paper-tape record. The below-deck installations at stations 19+60 and 7+20 con- 

 sist of 12-inch-diameter stilling wells with a 1-inch orifice and 0.5-inch 

 orifice, respectively, and an 8.5-inch float (Fig. 18). The stilling well acts 

 as a high-frequency filter to dampen the effects of waves, thereby giving accu- 

 racy to the measurements of water level fluctuations produced by the tide. The 

 float, which responds to any variation in the water column within the stilling 

 well, is coupled to a float-wire takeup drum and input shaft assembly via a 

 stainless-steel wire. The angular position of the input shaft is recorded on 

 paper tape every 6 minutes in a standard binary-decimal code. 



Figure 18. Stilling well. 



The Bristol and Metercraft pressure gages measure the hydrostatic pressure 

 created by a column of water above a submerged orifice. The orifice and the re- 

 cording instrument are linked by a nitrogen-filled, plastic supply line. Any 

 change in the hydrostatic pressure, such as that caused by the rise and fall of 

 the tide, is transmitted to the recorder where the pressure variations are re- 

 corded on a strip chart as changes in water depth. 



To monitor tide gage operation and provide datum information, electric tape 

 gages have been installed at stations 19+60 and 7+20 in lieu of a tide staff. 

 The operating principle of the tape gage is based on the electrical conductivity 

 of seawater. The gage consists of a stainless-steel tape on a takeup reel, 6- 

 inch-diameter stilling well, voltmeter, and battery. The weighted tape, grad- 

 uated to hundredths of a foot, completes an electrical circuit upon contact with 

 the seawater inside the metal stilling well. The distance to the water level at 

 that instant is measured below a reading mark on the tape. The reading mark 



22 



