Shortly after the subject has entered the chamber and in many instances
before the sealing-in process has begun, the ventilating air-current is
started by starting the blower. The air passes through one set of
purifiers during this preliminary period, and as no measurements are
made for this period it is not necessary that the weights of the
absorbers be previously known.
All precautions are taken, however, so far as securing tightness in
coupling and installing them on the absorber system are concerned.
During this period the other set of absorbers is carefully weighed and
made ready to be put in place and tested and about 10 minutes before the
experiment proper begins the residual analyses are begun. The series of
U-tubes, which have previously been carefully weighed, are placed on
small inclined racks and are connected with the meter and also with the
tube leading to the mercury valve. The pet-cock which connects the
return air-pipe with the drying-tower and the gas-meter is then opened
and the mercury reservoir is lowered. The rate of flow of air through
the U-tubes is regulated by a screw pinch-cock on the rubber tube
leading to the first U-tube. This rate is so adjusted by means of the
pinch-cock that about 3 liters of air per minute will flow through the
U-tubes, and as the pointer on the gas-meter approaches 10 liters the
mercury reservoir is raised at just such a point, gained by experience,
as will shut off the air-current when the total volume registers 10
liters on the meter. The pet-cock in the pipe behind the meter is then
closed, the U-tubes disconnected, and a new set put in place. A
duplicate and sometimes a triplicate analysis is made.
When the physical observer calls the time for the end of the period, the
switch which controls the motor is opened and the chemical assistant
then opens the rear valve of the new set of absorbers and closes the
rear valve of the old set, and likewise opens the front valve of the new
set and closes the front valve of the old set. As soon as the signal is
given that the oxygen connections have been properly made and that the
oxygen has been admitted to the chamber in proper amount, the blower is
again started. It is then necessary to weigh the U-tubes and disconnect
the old set of absorbers and weigh them. If the sulphuric-acid absorbers
have not exceeded the limit of gain in weight they are used again; if
they have, new ones are put in their place.
The first sulphuric-acid absorber is connected to the front valve, then
the potash-lime can, and then the last sulphuric-acid absorber; but
before connecting the last sulphuric-acid absorber with the
sodium-bicarbonate can, a test is made of the whole system from the
front valve to the end of the second sulphuric-acid absorber. This is
made by putting a solid-rubber stopper in the exit end of the second
sulphuric-acid absorber and, by means of a bicycle pump, forcing
compressed air in through a pipe tapped into the pipe from the valve at
the front end until a pressure of about 2 feet of water is developed in
this part of the system. This scheme for testing and the method of
connecting the extra pipe have been discussed in detail in an earlier
publication.[32] Repeated tests have shown that this method of testing
the apparatus for tightness is very successful, as the minutest leak is
quickly shown.
After the system has been thoroughly tested, the rubber stopper in the
exit end of the second sulphuric-acid absorber is first removed, then
the tube connected with the pump and manometer is disconnected and its
end placed in the reservoir of mercury. Occasionally, through oversight,
the pressure is released at the testing-tube with the result that the
air compressed in the system expands, forcing sulphuric acid into the
valves and down into the blower, thus spoiling completely the
experiment. After the testing, the last sulphuric-acid absorber is
coupled to the sodium-bicarbonate can. It is seen that this last
connection is the only one not tested, and it has been found that care
must be taken to use only the best gaskets at this point, as frequently
leaks occur; in fact, it is our custom to moisten this connection with
soapsuds. If new rubber gaskets are used a leak is never found.