A laser liquid level gauge for detecting the level of the surface of a fluid in a vessel includes an optical unit in the top wall of the vessel and a remotely located control unit. A laser diode at the control unit is amplitude modulated with a frequency tone to produce an incident beam which is carried by a fiber optic cable to the optical unit where it is directed through collimating optics to the fluid boundary.
Reflections of the incident beam from the fluid boundary are focused by receiver optics to a photodiode at the optical unit, the output of which is coupled by a cable to the control unit, which includes a phase detector for comparing the phase of the modulation tone with that of the detection signal to produce a level indicating signal for display.
1. Field of the Design
The present design relates to level sensing and, more particularly, to electro-optical devices for measuring the distance between a reference level and a variable surface, such as the surface level of liquid in a vessel.
2. Description of the Prior Art
There are various techniques for determining the level of a liquid in a vessel. One category of techniques involves physically measuring the level, such as with a dip stick, a float indicator or the like, but such intrusive techniques entail a number of disadvantages. They occupy space in the vessel, they contact the liquid in the vessel with the risk of either contaminating it or being contaminated by it, and they may require periodic access to the interior of the vessel.
This requires exposing the interior of the vessel to atmospheric pressure and may violate government regulations which prevent venting of the vessel to atmosphere.
Accordingly, there have been developed a number of nonintrusive level gauging techniques. One such technique involves the use of a light beam which is directed to and reflected from the surface of the liquid in the vessel. Such optical level measurement systems typically utilize the angle of reflection of the incident light beam from the surface of the liquid to indicate the distance of the surface from a reference level.
Typically the difference in incidence and reflection angles between full and empty vessels is very small and, therefore, accurate level measurement requires very precise mounting of the optical equipment; which can entail significant expense.
It is a general object of the present design to provide an improved optical liquid level gauge system which avoids the disadvantages of prior systems while affording additional structural and operating advantages.
An important feature of the design is the provision of an optical liquid level gauge apparatus which does not require extremely precise mounting of the optical equipment.
In connection with the foregoing feature another feature of the design is the provision of a gauge apparatus of the type set forth, which does not depend on differences in incidence or reflection angles for measuring the liquid level.
Another feature of the design is the provision of a gauge apparatus of the type set forth which is of relatively simple and economical construction and, yet, is quite accurate.
In connection with the foregoing feature, another feature of the design is the provision of a gauge apparatus of the type set forth, which includes a vessel-mounted unit and a remote unit interconnected by a transmission medium.
These and other features of the design are attained by providing apparatus for measuring the level of a light-reflective fluid boundary in a vessel relative to a reference level, comprising: source means for generating a light beam, modulation means coupled to the source means for modulating the light beam with a reference signal to produce an incident beam, transmitter means optically coupled to the source means for directing the incident beam from the reference level to the fluid boundary, detector means at the reference level for detecting modulated reflections of the incident beam from the fluid boundary, and means comparing the modulation of the incident beam with that of the detected reflections to produce a level signal indicative of the distance between the reference level and the level of the fluid boundary.
The design consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present design.
Referring now to FIG. 1 there is illustrated a vessel, generally designated by the numeral 10, which may in the nature of a tank or the like for containing a volume of liquid. The vessel 10 may be of any desired size and shape, and has a top wall in which is formed an internally threaded opening 13.
The vessel 10 contains a volume of liquid 14 having a surface 15, the level of which is to be measured relative to a reference level which corresponds substantially to the inner surface of the top wall 11 of the vessel 10. The surface 15 of the liquid 14 is the interface between the liquid and a gaseous phase above the liquid, which may be air or vapor, this interface being light-reflective, in a known manner.
The present design includes a gauge system, generally designated by the numeral 20, for measuring the distance between the reference level and the level of the liquid surface 15. The gauge system 20 includes an optical unit 30 and a control unit 40 remote from the optical unit 30 and coupled thereto by a transmission means 37.
The optical unit 30 includes a housing 31, which may be in the form of a cylinder which is externally threaded for threaded engagement in the opening 13 in the top wall of the vessel 10. The housing 31 is closed at its lower end by a transparent window 32, and may be closed at its upper end by a suitable wall (not shown).
Disposed within the housing 31 are transmitter optics 33, which are designed for collimating and focusing a laser beam onto the liquid surface 15 in a substantially vertical direction. Also disposed in the housing 31 is receiver optics 34, which is designed for focusing light reflected from the liquid surface 15 onto a photodiode 35, which converts the reflected light to an electrical detection signal.
The terminals of the photodiode 35 are connected by conductors 36 to the transmission means 37 and, more particularly to a cable 38, which may be a coaxial cable, the other end of which is coupled to the control unit 40. The transmitter optics 33 is optically coupled to one end of a fiber optic cable 39, the other end of which is similarly coupled to the control unit 40.
If desired, the transmitter and receiver optics 33 and 34 could be arranged in a coaxial configuration instead of the side-by-side arrangement shown.
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