transducer inside the hull, simplicity and reliability are obtained 
since no watertight housings, etc., are needed. The transducer is 
simply connected to the nearest sea chest by means of a high 
pressure line; the electrical leads are connected to the associated 
control and recording system. The pressure at the sea chest has 
a quasi-static component, and a fluctuating component due to the 
surface wave action. The latter pressure drops off exponentially 
with distance from the free surface and in usual operation is 
sufficiently attenuated. Near the surface, these fluctuating com- 
ponents would have to be filtered out to prevent aliasing when the 
pressure reading is sampled at intervals equal to, or longer than, 
half-surface-wave periods. 
A Statham Model PA208TC unbonded strain gage pressure 
transducer was used.* This was an absolute gage with a range of 
0-500 psia and a combined nonlinearity and hysteresis of less than 
+0.75 percent of full scale. The basic bridge circuit used with the 
pressure transducer, recommended by Statham, is shown in fig. 6. 
In this circuit there is only one adjustment necessary and this is 
the zero adjustment. Resistor, ? ,, is the coarse zero adjustment 
and the combination of 2 , and# "3 is the fine adjustment. In prac- 
tice, this adjustment was used in order to make this bridge coincide 
with the reading obtained on the ship's control room bourdon gage, 
with the submarine at zero bubble. Thus, comparisons continu- 
ing throughout the sea test provided one method of calibration. 
Another type of calibration consists of inserting the resistors 
Ro, throughf,, one at a time, by means of corresponding mo- 
mentary switches S ; through .. 
From the manufacturer's calibration data for the transducer, 
it is possible to calculate the value of the calibrate resistor which 
corresponds to a given pressure interval, and thus a known depth 
interval. For example, if the value of resistor, Ra), corresponds 
to a 25 foot interval, then, when it is inserted in the circuit, this 
will produce a change in voltage output, @9, corresponding to 25 
feet. It should be emphasized that this change is superimposed on 
the existing pressure signal. The depth calibration intervals 
*Statham Laboratories, Inc., 12401 W. Olympic Blvd. , Los Angeles 
64, California 
