housing the permanent magnet and solenoid. This air leak serves to balance 

 the long-term average pressure on the inside and outside pf the small 

 bellows and thus causes the solenoid to oscillate around the same static 

 point. Thus, if the pressure head is at sea level and is suddenly lowered 

 to 30-foot depth, there will be only a temporary displacement of "the small 

 bellows, the air leak serving to balance the pressure between the two 

 chambers within a period of some 7 or 8 minutes* This balancing arrange- 

 ment allows a small bellows of the greatest possible sensitivity to be 

 used and also, as will be explained later, brings the recorder pen to zero 

 for all existing average pressure conditions. Tidal fluctuations are com- 

 pensated for in this manner. 



This equipment is presently designed to measure accurately pressure 

 variations at the gage with periods of from U to 30 seconds. For periods 

 greater than 30 seconds, the air leak out of the bellows chambers causes 

 some error to be introduced into the record. Fluctuations with a period 

 of 5 minutes or more are not detected. The equipment is designed to 

 measure pressure fluctuations at the gage varying in magnitude from a 

 minimum of 1/2 foot of water to a maximum of 2S feet of water. It can 

 measure these pressures when operating under an average pressure of any 

 value between atmospheric pressure and $0 feet of water plus atmospheric 

 pressure. In any event the gage should not be operated under a pressure 

 greater than that produced by 60 feet of water. 



The pressure head and recorder are calibrated as a unit in a pressure 

 chamber built for this purpose in the laboratory of the Beach Erosion Board. 

 This chamber has provisions for applying a static pressiire of from 1 to UO 

 feet of water and then superimposing on this static pressure, varying 

 pressures of from 1 to 2^ feet of water at periods of from It to 30 seconds. 

 Provisions are also incorporated to determine the effect of water temperature, 

 although tests made in the chamber have shown that the response of the gage 

 is not noticeably affected by water temperature changes as great as UO°F. 



THE PRESSURE HEAD MOUNT 



The mount (Figure 2) is made up of a base, pressure head tripod and 

 triangular frame. The base of the pressure head mount is a concrete block, 

 weighing approximately 350 pounds submerged in salt water. The pressure 

 head is mounted on a tripod which rests directly on the concrete block 

 and is insulated electrically from it with a ring of Plexiglas or plywood 

 to prevent electrolysis. A triangular lifting frame is mounted on the 

 block, completely separated from the pressure head tripod, to provide a 

 means for raising and lowering the gage assembly and to protect the pressure 

 head from damage. A 3/8 inch lifting chain of 5,600 pounds breaking strength 

 runs from a ring at the top of the frame to the w ater surface where it is 

 attached to a buoy. Chain is preferred -to wire rope as it will not kink, 

 or lose its strength as quickly from rusting. The pressure head and mount- 

 ing assembly weigh about 750 pounds in air, and 500 pounds submerged in 

 salt water. 



