PRESSURE 



Fig. 10 10 Bourdon tube 



transmitting pressure through the hull to 

 the sensing element varies. In one instance 

 (.BE!>f FRANKLIIS) a soft reservoir external 

 to the hull was filled with mineral oil which 

 transmitted pressure changes by thru-hull 

 hydraulic lines directly to the sensing ele- 

 ment inside the hull. 



Resistance Type Pressure Transducers — TheSC in- 

 struments are composed of a pressure-sen- 

 sing element, such as the Bourdon tube, a 

 device to convert its tip motion to an electri- 

 cal parameter and a device to indicate or 

 record pressure changes. A major advantage 

 to this system is that there is no free or open 

 end leading directly to the sea, because the 

 depth indicating signal is passed electrically 

 through the hull. The most common form of 

 pressure transducer is a contact coupled to 

 the sealed end of the Bourdon tube which 

 slides along a continuous resistor. With a 

 resistor of constant cross section, the change 

 in resistance will be proportional to the 

 movement of the contact. 



The pressure transducer shown in Figure 

 10.11 is a bellows type in which the pressure 



from the bellows is exerted against a pre- 

 cisely designed spring which reacts and con- 

 verts the pressure to a linear motion via the 

 plate (moving contact) between it and the 

 bellows. The plate has a contact which wipes 

 the surface of the resistor and, if a constant 

 AC or DC potential is held across the resis- 

 tor, the measured voltage (at the voltmeter) 

 is a precise measure of the pressure. 



DS-4000 uses this principle as one means 

 of sensing depth, but a Bourdon tube is used 

 instead of a bellows. The unit (Hydro Prod- 

 ucts Model 404) is in a pressure-resistant 

 aluminum housing, the Bourdon tube is oil- 

 filled, and a rubber diaphragm separates the 

 pressure transducer from the external envi- 

 ronment to provide corrosion protection. A 

 potentiometer is contained in the sensor and 

 receives excitation voltage from a monitor 

 unit in the pressure hull. The output is volt- 

 age proportional to the external sensor. The 

 monitor (Hydro Products 402) indicates 

 depth to 1,600 meters at 20-meter increments 

 and is powered by a mercury battery that is 

 good for 200 hours. Maximum visual meter 

 indication error is 2 percent of full scale. 



Strain Gage Pressure Transducers — Chapter 5 



described the role of strain gages in provid- 

 ing data to compare calculated against meas- 

 ured hull strength. The same unit, by virtue 

 of its change in resistance as a function of 

 hull distortion from ambient pressure, can 

 also provide accurate depth measurements. 

 The U.S. Navy's SEA CUFF and TURTLE 

 employ three independent strain gage 

 (Wheatstone) bridges bonded to the interior 

 of the pressure sphere which are selected 

 individually to supply an electrical depth 



-7777P 



VOLTMETER 



Rg. 10.11 Resistive pressure transducer. 



482 



