SWISS HAIL TUNNEL 
mal inertia. External surfaces are additionally 
protected by a coating of aralidite, which pre- 
serves a certain elasticity even at very low tem- 
peratures. 
THE PERFORMANCE OF THE Hath TUNNEL 
During stable operation—The operation of the 
tunnel is very complex, in particular with regard 
to the regulation of the temperature. The various 
factors which have to be observed for the general 
case may therefore be listed as follows: 
(1) The basic temperature of an individual 
experiment is given by the lowest temperature in 
the tunnel, that of the air-cooler: ty . 
(2) On the way to the measuring section the 
temperature of the air is raised as a result of cold 
losses by conduction to the outside and also of 
indirect heat generated by the blower: values 
which depend on the temperature of the tunnel 
itself and on the air speed. 
(3) The tunnel temperature is further influ- 
enced by the excess energy injected through the 
medium of the humidity. 
Fic. 5—The measuring section of the tunnel, 
the gages for pressure and the control panel 
313 
(4) Ice-forming nuclei may also influence the 
temperature according to their activity in trans- 
forming energy, as freezing processes take place 
in the subcooled cloud, before it reaches the 
measuring section. 
The sum of these influences determines the 
actual temperature in the measuring section, 
which is the effective temperature of the experi- 
ment, ty. To regulate it by means of the com- 
pressor acting on the vaporizer temperature is 
extremely difficult and quite impossible where 
there is any sudden alteration in humidity. The 
procedure adopted was therefore to produce in 
each experiment always more cold in the vapor- 
izer than was actually needed. The surplus cold 
is then destroyed by the heating element, whose 
output can be altered almost instantaneously; so 
that a maximum temperature adjustment can be 
achieved by hand control, or alternatively be 
regulated automatically. 
The efficiency of the tunnel with regard to 
temperature and humidity input is a function of 
the vaporizer temperature as well as of the air- 
speed, which helps to determine the amount 
of the losses from the system. The effective per- 
formance available for a particular experiment 
can be ascertained directly, by observing the fol- 
lowing procedure: the refrigerating compressor is 
allowed to run at full capacity while the air- 
speed and heat-setting are constant. With time 
the stable vaporizer-temperature ty develops, and 
once this is arrived at it provides an operational 
point which describes the connection between 
vaporizer temperature, effective refrigerating ca- 
pacity (which equals heating capacity), and air- 
speed. The group of curves in Figure 6 were given 
by the sum of such measurements. They show 
| ] 
Fic. 6—The effective cold output LZ; available for the experiment, at 
different air speeds vy, , as a function of the vaporizer temperature ty 
