The complete removal of water-vapor and carbon dioxide from a current of
air moving at the rate of 75 liters per minute calls for large and
somewhat unwieldy vessels in which is placed the absorbing material.
This is particularly the case with the vessels containing the rather
large amounts of sulphuric acid required to dry the air. In the course
of an hour there is ordinarily removed from the chamber not far from 25
grams of water-vapor and 20 to 30 grams of carbon dioxide. This
necessitates weighing the absorbers to within 0.25 gram if an accuracy
of 1 per cent is desired. The sulphuric-acid absorbers weigh about 18
kilograms when filled with acid. In order to weigh this receptacle so as
to measure accurately the increase in weight due to the absorption of
water to within less than 1 per cent, we use the balance shown in fig.
29. This balance has been employed in a number of other manipulations in
connection with the respiration calorimeter and accessory apparatus and
the general type of balance leaves nothing to be desired as a balance
capable of carrying a heavy load with remarkable sensitiveness.
The balance is rigidly mounted on a frame consisting of four upright
structural-steel angle-irons, fastened at the top to a substantial
wooden bed. Two heavy wooden pieces run the length of the table and
furnish a substantial base to which the standard of the balance is
bolted. The balance is surrounded by a glass case to prevent errors due
to air-currents (see fig. 2). The pan of the balance is not large enough
to permit the weighing of an absorber, hence provision is made for
suspending it on a steel or brass rod from one of the hanger arms. This
rod passes through a hole in the bottom of the balance case, and its
lower end is provided with a piece of pipe having hooks at either end.
Since the increase in weight rather than the absolute weight of the
absorber is used, the greater part of the weight is taken up by lead
counterpoises suspended above the pan on the right-hand arm of the
balance. The remainder of the weight is made up with brass weights
placed in the pan.
[Illustration: FIG. 29.--Balance for weighing absorbers, showing general
type of balance and case surrounding it, with counterpoise and weights
upon right-hand pan. A sulphuric-acid absorber is suspended in position
ready for weighing. Elevator with compressed-air system is shown in
lower part of case.]
In order to suspend this heavy absorber, a small elevator has been
constructed, so that the vessel may be raised by a compressed-air
piston. This piston is placed in an upright position at the right of the
elevator and is connected with the compressed-air service of the
building. The pressure is about 25 pounds per square inch and the
diameter of the cylinder is 2.5 inches, thus giving ample service for
raising and lowering the elevator and its load. By turning a 3-way
valve at the end of the compressed-air supply-pipe, so that the air
rushes into the cylinder above the piston, the piston is pushed to the
base of the cylinder and the elevator thereby raised. The pressure of
the compressed air holds the elevator in this position while the hooks
are being adjusted on the absorber. By turning the 3-way valve so as to
open the exhaust leading to the upper part of the cylinder to the air,
the weight of the elevator expels the air, and it soon settles into the
position shown in the figure. The weighing can then be made as the
absorber is swinging freely in the air. After the weighing has been
made, the elevator is again lifted, the hooks are released, and by
turning the valve the elevator and load are safely lowered.
The size of the openings of the pipes into the cylinder is so adjusted
that the movement of the elevator is regular and moderate whether it is
being raised or lowered, thus avoiding any sudden jars that might cause
an accident to the absorbers. With this system it is possible to weigh
these absorbers to within 0.1 gram and, were it necessary, probably the
error could be diminished so that the weight could be taken to 0.05
gram. On a balance of this type described elsewhere,[20] weighings could
be obtained to within 0.02 gram. For all practical purposes, however, we
do not use the balance for weighing the absorbers closer than to within
0.10 gram. In attempting to secure accuracy no greater than this, it is
unnecessary to lower the glass door to the balance case or, indeed, to
close the two doors to the compartment in which the elevator is closed,
as the slight air-currents do not affect the accuracy of the weighing
when only 0.1 gram sensitiveness is required.