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The arms are three-inch diameter PVC pipe and fifty-two 

 inches in length. Each contains a pressure-sensing diaphragm near 

 its end. The extreme end of each arm is covered by a threaded 

 pipe-cap which can be removed to inspect the diaphragm and replace 

 copper scouring pads placed near the diaphragm to reduce biological 

 fouling. The arms are punctured with 11/16 inch diameter holes about 

 their ends to allow diaphragm communication with the environment. 

 Each arm is labelled with strips of white cloth tape to identify the 

 differential transducer channel corresponding to that arm. 



3. The Transducers 



A typical differential pressure gauge senses the pressure 

 difference between the diaphragm at the end of an arm and one of the 

 five diaphragms mounted inside the fuselage. The differential 

 pressure transducers are located inside the water-tight cylinder. 

 The differential transducers are Setra Systems Model 228. They are a 

 high line, low-differential wet-wet capacitance type sensor with 

 infinite resolution and a ±0.5 psid range. Transducer output noise 

 is rated as below 100 microvolts RMS. Burst pressure is 2500 psid 

 (either side) such that there is less danger of catastrophic failure 

 during construction or installation. The differential transducers 

 require 28 volts DC nominal excitation and feature to 5 volt 

 positive output. The transducer circuits have internal protection 

 against reversed excitation voltage for at least 5 minutes, 



