As of this writing, digital wave staffs iiave been built in tliree 

 different configurations. The first staff was built into a 10-foot length 

 of iJ^-inch pol yvi ny Ich I ori de (PVC) tubing. It has 96 contacts placed at 

 3 cm intervals along the tube. Its self-contained logic circuitry for 

 scanning the contacts is mounted on two printed circuit boards each 5 feet 

 long and I inch wide. These are positioned inside the PVC tube and may be 

 removed for repair if necessary. 



A few staffs have been constructed from commercial fiberglass fishing 

 rods. Each has 96 contacts placed at 2 cm intervals along the length of 

 the rod. Separate brass cylinders house the logic cards that constitute 

 the electronic circuitry. With these separate, universal electronics 

 packages, the "plug-in" staffs may be constructed cheaply enough to be 

 considered expendable. 



Two "miniature" staffs with rontact spacing of 2 millimeters have been 

 built for use in recording small waves in the Hydraulics Laboratory. (See 

 Figure 10). The miniature versions plug into the same electronics packages 

 as the larger models. They are produced on double-sided glass-epoxy, 

 printed circuit cards. Any number of staffs may be etched on cards from 

 a photographic master made from a single drawing. The copper circuitry is 

 gold plated to resist corrosion, and all but the contact edge is insulated 

 with a polyurethane coating. Once a staff is designed and a photo master 

 obtained, each miniature digital^ staff (called a "m i n i -d ig i t") can be 

 fabricated for a cost of about thirty dollars. 



2. V i brat i ng-Wi re Pressure Sensor 



Inaccuracy of measurement ^esu I ts when the transmitted data is 

 referenced to a voltage level as in the common strain gauge pressure 

 transducer. Variations in the amplitude of a signal occur with induced 

 noise on the transmission cabling, with temperature changes which may alter 

 amplifier gain, and with changes in power supply voltage. On the other 

 hand, when changes in a measured parameter are represented as corresponding 

 changes in signal frequency, or in pulse pattern, the only troublesome con- 

 ditions are those which may shift the phase of the signal. 



A vi brat i ng-wi re type sensor is commonly used when very accurate 

 measurements of the pressure field associated with waves are required (see 

 I^unk, [filler, Snodgrass, and Barber, 1963). Pressure variations change 

 the tension on a wire which is driven at its natural frequency of vibration. 

 A count of the resulting frequency in cycles per unit of time will give a 

 read-out that lends itself well to digital recording. Special circuitry 

 enables the vi brat i ng-wi re transducer to sense very small changes in 

 pressure even in a very high pressure environment. This circuitry, as 

 used in the acquisition system, is outlined in Figure II. 



A line-crossing detector, technically called a "Schmitt trigger" is 

 used to generate a train of electrical pulses, each coinciding with a cycle 

 from the output of the pressure sensor. An electronic gate is opened by 

 a pulse from a reset pulse generator, allowing the signal pulses to be 



15 



