BOURDON GAUGE — 
HOW A BT WORKS 
The bathythermograph is designed to obtain a record 
of the temperature of sea water at moderate depths. 
It can be operated while the ship is underway at 
speeds up to 18 knots. 
The temperature element, corresponding to the mer- 
cury column in a glass thermometer, consists of a 
long length of fine copper tubing filled with xylene. 
The tubing is wound around inside the tail fins 
of the BT and is in direct contact with the sea water. 
STOP PIN 
SYLPHON BELLOWS 
—_——_ —_—_ ——. 
I Tn 
Sali 
SMOKED SLIDE 
PEN LIFTER BAR 
As the xylene expands or contracts with the changing 
water temperature, the pressure inside the tubing 
increases or decreases. This pressure change is trans- 
mitted to a bourdon gauge, a hollow brass coil spring 
which carries a stylus at its free end. The stylus records 
the movements of the bourdon, as it expands or con- 
tracts with changes of temperature, on a smoked-glass 
slide. The slide is held rigidly on the end of a coil 
spring enclosed in a copper bellows or sylphon. 
Water pressure, which increases in proportion to 
water depth, compresses the sylphon as the BT sinks. 
This pulls the slide toward the nose of the BT at right 
