WO1982004316A1

WO1982004316A1 - Fluid level indicator

Info

Publication number: WO1982004316A1
Application number: PCT/GB1982/000165
Authority: WO
Inventors: Gordon Bryce Fraser
Original assignee: Gordon Bryce Fraser
Priority date: 1981-06-05
Filing date: 1982-06-04
Publication date: 1982-12-09

Abstract

A fluid level indicator for fluid level detection using laser beam interferometry. The indicator comprising a tube (1) for vertical arrangement in a fluid whose level is to be monitored, a reflector (5) arranged for movement longitudinally of the tube for reflecting a laser beam transmitted to the reflector (5) from the top of the tube and a float member (6, 6a) movable with the fluid, for moving the reflector (5) in response to fluid level changes. The reflector may be mounted on the float member in the tube which is provided with one or more inlets for the fluid or on a disc in a sealed tube which is moved with the float on the outside of the tube through magnetic attraction.

Description

Title: FLUID LEVEL INDICATOR

Technical Field:

The present invention relates to fluid level indicators.

Background Art: Various fluid level indicators are known which employ mechanical sensors for detecting the rate and extent of filling/discharge of containers, such as, for example, the tanks of an oil tanker. However, the sensitivity and speed of response of such systems are not sufficient to permit the accurate control of fluid supply to or discharge from very large containers,^" such as, those of a tanker, in which the rate of rise or fall of the level is very small, but the quantities of fluid concerned are large. Such control is particularly im- portant in tankers in order to ensure that the sequence and/or rate of loading/unloading does not set up structural stresses in the tanker's hull, leading to fatigue and eventual fracture.

It has now been found that light, and in particular a laser beam, can be used to accurately and rapidly monitor the rate and/or extent of filling/discharging of a con¬ tainer.

Disclosure of the Invention: According to the present- invention there is pro¬ vided a fluid level indicator for fluid level detection using laser beam interferometry, the indicator comprising a tube for vertical arrangement in a fluid whose level is to be monitored, a reflector arranged for movement long- itudinally of the tube for reflecting a laser beam trans¬ mitted to the reflector from the top of the tube and a float member movable with the fluid,for moving the reflector in response to fluid level.changes.

In one embodiment of the invention, the reflector is mounted on the float member which is located in the tube and the tube wall is provided with one or more inlets for entry of fluid from the tank into the tube.

In another embodiment, the float member is annular in shape and carries a magnet on its inner surface. The reflector is mounted on a disc containing or coated with iron inside the tube which is sealed. As the float member moves with the fluid on the outside of the tube, the disc with the re lector is carried along with it due to magnetic attraction between the magnet and disc. Alternatively, the reflector may be mounted on a magnet and the inner surface of the float be coated with iron. The tube is sealed at both ends, being provided at the top with a transparent plate or disc through which the laser beam is transmitted and reflected from the reflector. This tube preferably contains dry air or other gas at low pressure. Further, an attemporator is provided on the tube to regulate the temperature of the air/gas in the tube. These measures serve to maintain the refractive index of the air/gas in the tube substantially constant so as to mitigate errors in the measured path length travelled by the beam due to changes in the refractive index which occur with variations in humidity, temperature and pressure.

A further aspect of the present invention provides a fluid level indicator system comprising the fluid level indicator according to the present invention, a laser and an interferometric detector respectively aligned with the reflector to transmit a beam thereto and receive a beam reflected therefrom. Brief Description of the Drawings:

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram illustrating the principle of interferometric measurement;

Fig. 2 illustrates an embodiment of the present invention;

Fig. 3 illustrates a further embodiment of the present invention.

The fluid level indicator of the present invention is intended for incorporation in a laser beam inter¬ ferometric system. The high intensity and temporal coherence of laser beams offer advantages in inter- ferometric measurement over those of conventional light sources, and provide for high accuracy and speed of measurement.

An interferometric method based on A«_A. Michelson's interferometer will first be described in order to outline the basic operation of interferometric laser dis¬ tance measuring systems.

Figure 1 is a schematic diagram showing such an interferometer. A laser beam -i is split into two parts by a beam splitter A to produce a measurement beam on and a reference beam R. The measurement beam travels to a movable reflector B whose displacement is to be measured. The reflector B is usually a corner cube re¬ flector which provides an accurate return of the beam. The return beam *v and reference beam R are combined at the beam splitter A and travel to a detector C. The combined beams form an interference pattern. The amplitude of the light at the detector C depends on the phase difference between the reference beam and the measurement beam which in turn depends on the difference in the optical path that the two beams have travelled. The phase

OMPI difference ^ is given by = 2ks Cos θ where k = 2 ^ "X and S = wavelength; s = the path difference, and θ = the angle between the common axis of the beam and the direction of observ- ation which, in this case, is zero. When the moving part travels one half wavelength of light, the total difference in optical path goes through one wavelength and the fringe pattern obtained will go through one period, corresponding to a change from light to dark to light at the position of the detector. Thus, the motion of the moving part leads to amplitude modulation of the light which is sensed by the detector.

Electronic circuitry can then count the period of amplitude modulation detected by the interferometer and feed this information to a computer which calculates the distance through which the reflector has moved and/or velocity of motion of the reflector. The computer out¬ put can then be transmitted to- a display unit where it is displayed digitally or graphically, by for example a print out or cathode ray tube, and/or recorded on magnetic tape for subsequent analysis. The output of the computer may also or alternatively, be used to operate warning and/or control devices.

Various interferometic arrangements for measuring direction of motion have been developed. In one such system, two detectors are provided which respectively collect light from regions of the fringe pattern where the phase difference of the interfering beams differ by ^" -./2. The relative phase of the amplitude modulation viewed by the two detectors will therefore be different depending on the direction of the reflector motion and this is used to determine the correct displacement, and/or velocity.

In another such system, the laser beam is circularly polarized and split into a reference beam and measuring

beam. A^' series of reflectors of the reference and measuring beams result in their being of opposite circular polarization when they are combined. These beams combine to form a linear polarization vector whose orientation will depend on the relative phase of the two circularly polarized beams and which therefore rotates as a function of reflector position.

An alternative interferometer employs a two fre-quency laser system,in which the Doppler shift of the beam reflected from the moving reflector is measured. The laser emits light of two slightly different fre¬ quencies fl and f2 with different polarization properties allowing the beam to be split into these two frequencies. The frequency f2 travels to a fixed reflector and frequency f1 to the movable reflector whose displacement is to be measured. Light reflected from the moving reflector has a frequency shifted by an amount Δf, where:

> f/f = -s/

being the velocity of motion of the reflector and c the velocity of light. The beams are reflected from the fixed and movable reflectors are combined and produce an amplitude modulation of the light, of modulation fre¬ quency f2-(fl ^± _ f1) . A reference signal f2 - fl is generated and is fed with the modulation frequency to a converter which extracts Δf1 and hence v ^~ . The velocityt " can then be integrated to obtain linear displacements. In yet another type of interferometer the laser beam is amplitude modulated and the phase of the reflected light beam is compared with that of the emitted beam. The difference in phase occurs because of the finite time required for the light to travel to the movable reflector and return to the detector. The phase shift 0 is related

O PI to the .total path length L by the equation β — 2

(2n L/>"-g) where is the vacuum wavelength of the laser and n is the group index of refraction.

Generally, laser interferometers provide measure- ent of displacement from an arbitrary zero, rather than an absolute measurement of distance. Thus, the instrument reading is set to zero at the initial position of the movable reflector and its motion is measured relative to this present zero. However, there are systems on market such as, for example, the "Hewlett-Packard 3850A industrial distance meter" which measure absolute distance. The Hewlett-Packard meter is also self-correcting for ambient air conditions based on an operator's presetting. Any of the above interferometric measurement systems may be employed with the fluid level indicator of the present invention, the selection of the appropriate system being dependent on the specific application and accuracy of measurement desired.

Fr rerred Modes of Carrying Out the Invention:

Embodiments of the fluid level indicator of the present invention will now be described.

A tube 1 is shown in Fig. 2 located in a liquid containing tank 2. The bottom of the tube 1 rests on the tank bottom while the top is mounted in the top wall of the tank or the ships deck and is sealed by a transparent sealing plate 3, of, for example, glass or perspex.

Perforations 9 are provided in the wall of the tube 1 in the region of its bottom end through which liquid in the tank can pass. Preferably these perforations 9 are sufficiently small to act as a strainer to prevent entry of foreign bodies into the tube 1.

A safety gauze 4 is provided in the tube 1 above the deck and serves as an outlet/inlet for air displaced/ replacement as the liquid in the tube rises/falls. The

' OMP gauze 4 preferably contains silicate gel to dry the air drawn into the tube 1. , A reflector 5 is mounted on the top of a float 6

^ within the tube 1. The float 6 is located in the tube 1

5 on alignment rollers 7 which run on the inner surface of the tube 1. Seal and scraper members 8 extend obliquely from the side surface of the float 6 above the liquid surface level and are biased against the inner surface of the tube 1. The seal and scraper members 8 10 act to maintain dry air above the float 6, providing a substantially constant refractive index medium above the reflector, and to keep the tube wall clean above the liquid level.

A laser 10 and an interferometric detector 11 are 15 mounted directly above the transparent plate 3, respectivel aligned with the retroreflector 5. Lighttransmitted from the laser 10 is reflected by the retroreflector 5 and returned to a receiving lens of the detector 11 which surrounds the laser's transmitting lens. Electronic 20 circuitry determines the position of the retroreflector and this information is fed, for analysis, to a computer, such as, a desk-top computer in a single or multiple units. Information such as the amount and speed of filling/discharge may be computed either by height or 25 weight of liquid in the tank or by reference to permissible structural stress levels. This may then be recorded on magnetic tape for subsequent analysis , in the manner of an aeroplane flight recorded and/or be used to operate low and high level alarms and/or control the rate of 30 filling/discharge of liquid. Further, this information can be used in the testing of filling/discharge equipment, and in ascertaining the instantaneous weight of cargo in any tank, enabling tanker specific gravity corrections to be made during loading and unloading using ballast. 35 Figure 3 shows another embodiment of the present invention located in a liquid containing tank 2. In this case, the tube 1 is sealed and contains dry air or other

_O PI gas at low pressure throughout. The sealed bottom of tube 1 rests on the tank bottom while the top of the tube is mounted in the tank's top/ship's deck and is sealed by a transparent plate 3 as in the previous embodiment.

An attemporator 12 surrounds the top end of the tube 1 above the ships deck to control the temperature of the air/gas in the tube so as to maintain the temp¬ erature, and hence the refractive index of the air/gas constant.

The retroreflector 5 is mounted on an iron cont¬ aining/coated disc 13 which is free to move longitudinally of the tube. An annular float 6a surrounds the tube 1 and has a magnet 14 provided on its inner surface. The float 6a is preferably in two parts to facilitate its easy replacement. The float 6a_ is borne by and moves with the liquid in the tank. The magnet 14 attracts the disc 13 which therefore follows the movement of the float 6a_, carrying the retroreflector 5 with it. A laser 10 and.an interferometric detector 11 are mounted and operated as described in relation to the previous embodiment.

Claims

CLAIMS:

1. A fluid level indicator for fluid level de¬ tection using laser beam interferometry, the indicator comprising a tube for vertical arrangement in a fluid whose level is to be monitored, a reflector arranged for movement longitudinally of the tube for reflecting a laser beam transmitted to the reflector from the top of the tube and a float member movable with the fluid, for moving the reflector in response to fluid level changes.

2. A fluid level indicator according to claim 1 wherein the reflector is mounted on the float member which is located in the tube and the tube wall is provided with one or more inlets for entry of fluid into the tube.

3. A fluid level indicator according to claim 2 wherein the top of said tube has a transparent plate through which the laser beam is transmitted and reflected and an air inlet/outlet.

4. A fluid level indicator according to claim 3 wherein a gauze containing silica gel is provided across said air inlet/outlet.

5. A fluid level indicator according to claim 2, 3 or 4 wherein the float member is located in the tube on alignment rollers.

6. A fluid level indicator according to any of claims 2 to 5 wherein seal members for preventing evaporated liquid entering the region above the float, extend obliquely from the side surface of the float member above the liquid level line thereof and are biassed against the inner surface of the tube.

^TJRE

OMPI IP

7. ^'A fluid level indicator according to claim 6, wherein said seal members also act as scraper; members for maintaining the cleanliness of the inner walls of the tube.

8. A fluid level indicator according to claim 1, wherein the float member is annular in shape and located on the outside wall of the tube, co-axial therewith, and the reflector is mounted on a disc inside the tube which is sealed, the float moving the reflector through magnetic attraction between the float and the disc.

9. A fluid level indicator according to claim 8 wherein the tube contains dry air or other gas at low pressure.

10. A fluid level indicator according to claim 8 or 9, wherein an attemprator is provided on the tube to regulate the temperature of the air or other gas in the tube.

11. A fluid level indicator system comprising the fruid level indicator according to any preceding claim, a laser and an interferometric detector respectively aligned with the reflector to transmit a beam thereto and receive a beam reflected therefrom.