To bring away the heat produced by the subject, it is highly desirable
that a constant flow of water of even temperature be secured. Direct
connection with the city supply is not practicable, owing to the
variations in pressure, and hence in constructing the laboratory
building provision was made to install a large tank on the top floor,
fed with a supply controlled by a ball-and-cock valve. By this
arrangement the level in the tank is maintained constant and the
pressure is therefore regular. As the level of the water in the tank is
approximately 9 meters above the opening in the calorimeter, there is
ample pressure for all purposes.
[Illustration: FIG. 14.--Schematic diagram of water circuit for
heat-absorbers of calorimeter. A, constant-level tank from which water
descends to main pipe supplying heat-absorbers; _a_, valve for
controlling supply from tank A; B, section of piping passing into cold
brine; _b_, valve controlling water direct from large tank A; _c_, valve
controlling amount of water from cooling section B; C, thermometer at
mixer; D, electric heater for ingoing water; E, thermometer for ingoing
water; _d d d_, heat-absorbers inside calorimeter; F, thermometer
indicating temperature of outcoming water; G, can for collecting water
from calorimeter; _f_, valve for emptying G.]
The water descends from this tank in a large 2-inch pipe to the ceiling
of the calorimeter laboratory, where it is subdivided into three 1-inch
pipes, so as to provide for a water-supply for three calorimeters used
simultaneously, if necessary, and eliminate the influence of a variation
in the rate of flow in one calorimeter upon the rate of flow in another.
These pipes are brought down the inner wall of the room adjacent to the
refrigeration room and part of the water circuit is passed through a
brass coil immersed in a cooling-tank in the refrigeration room. By
means of a by-pass, water of any degree of temperature from 2° C. to 20°
C. may be obtained. The water is then conducted through a pipe beneath
the floor to the calorimeter chamber, passed through the absorbers, and
is finally measured in the water-meter.
A diagrammatic sketch showing the course of the water-current is given
(fig. 14), in which A is the tank on the top floor controlled by the
ball cock and valve, and _a_ is the main valve which controls this
supply to the cooler B, and by adjusting the valve _b_ and valve _c_
any desired mixture of water can be obtained. A thermometer C gives a
rough idea of the temperature of the water, so as to aid in securing the
proper mixture. The water then passes under the floor of the calorimeter
laboratory and ascends to the apparatus D, which is used for heating it
to the desired temperature before entering the calorimeter. The
temperature of the water as it enters the calorimeter is measured on an
accurately calibrated thermometer E, and it then passes through the
absorber system _d d d_ and leaves the calorimeter, passing the
thermometer F, upon which the final temperature is read. It then passes
through a pipe and falls into a large can G, placed upon scales. When
this can is filled the water is deflected for a few minutes to another
can and by opening valve _f_ the water is conducted to the drain after
having been weighed.
_Brine-tank._--The cooling system for the water-supply consists of a
tank in which there is immersed an iron coil connected by two valves to
the supply and return of the brine mains from the central power-house.
These valves are situated just ahead of the valves controlling the
cooling device in the refrigeration room and permit the passage of brine
through the coil without filling the large coils for the cooling of the
air in the calorimeter laboratory. As the brine passes through this
coil, which is not shown in the figure, it cools the water in which it
is immersed and the water in turn cools the coil through which the
water-supply to the calorimeter passes. The brass coil only is shown in
the figure. The system is very efficient and we have no difficulty in
cooling the water as low as 2° C. As a matter of fact our chief
difficulty is in regulating the supply of brine so as not to freeze the
water-supply.
_Water-mixer._--If the valve _b_ is opened, water flows through this
short length of pipe much more rapidly than through the long coil, owing
to the greater resistance of the cooling coil. In conducting these
experiments the valve c is opened wide and by varying the amount to
which the valve _b_ is opened, the water is evenly and readily mixed.
The thermometer C is in practice immersed in the water-mixer constructed
somewhat after the principle of the mixer inside the chamber described
on page 21. All the piping, including that under the floor, and the
reheater D, are covered with hair-felt and well insulated.
_Rate-valves._--It has been found extremely difficult to secure any form
of valve which, even with a constant pressure of water, will give a
constant rate of flow. In this type of calorimeter it is highly
desirable that the rate of flow be as nearly constant as possible hour
after hour, as this constant rate of flow aids materially in maintaining
the calorimeter at an even temperature. Obviously, fluctuations in the
rate of flow will produce fluctuations in the temperature of the ingoing
water and in the amount of heat brought away. This disturbs greatly the
temperature equilibrium, which is ordinarily maintained fairly constant.
Just before the water enters the reheater D it is caused to pass through
a rate-valve, which at present consists of an ordinary plug-cock. At
present we are experimenting with other types of valves to secure even
greater constancy, if possible.
_Electric reheater._--In order to control absolutely the temperature of
the water entering at E, it is planned to cool the water leaving the
water-mixer at C somewhat below the desired temperature, so that it is
necessary to reheat it to the desired point. This is done by passing a
current of electricity through a coil inserted in the system at the
point D. This electric reheater consists of a standard "Simplex" coil,
so placed in the copper can that the water has a maximum circulation
about the heater. The whole device is thoroughly insulated with
hair-felt. By connecting the electric reheater with the rheostat on the
observer's table, control of the quantity of electricity passing through
the coil is readily obtained, and hence it is possible to regulate the
temperature of the ingoing water to within a few hundredths of a degree.
The control of the amount of heat brought away from the chamber is made
either by (1) increasing the rate of flow or (2) by varying the
temperature of the ingoing water. Usually only the second method is
necessary. In the older form of apparatus a third method was possible,
namely, by varying the area of the absorbing surface of the cooling
system inside of the chamber. This last method of regulation, which was
used almost exclusively in earlier experiments, called for an elaborate
system of shields which could be raised or lowered at will by the
operator outside, thus involving an opening through the chamber which
was somewhat difficult to make air-tight and also considerably
complicating the mechanism inside the chamber. The more recent method of
control by regulating the temperature of the ingoing water by the
electric reheater has been much refined and has given excellent service.
_Insulation of water-pipes through the wall._--To insulate the
water-pipes as they pass through the metal walls of the calorimeter and
to prevent any cooling effect not measured by the thermometers presented
great difficulties. The device employed in the Middletown chamber was
relatively simple, but very inaccessible and a source of more or less
trouble, namely, a large-sized glass tube embedded in a large round
wooden plug with the annular space between the glass and wood filled
with wax. An attempt was made in the new calorimeters to secure air
insulation by using a large-sized glass tube, some 15 millimeters
internal diameter, and passing it through a large rubber stopper,
fitting into a brass ferule soldered between the zinc and copper walls.
(See N, fig. 25.) So far as insulation was concerned, this arrangement
was very satisfactory, but unfortunately the glass tubes break readily
and difficulty was constantly experienced. An attempt was next made to
substitute hard-rubber tubing for the glass tube, but this did not prove
to be an efficient insulator. More recently we have used with perfect
success a special form of vacuum-jacketed glass tube, which gives the
most satisfactory insulation. However, this system of insulation is
impracticable when electric-resistance thermometers are used for
recording the water-temperature differences and can be used only when
mercurial thermometers exclusively are employed. The electric-resistance
thermometers are constructed in such a way, however, as to make
negligible any inequalities in the passage of heat through the
hard-rubber casing. This will be seen in the discussion of these
thermometers.
_Measuring the water._--As the water leaves the respiration chamber it
passes through a valve which allows it to be deflected either into the
drain during the preliminary period, or into a small can where the
measurements of the rate of flow can readily be made, or into a large
tank (G, fig. 14) where the water is weighed. The measurement of the
water is made by weight rather than by volume, as it has been found that
the weighing may be carried out with great accuracy. The tank, a
galvanized-iron ash-can, is provided with a conical top, through an
opening in which a funnel is placed. The diagram shows the water leaving
the calorimeter and entering the meter through this funnel, but in
practice it is adjusted to enter through an opening on the side of the
meter. After the valve _f_ is tightly closed the empty can is weighed.
When the experiment proper begins the water-current is deflected so as
to run into this can and at the end of an hour the water is deflected
into a small can used for measuring the rate of flow. While it is
running into this can, the large can G is weighed on platform scales to
within 10 grams. After weighing, the water is again deflected into the
large can and that collected in the small measuring can is poured into G
through the funnel. The can holds about 100 liters of water and
consequently from 3 to 8 one-hour periods, depending upon the rate of
flow, can be continued without emptying the meter. When it is desired to
empty the meter at the end of the period, the water is allowed to flow
into the small can, and after weighing G, the valve _f_ is opened. About
4 minutes are required to empty the large can. After this the valve is
again closed, the empty can weighed, and the water in the small
measuring-can poured into the large can G through the funnel. The scales
used are the so-called silk scales and are listed by the manufacturers
to weigh 150 kilograms. This form of scales was formerly used in
weighing the man inside the chamber.[7]