- 9'^ 



from record to record. Such thermal effects accordingly affect the 

 mean of a pressure record. This could be mistakingly interpreted as 

 instrument tilt if two differential records taken at different 

 ambient temperatures are compared (Section III.C.4). It could also 

 affect the tide calculation by altering the back-fill pressure term, 

 P|jf , of the absolute sensor in Equation 4.35. Future development of 

 the DPG should more thoroughly investigate the effects of temperature 

 on the transducers and sensors. 



7. Absolute Sensor Back-Fill Pressure 



The absolute pressure sensor back-fill pressure, ?^^^ , is 

 obtained in the laboratory by either of two ways. (1) The instrument 

 is positioned such that there is no fluid head above the transducer 

 and the absolute pressure reported above the atmospheric is recorded. 

 (2) The instrument is oriented upright and the absolute pressure that 

 is reported beyond the atmospheric and sensor fluid head is recorded. 



D. CERC Wave Tank Tests 



1. Introduction 



The first DPG prototype was laboratory tested at the large 

 outdoor wave tank at the Coastal Engineering Research Center, Fort 

 Belvoir, Va. The instrument and cradle proved themselves easily 

 maneuverable — both above and below water. Although three of the 



