|Title of Invention||
AN ACOUSTIC TIDE GAUGE WITH PROVISION FOR IN SITU CALIBRATION
|Abstract||An acoustic tide gauge with provision for in situ calibration comprising a sounding tube with one end open for immersion in water and with the other end having an acoustic transducer for generation and reception of sound pulses, characterised in that the sounding tube is connected, laterally, with at least one branch tube at one end thereof, the her end of the said branch tube being closed, the said branch tube being fixed to the sidewall of the sounding tube of length L in metres (including Raleigh correction) equal to (2n-l) λ/4 = (2n-l)C /4f where n= 1,2,3.....λ is the wave length of sound (metres), C is the velocity of sound (metres per second) and f is the frequency of sound (Hz).|
This invention relates to an acoustic tide gauge with provision for in situ calibration.
The acoustic tide gauge (ATG) is used for high accuracy and reliable measurements of tide levels and operates on the principle of acoustic echo ranging.
In one type of ATG known to the art a sounding tube is used to guide the sound pulse towards the water surface. One end of this tube is open and is vertically immersed in water, while the other end, at the top, is provided with an acoustic transducer,, for the generation and reception of sound pulses.
The sound pulse generated by the transducer travels towards the water surface and is received by the same transducer after being reflected by the water surface. The time of flight of the pulse is measured for the two way travel. Knowing the velocity of sound in air under given conditions, this time can be converted into distance thus giving the tidal level.
However, the accuracy of the measurement depends on the
velocity of sound, which is a function of temperature.
The speed of sound in air is given by the formula:
C = 331 + 0.6 T, where C is the speed in metres per second and T is the temperature in degrees centigrade.
There is a temperature gradient in the tube, since one and of the sounding tube is in water and the other in air. Accordingly, it is not enough to measure the temperature of the air at one point for obtaining the sound velocity and thus the distance- In situ calibration for each measurement becomes necessary.
Conventionally, calibration is done by placing temperature sensors along the length of the tube, to obtain the temperature at select points along the tube. A suitable interpolation technique is then employed to obtain the correction for the sound speed and thus obtain an estimate of the tidal level.
However, in this technique, ensuring reliable operation of the sensors under marine conditions is very difficult; maintenance is equally difficult, since all
sensors,are not easily accessible: and data acquisition and signal processing software and hardware are complicated by reason of the increased number of data channels.
Another calibration technique that is used conventionally, consists in providing a hole on the side wall of the sounding tube at a known distance from the transducer. This acts like a partial reflector- A sma11 part of the sound pu1se is ref1ected back towards the transducer by the said hole. From the time of flight and the distance of the calibration hole from the transducer, the average nominal velocity of sound is calculated- This value is then used in the measurement of tidal level.
The drawbacks associated with the above technique are that the hole cannot be provided too far away from the transducer; otherwise it may get submerged at high tide. Besides, the reflected pulses at the hole and the water level can get mixed up, if both are too close together. The sound velocity correction that is
obtained is really valid for the length between the transducer and the hole, extrapolating the same value for the entire length leads to errors. A part of the signal energy is lost to the surroundings through the hole and also due to partial reflection at the hole. This leads to reduced signal to noise ratio and hence reduced accuracy.
On the other hand the ATG proposed herein reduces the
disadvantages associated with the known calibration
systems set out herein above as will be clear from
what follows hereinafter.
The acoustic tide gauge with provision for in situ calibration according to his invention , comprises a sounding tube with one end open for immersion in water and with the other end having an acoustic transducer for generation and reception of sound pulses, characterised in that the sounding tube communicates, laterally, with at least one branch tube at one end thereof, the other end of the said branch tube being closed, the said branch tube being fixed to the sidewall of the sounding tube and of length L in
This invention will now be described with re-ference to the accompanying drawings wherein
Fig.1 illustrates the known acoustic tide gauge showing the temperature sensors S according to one conventional system of calibration and the side wal1 calibration hole H according to another conventional system ai calibration already discussed herein above
Fig.2 illustrates, by way of example, one of possible embodiments of the AT6 proposed herein.
Fig.3 illustrates variation of amplitude transmission coefficient of a resonating branch tube of length n = 4 and f = 7-6 kHz pertaining to the above embodiment-
The AT6 proposed herein utilises the resonant characteristics of at least one (that is, one or more) branch tube B,, fixed to the side wall of the sounding tube T, The tube T communicates, laterally, with each branch tube, through one end El thereof, the other end E2 of the said branch tube being closed.
The branch tube B is of length L in metres (including Raleigh correction), where L = (2n-l)-A/4 = (2n-l)C/4f; n=l 2,3, ...... . A is the wave length of sound (metres),
C is the velocity of sound (metres per second) and f is the frequency of sound (Hz).
The reflection of sound at the junction of the branch tube B and the sounding tube T goes through a series of alternating maxima and minima as the centre frequency of the sound pulse is varied (Fig.3)
The reflection maxima occur when the effective length
of the branch tube (physical length plus Raleigh
correction) is an odd multiple of the quarter of the
wave length for a given frequency of sound.
The length of the branch tube for which this happens is given by the above equation.
It is possible to have two or more branches connected to the sounding tube T Thus by using properly tuned resonating branch tubes and signals of different frequencies;, for calibration and for measurement of the tidal level, it is possible to overcome several of the limitations of the conventional in situ calibration systems.
The advantages of the ATG proposed herein are:
*A true in situ calibration for the entire length of the "Sounding tube is possible, since multiple branch tubes can be provided
*No additional hardware is required (as in the case of temperature sensors)
*The fabrication and maintenance are easy.
*There . is no loss of signal energy and therefore the signal to noise ratio is improved,, leading to improvement in the accuracy of measurement.
The terms and expressions in this specification are of description and not of 1 imitation , there being no intention in the use of such terms and expressions of excluding any equivalents of the features illustrated and described, but it is understood that various other embodiments of the gauge proposed herein are possible without departing from the scope and ambit of this invention.
3.An acoustic tide gauge with provision for in situ calibration , substantial ly as herein described with reference to, and as i1lustrated by, the accompanying drawings.
Dated this the/st day of July 1997
|Indian Patent Application Number||1443/MAS/1997|
|PG Journal Number||07/2008|
|Date of Filing||01-Jul-1997|
|Name of Patentee||NATIONAL INSTITUTE OF OCEAN TECHNOLOGY|
|Applicant Address||IC & SR BUILDING, IIT CAMPUS, CHENNAI - 600036,|
|PCT International Classification Number||G 01 F 23/296|
|PCT International Application Number||N/A|
|PCT International Filing date|