angles to the direction in which the stylus moves 
to record temperature. When the BT is raised toward 
the surface, the spring expands the sylphon to its 
original shape. Thus the trace scratched on the 
smoked surface of the slide is a combined record of 
temperature and pressure, the pressure being propor- 
tional to depth. 
Since external pressure slightly affects the internal 
pressure of the xylene in the bourdon, and since 
temperature changes also influence the movement 
of the sylphon, each instrument must be carefully 
calibrated by the manufacturer. 
A special grid is supplied for each instrument for 
converting the stylus trace to temperature and ‘depth 
readings. From an examination of the grid, you 
will note that the temperature lines are not exactly 
straight and vertical, but curve slightly to the left 
with increasing depth. Likewise, the depth lines are 
‘not exactly arcs of circles with radius equal to the 
length of the stylus, but also allow for thermal ex- 
pansion of the sylphon. 
At a temperature of 105°F, the recording stylus 
brings up against a stop pin; if this temperature is 
exceeded, permanent deformation of the brass coil 
of the bourdon will occur, and the calibration of the 
instrument will be ruined. For this reason, the BT 
must always be kept out of the sun and away from 
the vicinity of fire rooms, steam pipes, and so forth. 
An instrument that has been overheated may often 
have the stylus arm jammed by the pen-lifter bar 
in the high-temperature position. If another BT is 
aboard, use it and turn in the damaged instrument 
for adjustment. If a spare is not available, gently lift 
the stylus arm from the pen-lifter bar and let the arm 
swing back toward the low-temperature side. 
The temperature calibration will henceforth be in 
error as a result of deformation of the bourdon. 
Record on the BT log sheet that the BT calibration is 
uncertain beginning with the next slide, and turn in 
the instrument for recalibration at the first opportunity. 
DEGREES FAHRENHEIT 
55 60 65 70 
Oe pee 8085 90, 
920k ae. pe reec 
at + : 
=e zs : : ae = co 
100 secezsssess == Het |00 
: : EES 
3 150 socessseassssseesseseee: iiss 150 
tet oa 
> 200 tt 200 
+ : 250 
= 250 i eessiesstesezseats Se 
& 300 iesssees 300 
© ssobseseeatas Seaeseeesee T° [350 
400HeiHs renee TB 40099|400 
+ th1e-59 
450% a5 a0 45 50 55 60 65 70 75 gO 65 ag5° 
BT grid 
