Abstract: A peripheral sealing assembly for connection with a buoyant panel unit usable in a liquid storage tank includes a fixing frame, an enclosure, and a buffer pipes. The fixing frame connects the buoyant panel unit. The enclosure is fixed to the fixing frame to sealingly contact an inner surrounding wall of the liquid storage tank. The enclosure defines a looped receiving space between the inner surrounding wall and the buoyant panel unit. The buffer pipes are inserted into the looped receiving space. Each buffer pipe is hollow, extends along the looped receiving space and has a lengthwise outer peripheral surface fixed to the lengthwise outer peripheral surfaces of two adjacent other buffer pipes.

Abstract: Tank lids include a first coated substrate including a first part having a first thickness, a first elastomer coating the first part, a second coated substrate positioned adjacent to the first coated substrate, the second coated substrate including a second part having a second thickness, and a second elastomer coating the second part. A first seam can be formed between the first and second coated substrates to allow for moisture to pass between the first and second coated substrates so that moisture is allowed to seep away from the top of the tank lid.

Abstract: A support platform for enabling a worker to service an oil well next to a wellhead having a valve tree includes: a) a platform having an upper floor, a lower floor and a cellar in between the floors; b) a platform having a periphery, a central opening and radially extending beams; c) a plurality of walls attached to the periphery of the platform, said beams connecting to said walls below said upper floor; d) a drain that enables fluid to drain from the upper floor to the lower floor; e) a coupler that spans between the floors for enabling a connection to be made between the platform and valve tree; and f) a liquid guard or other provision that disallows the escape of fluid from a floor via the central opening.

Abstract: Embodiments relate generally to a floating roof for use in storage tanks. A floating roof may comprise a plurality of pontoons arranged to form the floating roof. The pontoons may each comprise a lower panel, a plurality of outer walls disposed at the edges of the lower panel, and a plurality of stiffeners disposed adjacent the lower panel. Attachment points attached to the floating roof, wherein the attachment points are configured to couple the floating roof to a storage tank.

Abstract: A support platform for enabling a worker to service an oil well next to a wellhead having a valve tree includes: a) a platform having an upper floor, a lower floor and a cellar in between the floors; b) a platform having a periphery, a central opening and radially extending beams; c) a plurality of walls attached to the periphery of the platform, said beams connecting to said walls below said upper floor; d) a drain that enables fluid to drain from the upper floor to the lower floor; e) a coupler that spans between the floors for enabling a connection to be made between the platform and valve tree; and f) a liquid guard or other provision that disallows the escape of fluid from a floor via the central opening.

Abstract: An insulative cover for large tanks containing a fluid. The insulative cover utilizes rectangular panels to cover a majority of the fluid surface and trapezoidal or wedge shaped panels to form a ring around the rectangular panels. Each panel utilizes a locking rim to allow the panels to interlock when installed. In this way the rectangular panels cover the main surface but leaves spaces where the rectangular panels do not match with the curvature of the tank. These spaces are covered by the trapezoidal panels.

Abstract: A gas collection receptacle includes a segmented top portion and one or more side portions creating a volume for collecting gases. The gas collection receptacle, configured to float on a pond, lagoon, or other area, also includes interlocking side and end portions enabling multiple receptacles to connect, thus forming a substantially continuous covering.

Abstract: An insulative cover for large tanks containing a fluid. The insulative cover utilizes rectangular panels to cover a majority of the fluid surface and trapezoidal or wedge shaped panels to form a ring around the rectangular panels. Each panel utilizes a locking rim to allow the panels to interlock when installed. In this way the rectangular panels cover the main surface but leaves spaces where the rectangular panels do not match with the curvature of the tank. These spaces are covered by the trapezoidal panels.

Abstract: Systems and methods for covering the surface of a body of liquid, including a plurality of buoyant members, each having the shape of a rhombic dodecahedron including twelve identical quadrilateral faces, configured to float on the surface of the body of liquid and to substantially cover the surface of the body of liquid. The plurality of buoyant members naturally align when floating on the surface of the body of liquid with a quadrilateral face of a rhombic dodecahedron abutting a quadrilateral face of an adjacent rhombic dodecahedron and the plurality of buoyant members form a closely packed floating arrangement of face-to-face abutting rhombic dodecahedrons that substantially covers the surface of the body of liquid.

Abstract: A system for covering liquids stored within an open-topped liquid storage tank having a peripheral wall has a cover for extending across at least a portion of stored liquids, and two or more guide assemblies spaced about the peripheral wall for securing the cover to the peripheral wall and for permitting the cover to freely travel substantially vertically up and down within the interior space. Each assembly has a vertical guide, a clamping member, and an upper and a lower delimiting stop. The cover can be operatively secured to the assembly by a restraining tether.

Abstract: A floating cover roof for a storage tank has a frame assembly and multiple floating units. The frame assembly has a holding frame and an outer rim. The holding frame has multiple longitudinal beams and transverse beams. Each longitudinal beam has a connecting seat, a first holding segment and a second holding segment. Two of the transverse beams are connected to two adjacent longitudinal beams to form a locating area between the beams. The floating units are mounted in the locating areas of the frame assembly, are connected to the beams of the holding frame and each has an outer casing and a body. The outer casing is mounted in one of the locating areas of the holding frame, is connected to the adjacent beams and has a casing body and a top board. The body is mounted in the outer casing between the casing body and the top board.

Abstract: An oil tank floating roof device, includes a plurality of connecting bars and a plurality of floating units, which are respectively secured to the plurality of connecting bars by means a plurality of longitudinal and crosswise holding spaces collectively encircling the floating units. A plurality of locking members are fitted on the peripheries of the floating units, and two sides of the connecting bars respectively form at least one joining groove. The joining grooves respectively enable each of the floating units to longitudinally or crosswisely insert therein. The top portions of the connecting bars form at least one pressure resistant reinforcing plate, and the pressure resistant reinforcing plate is fitted with a plurality of locking portions. Bolts are used to respectively correspondingly pass through the locking portions and bolt into the locking members on the longitudinal or crosswise sides of the base of the floating units.

Abstract: A floating modular cover for a water storage consisting of a plurality of modules in which each module includes a chamber defined by an upper surface and a lower surface there being openings in the upper surface to allow ingress of water into said chamber and openings in the upper surface to allow air to flow into and out of said chamber depending on the water level within said chamber to provide ballast for each module floats. The modules prevent water evaporation from the area covered and the shape and size is selected to ensure that the modules are stable in high wind conditions and don't form stacks. The modules may be made from identical hexagonal or octagonal halves by blow molding or thermoforming.

Abstract: A multi-purpose floatable container includes a casing, a cap, at least one linkage disc, and at least one buckling bolt. The casing has four corners and each corner has a linking groove, an upper hole, an upper enlarged hole, and a lower hole. The at least one linkage disc is partially and respectively inserted into at least one of the linking grooves. Each one of the at least one linkage disc has four connecting orifices longitudinally defined through the linkage disc. Each one of the at least one buckling bolt is inserted into the top hole of one of the corners, the upper hole, the connecting orifice, and the lower hole for fixing the cap on the casing. The container can be utilized as a single unit for storing objects in the casing. Multiple containers can also be assembled to form a floating platform for diversified purposes.

Abstract: Disks configured to float on the surface of a body of liquid, including a top member configured to float substantially above the surface of the body of liquid, a bottom member spaced from the top member and configured to float substantially below the surface of the body of liquid, a sidewall extending between the top member and the bottom member along the periphery of the top member and along the periphery of the bottom member, wherein the top member, the bottom member, and the sidewall collectively define a cavity, a chamber mounted within the cavity, the chamber enclosing a predetermined volume of a gas, and a port defined in the sidewall to allow liquid from the body of liquid to enter the cavity, wherein the predetermined volume of gas enclosed in the chamber is selected to impart a buoyancy force sufficient to maintain the disk afloat on the body of liquid with the bottom member a predetermined depth below the member of the body of liquid.

Abstract: A modular floating impermeable rectangular module, with a connective clustering and solar collector carrying capability. The modular configuration is applied to a water surface, in a synergetic combination for the solar generation of power and the prevention of evaporation and/or airborne water and particulate contamination of the water body. Each module is adapted to support a solar collection panel for converting solar energy into electrical energy in which each flotation module is formed from two half shells which connect together to form a module, the outer surfaces of at least one shell being adapted to support a solar collector and each module is adapted on two opposed edge sections for connection in line to form a chain of modules and each solar collector in said chain being connected in electrical series and each chain of modules being connectable laterally to form arrays of modules and each chain of solar collectors being electrically connected in parallel.

Abstract: A liquid receptacle made of a wire frame and a disposable and biodegradable material that is substantially impervious to water. The liquid receptacle is capable of being expanded and collapsed for ease of transportation and storage. For ease of use, the liquid receptacle is shaped with a flat bottom allowing it to stand without support, and equipped with a flap surrounding a top opening of the receptacle. For ease of transportation after use, a cord is supplied around the flap for closing the top opening of the liquid receptacle.

Abstract: A device and method of making a full contact floating roof for use in covering fluid bodies, such as storage tanks containing hydrocarbon fluids, allowing ease of construction, high integrity, and low maintenance cost.

Abstract: The present invention relates to an oil storage tank equipped with a floating bed type inner floating roof. The oil storage tank structure has a tank body and an inner floating roof which lies on the liquid surface inside the tank. The inner floating roof includes filling bags and a resilient sealing tube. The filling bags, separated by isolation sheets, are filled with floating plates. The resilient sealing tube is disposed around the entire circumference of the filling bags, and is in contact with and biased against the inner wall of the tank body.

Abstract: A hatch cover for deployment over a hatchway formed in a floating roof liquid storage tank permits rapid loading and unloading of inspection or maintenance equipment and minimizes vapor emissions from the tank. The portable hatch cover has two base plates movably connected at a hinge and each defining complementary recesses. A circular flange extends from the bottom surfaces of the base plates to seat the hatch cover in the hatchway. Two door panels are movably connected at hinges to the top surfaces of the base plates. When one or both of the door panels are opened, equipment may be loaded into the tank through the complementary recesses. When closed, the door panels cover the recesses in the base plates. Complementary notches in the door panels provide an opening through which equipment tubing or wiring may extend when the door panels are closed. A collar gasket optionally may be wrapped around the tubing or wiring as a further means to limit vapor emissions from the tank.

Abstract: A full contact floating roof for use in covering fluid bodies, such as storage tanks containing hydrocarbon fluids, allowing ease of construction, high integrity, and low maintenance cost.

Abstract: A hatch cover for deployment over a hatchway formed in a floating roof liquid storage tank permits rapid loading and unloading of inspection or maintenance equipment and minimizes vapor emissions from the tank. The portable hatch cover has two base plates movably connected at a hinge and each defining complementary recesses. A circular flange extends from the bottom surfaces of the base plates to seat the hatch cover in the hatchway. Two door panels are movably connected at hinges to the top surfaces of the base plates. When one or both of the door panels are opened, equipment may be loaded into the tank through the complementary recesses. When closed, the door panels cover the recesses in the base plates. Complementary notches in the door panels provide an opening through which equipment tubing or wiring may extend when the door panels are closed. A collar gasket optionally may be wrapped around the tubing or wiring as a further means to limit vapor emissions from the tank.

Abstract: A cover assembly for an organic waste lagoon basin and a method of making and deploying such a cover assembly. A cover includes connected cover panels or sections. In one form of the invention the panels are comprised of a single layer of geotextile fabric. In another form of the invention the panels are comprised of interconnected strata including a sacrificial layer which faces the sun and protects the cover from ultraviolet light degradation; a foam layer that floats the cover; and a geotextile fabric layer. The geotextile fabric is porous and suppresses the release of malodorous gases from the lagoon basin. The cover can be constructed and deployed by laying out cover panels in fanfold fashion along a side of the basin; connecting adjacent edges of the panels; attaching flotation to the free edge of the top panel; and using deployment lines and a mandrel to move the cover across the filled basin.

Abstract: A floating cover having a plurality of floating panels is provided. The floating panels are assembled to form the floating cover disposed in a storage tank. The floating panels are provided with extruded channels formed on each peripheral outside of a honeycomb. A trough or hollow inside is formed within the extruded channels. An inspection port is formed on the extruded channels to provide a user with an inspection for leakage without disassembling the floating cover and without taking out the floating cover from the storage tank. The inspection port provides the inspection of presence of moisture and condensate trapped in the inside of each of the floating panels and also provides the replacement of damaged or degraded one among the floating panels.

Abstract: The present invention relates to a floating roof (1) with non-structural characteristics, which basically comprises a peripheral ring (7) formed by the joining of a plurality of plates and which has a plurality of floats (2), and a central platform (8) formed by the joining of a plurality of floats (2), and a central platform (8) formed by the joining of a plurality of plates and which has floats (2).

Abstract: An ice rink covering system utilizes a plurality of support mechanisms for sets of non-flexible insulated panels which are interconnected by a hinge mechanism between each pair of adjacent panels. The support mechanisms are connected to the panels by lifting units for selectively moving the panels in an accordion fashion for either edge to edge contact when the panels cover a rink or side to side contact when the panels are in their stored condition.

Abstract: A full contact floating roof for a storage vessel includes a plurality of panels wherein each panel includes a core disposed between upper and lower skins. A plurality of closures are provided wherein each closure is secured to perimeter edges of a panel by welding the closure to at least one of the upper and lower skins. A plurality of fasteners connect adjacent closures thereby forming the floating roof.

Abstract: The present invention relates to a ring seal for between a floating roof and a side wall in a liquid and vapor storage tank. The ring seal includes a number of overlapping shoe plates hung from the floating roof by pantagraph hanger assemblies. The pantagraph hanger assemblies urge the shoe plates against the side wall and a gap seal prevents tank vapor loss along a lap joint between adjoining shoe plates.

Abstract: A clarifier is disclosed with a cover or roof configured and disposed to minimize headspace between a top level of liquid in a clarifier vessel and a bottom of the roof. In one aspect the roof or cover includes one or more removable panels which permit access to a clarifier vessel's interior; the panels removably secured to a purlin system which is connected to a truss support secured to the vessel. In one aspect one or more circumferential purge air inlets are provided through which flow purge air. Purge air and gaseous material emitted from a mixture in the clarifier flow to a vent or vents through which they exit the clarifier. In one aspect the air inlet or inlets are disposed to provide uniform flow from the inlets to the vents. In one aspect one or more roof panels is connected with a known force and has a known rupture strength so that before explosive damage to other parts of the clarifier the panel is explosively disconnected and ruptured to relieve pressure in the clarifier.

Abstract: An overflow protector for a sump having a wall defining an open top reservoir includes a stop member forming an abutment along the wall of the reservoir, a floating sleeve member movable between a non-operable position contained within the reservoir and an operable position projecting from the reservoir for increasing the storage capacity of the reservoir, and a sealing arrangement for sealing the sleeve member against the abutment when the sleeve member moves to its operable position.

Abstract: There is disclosed a cover structure for use with fermentation and the like containers. The cover structure includes a lower cover having a plurality of layers of strips of material covering the container, with the strip layers angularly oriented to each other. Spaces can be formed between strips, and in one embodiment the second strips are connected along their sides at spaced intervals to form the spaces. This can provide gaps in the sheet covering the container, allowing gases to pass through. Additionally, spacers may be positioned on the lower cover with a further membrane extending thereover. The spacers create passages for collection of gases escaping through the lower cover. Floating gas collectors may also be provided for collection of gases for further treatment. The cover can provide insulation, to a variable degree.

Abstract: Discloses a kind of built-up inner floating ceiling for use in an oil storage tank, with a view to removing electrostatic effects. The ceiling comprised a skeleton and plastic buoys concentrically arranged thereon, under which there is provided a metallic net in contact with oil, said buoys being pressed from both sides by radial beams of said skeleton, and placed between peripheral beams at rear and front, with a cover plate at top and said metallic net at bottom, so that they are in contact with conductors at any side. Further, wires leading to the top of the oil tank are connected to said cover plates, and said buoys are filled with a conductive additive to remove accumulation of electrostatic charge. Such a floating ceiling can, therefore, be free from any electrostatic effects.

Abstract: An apparatus for covering a basin for liquids, for instance basins used for purification of waste water. The apparatus according to the invention comprises a covering element, with a regular circumferential shape, so that they can be located adjacent, which covering element floats unto the liquid to be covered. For making the element float a number of floating elements is provided at the lower side of the elements, but these floating elements are not adjacent. Consequently the resulting upward floating force of the floating element keeps the lower side of the covering element on a distance from the surface of the liquid. This distance allows draining or removal of bodies floating on the liquid. Preferably the upper surface of the covering element has the shape of a deepened tray to allow the collection of rain water, so that the elements cannot be blown away.

Abstract: Flotation covers for large bodies of liquid, e.g. reservoirs usually consist of large sheets or panels adapted to float on the surface of the liquid. Such covers are adapted to follow changes in liquid level. However, in general, the problem of covering the surface of the liquid around the periphery of the cover is not addressed. A solution to this problem involves a panel covering all but the peripheral edges of the liquid surface, cables connected to the edges of the panel and to counterweights for maintaining the panel centered on the surface, and floats for supporting the edges of the panel and the bottom edge of a skirt, which extends upwardly to the wall of the reservoir, the skirt being connected to the cables by slip rings, so that the skirt folds and unfolds as the liquid level rises and falls.

Abstract: A composition for protection of the liquid surface from heat- and mass-trfer, for example from evaporation, contains a polymer limitedly swelling in the protected liquid or a mixture of said polymer with a neutral filler having a density lower than the density of the protected liquid.A method for protection of the liquid surface from heat- and mass-transfer comprising application of said composition on the surface of a liquid, for example petroleum, in an amount of from 0.1 to 2 kg per m.sup.2. Upon contacting of the polymer with the liquid the polymer rapidly swells and forms a continuous gel-like covering which hinders the heat- and mass-transfer on the liquid surface.

Abstract: A storage tank for liquid bitumen including a steel shell having heat insulation adjacent to its interior surface. The insulation is glass foam. Preferably, the insulation is in the form of glass foam panels which are initially arranged with gaps between them. When the tank is filled for the first time, the liquid bitumen penetrates into the gaps and solidifies therein to permanently interconnect the glass foam panels. A glass foam insulating cover floats on the surface of the liquid bitumen, and a heating unit is located within the tank.

Abstract: A double secondary seal for a floating roof type of liquid storage tank is disclosed. The secondary seal includes generally a weather shield of edge overlapping steel panels which extends upwardly and outwardly from the peripheral edge of the floating roof. The panels terminate upwardly in flexible wipers which are normally biased into sealing contact with the tank sidewall. A flexible vapor barrier underlies the steel panels to prevent the escape of vapors therethrough and defines a peripheral log space below the panels. A resilient foam log is positioned within the log space to provide a reserve seal. The dimensions of the foam log are designed to maintain the foam log away from contact with the tank sidewall when the wipers are in contact with the sidewall and assure sealing engagement of the foam log and the sidewall whenever the tank is filled sufficiently to raise the wipers above the top of the sidewalls.

Abstract: A cover adapted to float on the surface of liquid in a storage tank employs a deck and first and second generally horizontal elongated pontoons disposed essentially along a common axis of symmetry. The pontoons are adapted to float support the deck on the surface. A vertical hollow member open at top and bottom is disposed between adjacent ends of the adjacent pontoons.First and second devices each resiliently and flexibly secure the end of the corresponding one of the pontoons adjacent the member to the member. The first and second devices are disposed on opposite sides of the member.A vertical leg extends downwardly through the hollow member and is secured thereto.

Abstract: An apparatus for sealing floating roof tanks including a sealing element disposed around the circumference of the floating roof between the floating roof and the tank wall, and a plurality of baffles adapted to substantially inhibit or prevent the circumferential flow of vapors through a vapor space between the sealing element and the body of stored volatile liquid.

Abstract: A floating cover for a tank is characterized by a pivotal interconnection between confronting ends of longitudinally adjacent flotation pontoons such that one pontoon is pivotally movable with respect to the longitudinal axis of the other pontoon when the deck of the cover flexes in response to turbulence in the liquid on which the cover is disposed or in response to any other non-uniform loading on the deck.

Abstract: The disclosure herein describes a modular unit for use in the construction of floating decks used in horizontal or vertical liquid storage tanks; the unit consists of a core of cellular foam plastic and of an envelope sealingly encasing the core, the envelope consisting of a layer of electrically conductive flexible metal and of at least one layer of plastic.

Abstract: A sectional floating cover used to control vapor losses from storage tanks and other similar vessels containing volatile organic compounds, is disclosed. The floating cover comprises a plurality of buoyant steel chamber units secured together to form a continuous deck floating on the surface of the liquid in a storage tank. Each chamber unit includes an upper flat sheet, a corrugated lower sheet and sheet strips closing the ends of the corrugations. These parts are welded together in fluid-tight manner, so that the upwardly opening corrugations form independent fluid-tight cells. The units are easily assembled within the tank by simple bead connections readily accessible for crimping and resisting vertical shearing stresses.

Abstract: An open top tank for storing volatile liquids has a circular floating roof buoyantly supported on the surface of the stored liquid within a confining cylindrical shell of the tank. Adjustable sealing means extends between the roof and the top of the shell for sealing the gap between the roof and the surrounding shell. The adjustable sealing means includes a plurality of cylindrical annular sections of different diameters nested one inside another in telescoping relationship, the sections being coaxial with the shell and positioned above the floating roof. Limit means interconnects adjacent ones of the sections for limiting the vertical movement of each section relative to the next adjacent outer section and the outermost section relative to the surrounding shell. The limit means transfers the weight of any section in its lowest position through the limit means to the tank shell.

Abstract: The disclosure is of a buoyant, air-filled, plastic cover for swimming pools, large ponds, and the like, and to facilities by which these may be constrained to float on the water (or other liquid) without proneness to disarrangement, especially in the regions of skimmers formed in the sides of the pool.There is disclosed a pool cover subdivided into interfitting sub-sections, preferably of a rectangular shape. These are provided with coacting marginal abutments which interengage to prevent one cover section from overriding another. These abutments are in some cases of sufficient specific gravity to sink the edge portions of the cover sections somewhat deeper in the water than the remainder of the cover, which is airfilled, and of low specific gravity. Abutting, or interengaging edge elements are disclosed which mechanically engage in a confining manner, preventing overriding.

Abstract: Open top tanks or ponds are provided with a thermally insulating floating cover which comprises a plurality of individual generally flat polygonal plates floating in edge-to-edge relationship. Each of the plates has at least one peripherally disposed downwardly projecting flange and means to vent gases from beneath the plate-like element.

Abstract: A floating cover assembly encompassing in a first condition a method and apparatus for tensioning a floating cover for a liquid storage reservoir wherein the surface level of the stored liquid varies and in a second condition, a method and apparatus for maintaining a liquid storage reservoir. In the apparatus of the above inventions, a peripheral envelope formed at least adjacent a portion of the peripheral edges of the floatable sheet is adapted to be inflated with a compressible fluid, such as air, for proper tensioning of the sheet as the level of the stored liquid varies in the reservoir and for preventing wrinkling of the sheet while, in the second condition, the peripheral envelope is adapted to be filled with a non-compressible fluid for providing a ballast for the sheet during maintenance operations thereof while the sheet is inflatably supported for maintenance of the sheet and/or reservoir.

Abstract: A floating roof of a tank for storing liquids consisting of a resilient disc-shaped diaphragm attached, leakproof fashion, all the way around its circumference to an annular seal and further attached to a floating chamber at the center which is fitted to an upright with provision for displacing along same when the level of liquid is changing. The resilient disc-shaped diaphragm is made hollow and is corrugated in the radial and circumferential directions so that the corrugations form a plurality of cells. The diaphragm rests on a series of radially disposed telescopic arms each attached with one of its ends to the floating chamber and resiliently connected at its free end to the annular seal. A rool of this type can practically be used with tanks of any shape.

Abstract: The evaporation of volatile non-aqueous liquids is inhibited by a floating layer of expanded cereal grains optionally treated with fluorocarbons or perlite treated with fluorocarbons.

Abstract: A floatable ball, a plurality of which floatable balls are for providing a gapless floating ball blanket, wherein the floatable ball includes a modified spheroid constructed of a floatable material and which modified spheroid is provided with a plurality of flat surfaces around the equator which flat surfaces provide the modified spheroid with an equatorial cross-section the outline of which is a closed plane figure comprised of interconnected straight lines equal in number to the plurality of flat surfaces, and a plurality of which floatable balls are engageable in flat surface-to-surface contact at the flat surfaces around the equators thereof to provide the gapless floating ball blanket. The floatable ball may further include means for absorbing and transferring radiant energy whereby the floatable ball can be utilized to heat liquid covered thereby.

Abstract: Means for substantially reducing evaporation and heat loss from an open liquid surface comprising a plurality of pentagonal dodecahedrons which substantially cover the surface of the liquid. The dodecahedrons may be hollow or solid but are preferably made of structural foam having a specific gravity which is approximately one-half the specific gravity of the liquid.