechanical t v ~ eThis . studv was undertaken to learn about the cost effeiiiveness of thk halances, and also to find out how many electronic balances would be needed for our freshman classes. Our study was run on the more advanced of the two large freshman chemistry labs a t Princeton. Students were first trained on Mettler H15 mechanical balances. At the beginning of the third weighing experiment, four electronic balances (Mettler AC100's) were made available. Students were assigned a t random to mechanical or electronic halances. Students were not explicitly trained on the electronic balances but worked from a sheet of oosted instructions. The time recorded was the time from the moment the student first touched the balance to the time when the weighing of a single sample was complete. The weighing was done by the difference method; that is, a vial containing the solid was weighed, a little of the solid decanted, and the vial reweighed. Results and Discussion The results are summarized in the table. The following conclusions are striking 1) The electronic balances are about 3 to 4 times faster than the mechanical balances which saves over one minute per
weighing.
2) The electronic balances are simple enough that virtually all
students can learn to operate them without explicit training. The average time taken for a weighing does not decrease after the first week on the electronic balance. This indicates that maximum efficiency has been reached very quickly. For the mechanical balances, the average time is still declining after the fifth weighing experiment. We find that even in the less advanced of the two freshman lab courses there has been no difficulty in training students touse the balances, even those who have major difficulty coping with mechanical balances. We note parenthetically that there appears to he no difference between times for men and women on either the mechanical or electronic halances. We found that the only disadvantage of the electronic balance was that the fluctuations due to leaning on the table
418
Journal of Chemical Education
and similar technique faux pas were much more visible because they were digitally displayed rather than read from an analog display. Once students got used to this, the balance enjoyed excellent student acceptance, and students now often wait in line to use an electronic balance rather than using available mechanical ones. I t is clear to us that the electronic balance is as much an advance over the single pan mechanical balance as that was over the "double pan counting swings" method. The electronic balance removes the onerous task of teaching people how to weigh. a task that has been dreaded bv everv analvtical chemistry instructor since the time of ~erzeliu;. he-electronic balance frees uo vast amounts of time. not onlv because each weighing is veryhuch quicker, hut also because precious lab time previously used for training can now be devoted to real lab work. These balances will probably turn out to he one of the real revolutions in lab training in the 1980's, akin to the introduction of standard taper glassware and single pan balances in student labs in the 60's and 70's. Acknowledgment We are grateful for the help of one of our graduate teaching assistants, Brian Humphrey.
' Author to whom correspondenceshould be addressed. Comparison of Time in Seconds to do a Single Weighing by Difference on an Electronic versus a Mechanical Balance
Week
Electronic balance Average
1
31
2 3
23 23 26
Average Male Average Female Overall Average b-t
26.5 26
I'
10.2 9.8
Mechanical balance Average
s
WEratla 3.84 4.35 4.04 4.00
9.4
93
11.2
104
47.4 25.6 37.0 28.8
7.6
106
42.9
4.00
10.7
104
37.9
4.00
estimate of standard deviation
119
100
