A Convenient Multiple-Unit Calorimeter Installation Page: 44
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44 CONVENIENT MULTIPLE-UNIT CALORIMETER INSTALLATION.
surface and condense on the cooler walls of the jacket when the tem-
perature of the calorimeter is positive. Thus an amount of heat
proportional to the amount of water evaporated is carried from the
calorimeter system. That the weight of water evaporated is about
proportional to the time for a given temperature difference with the
bureau's calorimeters has been shown by making a number of com-
bustions with the water seal removed. Thus, for a difference of ap-
proximately 1.50 C. the normal rate of temperature drop is 0.0045-
0.0050 (corresponding to 0.00220--0.0025o C.) per minute, while with
the seal removed the figure was found to be 0.00350 per minute.
It would seem that serious error would not be introduced by dis-
pensing with the seal as long as combustion standardization is per-
formed under the same conditions or as long as the calorimeter tem-
perature is always below the jacket temperature. The apparent
cooling correction would, of course, be larger because of the evapora-
tion factor, and in that respect would be subject to larger error.
For this reason alone the seal is perhaps justified, as it involves little
added expense and does not affect in the least the manipulation of
the instrument. The calorimeter must be perfectly dry when placed
in the jacket or an uncertain evaporation factor will be introduced.
This factor will be larger than that just mentioned because of the
larger surface exposed and will be uncertain because the excess
moisture may all have evaporated during the combustion period,
causing an abrupt change in the heat transfer equilibrium. Such a
condition exists when the water charged into the calorimeter is below
the dew point of the atmosphere of the room; then a considerable
amount of moisture will deposit on the calorimeter can before this
is placed on the jacket. On numerous occasions the authors have
found erratic results due to this cause, and for the past year working
temperatures have been kept well above the dew point. Now, if the
jacket walls are not kept dry, water will distill to the calorimeter,
causing an input of heat while the calorimeter temperature is nega-
tive and an outflow of heat when the calorimeter temperature changes
to positive. Hence, it is considered much better practice to work well
above the dew point, with calorimeter and jacket surfaces dry, than
to permit evaporation and attempt. to correct for it.
ACCURACY ATTAINABLE WITH THE CALORIMETER.
The probable error of a single determination with bomb No. 3 of
the equipment described may be taken as established by the following
series of experiments:
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Davis, J. D. & Wallace, E. L. A Convenient Multiple-Unit Calorimeter Installation, report, 1918; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc66440/m1/56/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.