3B Scientific Heat Equivalent Apparatus - Manual (Page 6)

6
3. Operation and maintenance
•
The equivalent of heat apparatus is attached to a
stable workbench using its table clamp. The fric-
tion cord is then wrapped around the friction cyl-
inder 4½ to 5½ times with the counterweight sus-
pended at the rear and the loose end of the cord
hanging down at the front.
•
The bucket provided can be filled with water or sand
etc. (total weight approx. 5 kg) and used as a weight.
The loose end of the friction cord is attached to the
weight while the latter is resting on the ground. It
should be observed that the counterweight should
be no more than about 5 cm above the ground
when the cord is taut. This prevents the weight be-
ing raised by more than about 10 cm during the
experiment.
•
If it is observed that the cord moves to the right
when the crank is turned or fails to remain in its
groove, then the cord should be wrapped around
the cylinder so that the end of the cord with the
weight is on the right and that with the counter-
weight is on the left.
•
The temperature sensor should be wetted with a
drop of oil (important!) and inserted into the se-
lected friction cylinder according to Fig. 1 until it is
felt to snap into place and can be turned easily (if
it is inserted too far or not far enough, it is not easy
to turn it). The two connections of the temperature
sensor are attached to a resistance meter (multim-
eter) operating in the range 2 k
Ω
to 9 k
Ω
with a
display accurate to at least three figures. The con-
version of the resistance so measured into a corre-
sponding temperature can be performed either
with the help of the conversion table on the last
page of these instructions or by using the follow-
ing equation:
T
R
= −
217
151
0.13
(1)
where R must be given in k
Ω
to obtain T in °C. This
equation agrees with the table provided by the NTC
thermistor manufacturer in the range from 10 - 40 °C
to an accuracy of approximately ± 0.05 °C.
•
Before an experiment the friction cylinder should
be cooled to about 5 - 10°C below the ambient tem-
perature. This can be achieved by putting it in a
refrigerator or by dipping it in cold water. In the
latter case the hole for the temperature sensor
should point upwards and the cylinder may only
be immersed to a depth of about 2/3 the height of
the cylinder (tip: if the friction cylinder is dipped
in water inside a plastic bag, it will not need to be
dried off again when it has finished cooling).
•
The rise in temperature during an experiment
should continue until the friction cylinder’s tem-
perature has been raised to about 5 - 10°C above
the ambient temperature. The more precisely the
temperature differences for cooling and heating
(with respect to the ambient temperature) are simi-
lar, then the smaller is the net exchange of heat
with the environment.
•
For heating the friction cylinder electrically, adapter
cables are provided with plugs of 2 mm diameter
at one end and conventional 4 mm lab plugs at the
other. The power should be provided by a power
supply where voltage and current limiting can be
regulated. The maximum voltage from the power
supply may not exceed 40 V. The positive pole of
the power supply is connected to the isolated socket
(identifiable due to the round, gray plate beneath
the socket) and the negative is connected to the
other socket.
•
The heating elements for the friction cylinders do
not behave like ohmic resistors. From about 10 V
the current rises rapidly until it reaches the per-
mitted maximum of 3 A at about 12 V. To calibrate
an operating point it is recommended that the cur-
rent limiting initially be set at 2 – 3 A then the volt-
age raised until about 11 V, in which case the cur-
rent should rise to about 1 A. The current limiting
is then regulated until about 0.8 A flows. These set-
tings are not altered thereafter until the current is
broken simply by pulling out the cables at the time
of the experiment. The active current limitation and
non-ohmic characteristic mean that the voltage and
current remain nearly constant in spite of the tem-
perature change during an experiment.
•
Maintenance: the equivalent of heat apparatus in
principle requires no maintenance. It can be wiped
clean with soap and water. Solvents should not be
used. Immersion in water should also be avoided.
•
The friction cylinders should be plain naked metal.
If a coating has formed on them, this can be re-
moved using metal cleaner.
•
The friction cord can be washed if necessary. For a
good value alternative, woven nylon cord can be
used as a replacement.
4. Experiment procedure and evaluation
4.1 Conversion of mechanical work into heat
4.1.1 Experiment procedure
•
First the various masses are measured:
Primary weight (e.g. bucket with water) m
H
= 5.22
Counterweight (attached to friction cord)
m
G
= 0.019 kg
Aluminum cylinder m
A
= 0.249 kg
•
Other values to be measured in advance:
Ambient temperature T
U
= 23,2 °C
Diameter of cylinder where friction occurs
D
R
= 45.75 mm
•
After cooling the cylinder, it should be screwed to
the base, the temperature sensor should be in-
serted and the friction cord should be wrapped
around it. (cf. Section 3). After a few minutes, that
should be ignored for the sake of a homogenous