AN OCEANOGRAPHIC MODEL OF PUGET SOUND 691 



Instrumentation 



The design and development of instruments for measuring and re- 

 cording tides, tidal currents, and the density structure in the model is 

 complicated by the small scales and distortion. The sensing ele- 

 ments must be such that they do not significantly alter the properties 

 being measured or interfere with the operation of the model. Further, 

 because of the short time scale, it is necessary to obtain measurements 

 rapidly in order to adequately differentiate conditions during the tidal 

 cycle. 



A portable recording tide gauge and an instrument for recording 

 the salinity or density structure in the model have been designed and 

 constructed. The recording tide gauge will resolve the model tides to 

 within about 3 inches or 0.003 inch actual change in water level. The 

 gauge is not affected by surface tension or factors other than change 

 in the water level in the model. In principle it consists of an inter- 

 connected probe and recording arm operated by a synchronous motor. 

 The prqjbe alternately rises and lowers as the recording arm sweeps a 

 stylus across electrosensitive paper being moved by a standard recording 

 tape puller. As the probe, tipped by a fine platinum -^vire, contacts the 

 surface of the water, an electrical circuit is completed which fires a 

 thyrotron tube discharging a condenser. The charge from the condenser 

 passes from the stylus through the paper to a second electrode consist- 

 ing of a small roller extending across the underside of the paper. The 

 discharge causes a small spark to burn a fine hole in the paper making 

 a permanent black dot. The probe operates at a frequency of once per 

 second. 



The instrument for recording the salinity structure is based upon 

 the conductivity of the water. The variations in the conductivity of a 

 vertical section are electronically transposed to a plot of conductivity 

 versus depth on an oscilloscope which may be photographed for a 

 permanent record. The conductivity cell consists of a capillary 0.025 

 inch in diameter and 3/4-inch long, having electrodes at each end 

 forming an integral part of the capillary. A maximum potential of 

 about 50 volts at 10,000 cycles may be applied across the cell. Alter- 

 nating current is used to reduce the effects of polarization of the elec- 

 trodes. Water is draw'n through the cell at a rate equivalent to the rate 

 of displacement of water by the cell and tube to which it is attached, 

 as it is lowered through the water. It has been found that measure- 

 ments must be made while lowering the probe since some mixing and 

 turbulence occur when the probe is raised, leading to fictitious values. 

 Response is fast because of the rapid flushing of the cell. 



