This paper not to be cited without prior reference to the author

Transcription

1 This paper not to be ited without prior referene to the author International Counil for the Exploration of the Sea CM 1971/C:24 Hydrographi Committee An ultrasoni urrent meter by Trygve Gytre ~: ~: Institute of Marine Researh Diretorate of Fisheries Bergen, Norway

2 Disadvantages with onventional urrent meters Current meters that are based on rotating sensors (rotors, propellers) have several disadvantages. The most serious ones are: Low sensitivity (usually unreliable at urrents smaller than about 5 m/se. Slow response (unable to detet rapid hanges in urrent values). Poor diretional sensitivity. High risk for instrument derangement from impurities and algae growth in the water. To obtain better urrent measurements a team from the Norwegian Institute 02 Marine Researh and Chr. Mihelsen Institute - both in Bergen, ~orway has developed an ultrasoni urrent meter with no moving parts. Priniple of operation Figure 1 shows the priniple of operation. The sensor onsists of two piezoeletri diss with a diameter of 10 mm and a resonant frequeny of 3 MHz. The sensors are spaed m apart and emit short bursts of ultrasound towards eah other times per seond. If a urrent omponent with veloity vex) flows in the diretion of the sound, the sound propagandation time over the distane dx equals dx! v(x) where is the veloity of sound in the water, and the! sign depends on whether the sound moves with or against the urrent. The transit time over the distane i (Assume vex) «1) between the sensors will be T f o i dx! vex) (1) 1 i f o -~- (1 + vex) dx i i f v(x)dx o (2)

3 - 2 - Tl - T2 [)'T 2 2.!I. 2.!1. f v(x)dx = o 2 v (3) The mean veloity over the sound path v [)'T C 2 ~ (4) In order to ompensate for variations in the sound veloity, the sum of Tl + T2 -;::j 2Tl 2.!1. an be measured. This gives v [).T Jl.2 2J/. T12.!I. 2T12 [).T (5) whih shows that urrent an be measured ompletely independent of the sound transmission properties of the water. To obtain an output signal Tl and T2 are onverted to analogue voltages and their differene is amplified. Use of high speed logi siruits permits measurements of time differenes in the subnanoseond range allowing a reording of urrents down to 1 mm/se. to be easily measured. As this instrument has no moving parts it an be given a rugged design making it insensible for algae growth around the sensors.

4 ~, -.) - Pratial results A prototype urrent meter is shown on figure 2. The meter has been tested for several weeks to ontrol the urrents flowing in and out from a flsh farming inlet. Figure 3 shows a reorded setion from an experiment on improvlng the natural water exhange in the inlet by setting up artifiial urrents with propellers. In this experiment onventional rotating sensors failed to work in a short time beause of intense algae growth. Further davelupment The present instrument has been made to measure one dimentional urrent, Planned prototypes will be designed tp measure urrent omponents in the x, y and z-diretions referred to magneti north and true vertial dlretlon To obtain a ontinuous reording of the instrument orientation an ultrasonl.c ompass that utilizes the same eletroni iruits as the urrent meter that has bee. devied. It onsists mainly of an ordinary magneti ompass and obtains its output signal by measuring the ompass needle orientation aoustially. Prototype speifiations Heasuri:g range ~ Adjustable in steps from m/se Output signal~ Analogue.::: 10 V Linear-ity: Band vjidth: Zero drift~ Power onsumption: Better than 1% o Hz Less than 0, eliminated by zeroing) w V/week (an be inluding automatl

5 -4 - Disussion Ultrasoni urrent meters are first of all superior to rotating meters when onerning sensitivity and response time. They need however more ompliated eletroni iruitry, more power and more skill in adjustment and serviing. In order to obtain better measurements more ompliated instruments will be neessary, and it is very probable that ultrasoni urrent meters will play a very important role in future urrent instrumentation systems 0

6 =M~~~=~=r ~"~ =~~--~~~=r--=~=r~=~ Jr " / Transduers l ~ ~-j ~j H ~ f{t-sl C V! )( ) -' - C Il g «,.. -- s."'='...,.,... Figure 1 Priniple of operation. Figure 2 Prototype urrent meter. CURRENT VELUCITi mh 1 30 Propellers on 201\\rooell<, rs ott I~ 1 I 20 I I )! '\ v / 151 ' r-'. /\ J'" 1-' \ I I '" '\("0 It ~ 01 '\ ~~/ljv~~l 10 l' 2C~ J FORCED CURRENT i NATURAL CU.~RENT FORCED CURRENT _nnn W SO C 70 O 90 10C l~o 'LO 16(' "'.In Figure 3 Reorded urrents from prototype instrument.