A RESISTANCE WIRE WATER LEVEL MEASUREMENT SYSTEM 



R. A. AYERS* 



D. J. CRETZLER 



Hytech Division of Bissett-Berman Corporation 

 San Diego, California 



INTRODUCTION 



Resistance wire elements have long been used 

 for measurement of water level variations. 

 Because both amplitude and time response are 

 normally degraded by biological growth most 

 resistance wire transducers have been used in 

 the laboratory. The transducers that have been 

 used in oceanography have consisted of small 

 diameter single wire elements, either enclosed 

 in a protective slotted tube 1 or suspended tautly 

 between two supports without protection. The 

 former configuration suffers loss of response and 

 precisi'on due to interference of the tube and the 

 latter is susceptible to being broken by drifting 

 objects or foulants. These elements have the 

 additional disadvantage of a low full scale resis- 

 tance range, thus making it difficult to achieve 

 high sensitivity with relatively simple low 

 power circuitry. 



The measurement system described in this paper 

 was designed to avoid or reduce the major disad- 

 vantages mentioned above. The initial set of 

 requirements for this system was imposed by a 

 particular application; namely, suspension on 

 the exposed side of piers and offshore platforms. 

 For this purpose a staff was needed that was 

 light enough for easy installation and not too 

 delicate to withstand normal rough treatment in 

 the natural environment . Staff lengths up to 

 kO feet were required with a measurement pre- 

 cision of 0.2$ of full scale or better over the 

 frequency range of natural water motions from 

 short chop (about 1 cps) to tides (about 1 cycle 

 per day) . It was further considered necessary 

 that the system consume a minimum of power and 

 be insensitive to wide fluctuations of line vol- 

 tage and frequency. Transducer output was 

 required to be high level at to 5 volts DC 

 full scale and directly proportional to water 

 elevation. 



SPECIFICATIONS REQUIREMENTS 



The first consideration of specifications was 

 the range of wave and tide heights that the 

 instrument should accommodate. Maximum wave 



height seems to be of the order of 100 feet in 

 .the open sea. In areas of minimum tide range and 

 exposure as little as h feet can suffice for 

 total staff length. To encompass multiple pur- 

 pose use within this range an incremental length 

 of 5 feet was selected for total lengths up to 

 100 feet . The greatest demand seems to he for a 

 ^0-foot staff. For the purpose of this paper, 

 the i)-0-foot unit is applied to specifications of 

 range and precision. 



A second parameter is overall accuracy required. 

 For purposes of adequately measuring short chop 

 superimposed on low seiche or swell, resolutions 

 of a fraction of an inch are necessary. This 

 means the instrument system must have an overall 

 precision of at least io.2$ of full scale, 

 including both the staff and electronics errors, 

 if staffs ranging up to ^0 feet are to be used. 



A third parameter of importance is response. 

 Since both tides and waves are to be measured, 

 the instrument must respond accurately to long 

 and short periods, ranging in the extremes from 

 months to a fraction of a second. A nominal high 

 frequency cutoff has been selected at 5 cps. 



The fourth major design parameter is output 

 signal characteristics. The most standard 

 telemetry voltage controlled oscillator input is 

 to 5 volts DC so it seems expedient to make 

 the output from the instrument fall in this range. 

 The output resistance should be low enough to 

 enable the instrument to drive most recorders, 

 voltmeters and various devices that may be con- 

 nected to its output terminals. This system was 

 designed to drive its full 5 volt output into 

 10,000 ohms. To meet the preceding accuracy 

 requirements, the ripple on the output should be 

 less than 5 millivolts .peak to peak. The power 

 source for the instrument to accomplish the 

 above mentioned tasks is most readily available 

 from a 115 volt AC, 60 cps power line. Power 

 consumption for the instrument is approximately 

 25 watts . For operation in areas without 115 volt 

 AC power facilities, a static inverter with bat- 

 teries may be employed. 



^Presently with General Atomic, San Diego, California 



Superior numbers refer to similarly numbered references at the end of this paper. 



93 



