A vertical-difference measuring instrument comprising an entirely closed liquid system containing, as the only fluid, only substantially noncompressive liquid, the liquid system comprising a variable-volume reference reservoir and a sensing reservoir interconnected by a flexible hose, a flexible-walled gas container in the sensing reservoir and a pressure gauge connected in fluid communication with the flexible-walled gas container in the sensing reservoir for giving indication of the vertical distance between the reference reservoir and the sensing reservoir is disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to instruments for measuring vertical distances and 
for assuring that various components of a building structure are at the 
same level or at specified, predetermined differences in level. 
In the construction industry it is often necessary to place a plurality of 
structures at the same level or at different levels at known and 
predetermined differences between the various levels. For example, in 
laying tile, positioning windows vertically, positioning wall panels 
vertically, and the like, it is necessary that all of the particular 
components be at exactly the same level or at precisely predetermined 
level differences. 
There are various methods and apparatus used in ascertaining such levels. 
The conventional spirit level or bubble level may be used, but when large 
distances between the components are involved, this approach is difficult. 
In addition, when one starts at a given point on a long wall, particularly 
a wall with corners, it is possible to accumulate errors as one progresses 
along the wall and end up with a non-level panel, paint line, tile line, 
or the like. 
Most often, these craftsmen measure from the floor or some other structure 
which is presumed, or at least hoped, to be level. While this is 
satisfactory some instances, it is not reliable in many structures. 
The well-know surveyor's transit is unsuited to use by tile layers, 
plumbers, carpenters and others who work in confined spaces and often 
alone. These instruments require, for efficient use, two workmen and are 
expensive and cumbersome. In addition, one cannot assure that a tile line, 
for example, which extends into several room will be level because these 
rooms may not be visible from a given site. 
Examples of prior art leveling devices are disclosed in the following 
patent documents: 
Japanese Patent Application 54-10409, Hitratsuka, which discloses the use 
of a reflected light beam to measure the surface of a liquid in two tanks 
in which the liquid level is always the same. 
Offenlegungsschrif 27 39 975, Wittmann, which discloses a device involving 
two hermetically sealed chambers with interconnected pressure sensors and 
a device for measuring differential pressure between the chambers. 
French Patent 2,515,337, Legris, which discloses a complex solid state 
electronic system involving two mercury filled, sealed reservoirs sealed 
by stretched membranes with a semiconductor film on it. 
French Patent 2,542,866, Legris, which discloses an exceedingly complex 
electromechanical system utilizing a reservoir closed by a membrane which 
closes a first space containing air which limits movements of the 
membrane. The first space is connected to a second space which is divided 
by a membrane which operates switches. 
U.S. Pat. No. 2,557,021, Williams, discloses systems which require a closed 
loop between the bottom and top of the reservoir and the gauge, an 
artificially applied positive pressure, or a thermal insulator jacket. 
U.S. Pat. No. 2,587,998, Heath, discloses a leveling device which has a 
reference reservoir which is open to the atmosphere, and hence subject to 
spillage or leakage at the reference point, and measuring tube which is 
also open to the atmosphere, which requires filling for use and emptying 
for transport or storage. 
U.S. Pat. No. 2,804,692, Karstens, which discloses an electro-mechanical 
system involving a plurality of mercury filled chambers with floats on the 
mercury and plural electrical connections between the chambers. 
U.S. Pat. No. 3,310,880, Watts, discloses a system using two manometers and 
a liquid system comprising a body of mercury and a body of less dense 
colored liquid. 
U.S. Pat. No. 3,645,135, Hadley, discloses an electro-mechanical system 
utilizing a pair of cisterns with interconnected pools of mercury and a 
float on the mercury in each cistern which serves as a capacitor. 
U.S. Pat. No. 3,815,423, Gearhart, discloses a ruggidized altimeter one or 
more diaphragms as the pressure sensing device, the diaphragms separating 
liquids and pressurized gasses, and error compensating structures of 
several types. 
U.S. Pat. No. 3,835,548, Gearhart, discloses a leveling apparatus including 
a sight tube and a connecting liquid conduit to prevent spillage of liquid 
or entrapment of gases. 
U.S. Pat. No. 4,106,342, Sormunen, discloses a barometric type device which 
uses two variable volume vessels, a pressure indicator for one vessel with 
temperature error compensation by means of a closed vessel acting upon the 
variable volume vessels. 
U.S. Pat. No. 4,379,367, Legris, discloses a device relies upon a very 
dense liquid, e.g. mercury, and the stretching of diaphragms, membranes 
against the resisting force of the diaphragm material, whereas applicant 
relies upon a substantially non-resistant flexible bag which permits 
horizontal flow. 
U.S. Pat. No. 4,384,410, Melugin et al, discloses a system including 
indicator heads connected by a liquid hose for equalizing static pressure 
in mercury pools and a gas hose for equalizing pressure within the heads 
above the mercury pools. 
U.S. Pat. No. 4,563,822, Legris, discloses a masonry level which utilizes a 
chamber divided by a stretched diaphragm separating liquid in the lower 
part of the chamber and gas in the upper part of the chamber. The pressure 
of the liquid is reflected by the stretching of the diaphragm which 
compresses air which, in turn actuates a switch or indicator. 
U.S. Pat. No. 4,581,931, Robotti et al, discloses a system for measuring 
differences in levels using plural reservoirs with floats in each, the 
float and an electrode forming a sensor for the level of liquid in the 
reservoir. 
U.S. Pat. No. 4,651,433, Mohr, which discloses a system which uses pressure 
cells connected by flexible tubing and a strain gauge for sensing 
differential pressure, with various compensating devices such as a bellows 
to reduce hysteresis errors. In the Mohr device, accuracy depends upon 
exact identity of diaphragm structures and/or upon exact repetition of 
movement of such structures over the life of the instrument. Errors in 
flexing result in erroneous readings. 
U.S. Pat. No. 4,759,134, Chrisley, discloses a devices which uses two 
flexible bags, but these are both filled with air, or gas, and neither 
serves as a reference reservoir. The bags of Chrisley are intended, 
apparently, only to permit the transmission of atmospheric pressure to the 
liquid column without permitting evaporation or contamination of the 
liquid. 
Laser transits are even less suited to applications as described because of 
its great expense and the great amount of time required to set up the 
equipment. 
There is a great and long-standing need for an inexpensive but accurate 
measuring and leveling device which will assure that components are 
exactly level with each other, vertically, or are at exactly predetermined 
differences in vertical position, as may be desired in particular 
instances. It is to a solution of this long-standing problem and meeting 
this serious need in the building and construction industries that the 
present invention is directed. 
SUMMARY OF THE INVENTION 
The present invention is a vertical-difference measuring instrument 
comprising an entirely closed liquid system containing, as the only fluid, 
only substantially non-compressive liquid. The system includes a 
variable-volume reference reservoir and a sensing reservoir interconnected 
by a flexible hose, a flexible-walled gas container in the sensing 
reservoir and a pressure gauge connected in fluid communication with the 
flexible-walled gas container in the sensing reservoir for giving a 
visible indication of the vertical distance between the reference 
reservoir and the sensing reservoir is disclosed. 
As a leveling instrument for assuring that plural points on vertical 
surfaces in a building structure are at the same level, the invention 
comprises two basic structural assemblies. An entirely closed liquid 
system containing, as the only fluid, only substantially non-compressive 
liquid, the liquid system comprising a variable-liquid-volume reference 
reservoir, a sensing reservoir, and a flexible conduit having a first end 
and a second end for connecting said reservoirs in liquid communication 
with each other. The liquid-containing system also includes means 
connecting the reference reservoir at the first end of and in fluid 
communication with the flexible conduit means connecting the sensing 
reservoir at the second end of and in fluid communication with the 
flexible conduit, and liquid filling the flexible conduit. The instrument 
also includes a fluid-pressure gauge having a visible pressure indicator 
and indicia for giving a visual quantitative indication of the pressure 
applied to the gauge. Means are provided for connecting the fluid pressure 
gauge in fluid communication with the sensing reservoir. The 
fluid-pressure gauge and liquid-containing system are so connected, 
constructed and configured that the pressure measured by the pressure 
gauge is proportional to the vertical distance of the sensing reservoir 
above or below the reference reservoir. The leveling instrument includes 
indicia on the gauge, including a zero point and indicia indicating 
pressures above and below the zero point, and means may be provided for 
adjusting the indicator to the zero point regardless of the actual 
pressure applied to the pressure gauge. At the zero point, when so 
adjusted, the sensing reservoir is always at the same vertical elevation, 
even though moved from one wall to another or one room to another. 
In a preferred embodiment, the means connecting the fluid-pressure gauge 
with the sensing reservoir for communicating the pressure in the sensing 
reservoir to the gauge comprises a flexible-walled gas container in the 
sensing reservoir. The pressure gauge is connected in fluid communication 
with the flexible-walled gas container thus transmitting the pressure in 
the sensing reservoir to the gauge. In this embodiment, the fluid-pressure 
gauge, flexible-walled gas container and liquid-containing system are so 
connected, constructed and configured that the pressure measured by the 
pressure gauge is the pressure of the gas in the gas container which is 
proportional to the vertical distance of the sensing reservoir above or 
below the reference reservoir. 
The invention also serves as a vertical-difference measuring instrument 
comprising an entirely closed liquid system containing, as the only fluid, 
only substantially non-compressive liquid, the liquid system, in turn, 
comprising a variable-volume reference reservoir, a sensing reservoir, a 
flexible hose having one end connected in fluid communication with the 
reference reservoir and the other end connected in fluid communication 
with the sensing reservoir, a flexible-walled gas container in the sensing 
reservoir, a pressure gauge connected in fluid communication with the 
flexible-walled gas container in the sensing reservoir, the pressure gauge 
having a visible indicator, and liquid in the reference reservoir and hose 
and that portion of the sensing reservoir not occupied by the gas 
container, the reservoirs, hose, gas container and pressure gauge being so 
constructed and connected that the pressure gauge gives a visible 
indication of the pressure in the gas container which is proportional to 
the vertical distance between the reference reservoir and the sensing 
reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following description, reference will be made principally to the 
instrument as a leveling device, but as explained above, the instrument 
serves just as well for measuring different vertical distances. Also, the 
following description will use particular reference to convenient 
structural materials and particular structures, it being clearly 
understood that these are not limiting and that many alternatives to the 
various structures and embodiments described below may be used without 
departing from the spirit and scope of the invention. 
Referring first to FIG. 1 for an understanding of the use and general 
operation of the invention, the invention is shown in use in a building 
structure which comprises a floor F and a wall W. The invention 
conveniently comprises a case 10, although that certainly is not a 
necessary or an important part of the invention. The case 10, in the 
preferred embodiment, has a variable-volume reference reservoir 12 
connected by means of a tube 14 in fluid communication with a hose 20. The 
reference reservoir 12 is a bag containing only liquid. The bag in a first 
configuration wherein the bag is not fully expanded to its maximum 
capacity, having the capability of expanding to a second configuration 
either by resilient or non-resilient stretching of the walls of the bag or 
by the unfolding or expanding of folds in the bag to a configuration 
having a greater volume than that of the first configuration. The bag is 
sufficiently large, in its full-capacity configuration, and is so 
incompletely filled with liquid that the bag will freely expand with 
negligible resistance, i.e. resistance so low as to be insignificant. For 
example, a practical plumber's level is satisfactory if it is accurate to 
one-tenth of and inch of water. The bag is sufficiently large to expand 
sufficiently to absorb the liquid forced by the change of volume in the 
gauge bourdon tube or other diaphragm through the conduit into the bag 
with a change in liquid level of no more than about one twentieth or less 
of an inch. Stated differently, the expansible volume of the bag with 
negligible resistance is at least equal to the maximum volume of liquid 
displaced by the gauge bourdon tube or diaphragm or other from its fully 
expanded configuration its fully compressed configuration. The bourdon 
tube is, of course, merely a special case of the more conventional 
pan-shaped diaphragm. The bag contains only liquid in all configurations, 
i.e. there is no gas in the bag. Obviously, small solid objects in the 
bag, while not desirable, may be tolerated. Terminology indicating the bag 
is incompletely filled with liquid but containing only liquid defines a 
bag in a first configuration containing only liquid, i.e. the only fluid 
in the bag is substantially non-compressible liquid, e.g. water, 
anti-freeze, etc., the bag being capable of conforming to a second 
configuration of greater volume than the volume of the first configuration 
with negligible resistance. A plastic bag or balloon incompletely filled 
with water meets this criteria perfectly and unambiguously. The bag may, 
however, be of any convenient construction or configuration which meets 
the stated requirements. A resilient wall rubber or plastic bag or a 
fluted or folded plastic bag are quite suitable. In FIG. 2, for example, 
the reference reservoir is simply a flexible, non-resilient bag which is 
incompletely filled with liquid but contains only liquid, i.e. does not 
contain any gas, but is capable of expanding generally horizontally as 
well as vertically. In practice, virtually no vertical expansion occurs, 
however. The liquid conveniently used and which will be referred to 
hereinafter is water, but any liquid may be used. Liquids with a high 
coefficient of thermal expansion should be avoided for greatest accuracy. 
Liquids should be free of dissolved gases. 
In the preferred embodiment, the variable-volume reference reservoir 12 is 
an accordion-fluted balloon-like structure, i.e. a resilient container 
having accordion-like fluted walls for easy expansion. If desired, the 
bulb may be constrained at its upper level and allowed to expand 
horizontally. 
A housing 30 formed of an injection-molded plastic having walls such as 
indicated at 32 defining an upper chamber 34 and a lower chamber 36 
divided by a wall 38 may be used. Actually, however, the size and 
configuration of the housing is not critical, and any housing may be used. 
Indeed, it is not necessary to use a housing, but it certainly is a 
convenient form of the invention. 
The housing 30 carries a gauge 40 which, as shown in FIG. 3, may have 
indicia on the gauge and will have a visual indicator of the pressure or 
force being exerted on the gauge. 
The lower chamber 36 of the housing, in the preferred embodiment of the 
invention under discussion, constitutes a sensing reservoir. Also, in the 
preferred embodiment, a flexible-walled air container, such as a fluted 
balloon, shown at 50, is positioned and is connected by means of a conduit 
52 in fluid communication with the pressure gauge 40. The sensing 
reservoir is connected through a conduit 54 and through the hose or other 
flexible conduit 20, thereby forming a fluid communication path. 
It will also be noted, in reference to FIG. 1, that the hose may follow any 
path, such as over a beam B, without introducing errors in the 
measurement. This permits placement of the reference reservoir at any 
desired reference location and movement of the sensing reservoir to any 
location, with the hose running under or over objects, whether or not the 
reference reservoir is visible. For example, in health clubs and the like, 
multiple shower and dressing rooms are often tiled. The instrument of this 
invention permits the craftsman to assure that a line of tile is perfectly 
level throughout all rooms, thus avoiding the embarrassing circumstance of 
having a tile joint which is off-set vertically. 
While it is preferable, for economy and accuracy, to provide the 
flexible-walled container 50 in the sensing reservoir 36, it is not 
necessary to do so. As shown in FIG. 4, the gauge 40 may be in fluid 
communication directly with the sensing reservoir 36. In either instance, 
the gauge 40 will measure the liquid head, i.e. the distance above or 
below the reference reservoir of the sensing reservoir. Stated 
differently, the gauge 40 will measure and indicate the weight of the 
water being supported by the reservoir 36 or the weight of water above the 
level of the constant-level sensing reservoir 36, if the sensing reservoir 
is below the reference reservoir. 
In some construction operations it may be desirable to establish a "zero" 
level or reference level and to assure that all similar structures are at 
exactly that zero level or at some predetermined distance above or below 
that zero level. It is, therefore, desirable to be able to "zero" the 
gauge regardless of the force or pressure being exerted upon the gauge. 
FIG. 5 depicts a simple mechanism for accomplishing this desired result. 
The shaft 60, which is connected to the pressure measuring device, extends 
through a wall or face plate 62 upon which the indicia may be formed. The 
visual indicator 64 is provided with a hub 66 having a hollow, cylindrical 
aperture therein which slips over the end of the shaft 60. The hub will 
cause the indicator to turn as the shaft turns, but will also permit the 
hub to turn relative to the shaft, thus permitting the indicator 64 to be 
moved to the zero position, thereby "zeroing" the gauge. The movement may 
be accomplished by a shaft 68 connected to a knob 70 on the outside of the 
gauge, the shaft extending through a glass or other transparent gauge 
cover 72. 
Having described the basic features and structure of the invention, the 
various components will be discussed in somewhat greater detail. As 
previously noted, it is not necessary to provide an enclosed reference 
reservoir; all that is necessary is that the volume of liquid which may be 
held by the reservoir is variable. In a convenient embodiment, the 
reference reservoir is a rubber balloon or even a non-resilient plastic or 
polymeric bag which is held in a tray or container and is incompletely 
filled to define a first configuration, liquid, however, being the only 
fluid in the bag in said first configuration, there being no gas in the 
bag. The first configuration of the bag is such that additional liquid may 
flow into and out of the bag. It is important that the liquid in the 
reference reservoir be able to change in volume with negligible 
resistance; otherwise, the structure of the reference reservoir is 
insignificant. 
The hose 20 may be of any conventional design. Many such hoses are 
available from any industrial supply house. A typical garden hose, 
small-diameter rubber, chlorinated rubber or other polymeric hose, or a 
high-pressure hose such as is used for filling pneumatic tires and the 
like may be used, for example. 
The housing 30 may be made of metal or any other convenient material. 
Molded plastics are exceptionally convenient in forming housings of this 
type. The sensing reservoir 36 may be of any design which will assure that 
the weight of the water, or water head, is transmitted accurately to the 
gauge. The weight of the water or water head may be transmitted directly 
to the gauge, i.e. the gauge may be in continuous liquid communication 
with the reference reservoir, as depicted in FIG. 4. In the preferred 
embodiment, however, the weight of the water is transmitted by means of 
air or gas pressure differential, the gas pressure differential being a 
function of the expansion or contraction of the flexible-walled gas 
container 50 in the sensing reservoir. In the preferred embodiment a 
balloon-like structure is preferred but any other flexible-walled 
container which may be contained substantially entirely in the reference 
reservoir and which can expand or contract in volume with negligible 
resistance, may be used. A fluted rubber or plastic bag, for example, may 
be used. 
The gauge 40 may be of any convenient construction or mode of operation. 
The conventional Bourdon tube pressure gauge, the diaphragm-type pressure 
gauge, etc., may be used. In all such pressure gauges it will be noted 
that there is a mechanical movement system which responds to differences 
in pressure which differences are transmitted via any of many various 
mechanical movements to an indicator. The indicator may, as depicted in 
FIG. 3, be a simple needle on the face of a gauge with numerical or other 
indicia thereupon, or it may be a digital or electronic display, or any 
other display. For simplicity and accuracy and convenience, the well-known 
DWYER.RTM. gauge is highly accurate and is conveniently available through 
industrial supply houses. DWYER.RTM. gauges are conveniently used in 
connection with the present invention. In a preferred embodiment, a 30 
inch gauge (scale of 0 to 30 inches of water) manufactured by MARSHALTOWN, 
Hastings, Nebr., USA which uses a phosphor-bronze diaphragm has been found 
particularly satisfactory. It will also be noted that the movement of the 
mechanical components of the gauge make the use of the flexible-walled gas 
container unnecessary, since such movement can be accomplished by liquid 
filling of the gauge tube or diaphragm chamber, etc. It is difficult, 
however, to fill some gauge mechanisms fully with the liquid and, 
therefore, it is sometimes more convenient or more accurate to use a gas 
transmission mechanism such as the flexible-walled gas container which is 
in fluid communication with the gauge, rather than connecting the liquid 
directly to the gauge mechanism. Various types of indicating instruments, 
many of which are quite suitable for use in the present invention, are 
known in the prior art; see, for example, U.S. Pat. Nos. Re. 19,709 and 
1,848,053. 
It is also well known that many gauges have built-in "zeroing" mechanisms 
which work through clutches, gears, or other mechanisms to allow the 
calibration of the gauge in setting the indicator at zero. The adjusting 
mechanism depicted in FIG. 5 is provided simply for simplicity to 
illustrate the principal of operation and not necessarily to indicate a 
preferred mechanism of operation. Any gauge which can be calibrated may be 
used. 
Returning to the mode of use, for further discussion, it will now be 
understood that the invention can be used in connection with a marking 
pencil or any other marker, to mark a line around a plurality of walls 
which will be perfectly level, or to mark particular points on walls in 
different rooms which will be perfectly level and will all be the same 
vertical distance above the floor or other reference point where the 
reference reservoir is located. By appropriate marking, the exact vertical 
location of windows, paint lines, wall boards, and other structures may be 
determined, and high accuracy and leveling achieved. In some plumbing 
insulations, it is desirable to maintain a high level of accuracy as to 
vertical height of various drain pipes and to assure proper sloping of the 
pipes. This invention may be used by plumbers for accomplishing these 
results. 
In like manner, the invention may be used to position various structures at 
precisely known elevations that are different one from another. For 
example, if it is desired to position one window a precisely determined 
number of inches above another window, the instrument may be zeroed at the 
level of one window and then moved to the level of the other window, the 
level of the other window being determined by the reading on the gauge. 
The gauge indicia may be made to read in centimeters, inches, feet, etc. 
Most conveniently for most structural operations, the gauge will read in 
inches and in fractions of inches, in English and American construction. 
William, U.S. Pat. No. 2,557,021, discloses systems which require a closed 
loop between the bottom and top of the reservoir and the gauge, an 
artificially applied positive pressure, or a thermal insulator jacket, 
which are subject to temperature errors for which Williams seeks a 
solution. The present invention overcomes the temperature error addressed 
by Williams in an entirely different manner, i.e. the bag contains no gas, 
only liquid. 
Health, U.S. Pat. No. 2,587,998, discloses a leveling device which is 
simple in concept and reliable in operation, but is impractical for most 
industrial construction and other projects, because both the reference 
reservoir and the measuring tube are to the atmosphere, easily spilled, 
and require filling for use and emptying for transport or storage. In 
addition, the user always faces the very common circumstance that gas may 
come out of solution or otherwise be entrapped in the liquid. The presence 
of gas bubbles presents one or more, frequently a great many, compressible 
pockets which may block free-flow of liquid or prevent accurate 
measurement by effectively reducing the density of the liquid by 
interspersing small bubbles in the liquid and dissolving gas in the 
liquid. 
The principal of the invention may be embodied in many structures, and many 
types of components may be assembled to accomplish the purpose of the 
invention without departing from the spirit of the invention. The 
invention is also embodied in a method of determining relative vertical 
differences or in assuring that two or more components are at the same 
level. In carrying out this method, a variable volume reference reservoir 
is positioned at any desired reference point from which other measurements 
are to be taken. The reference reservoir is in fluid communication with a 
sensing reservoir to which a gauge is connected for measuring the 
difference in vertical elevation of the sensing reservoir from the 
reference reservoir. The sensing reservoir is then moved to obtain a 
predetermined reading on the gauge, or to a predetermined point at which 
the gauge is set to read zero. By moving the reference reservoir to 
different points on a wall or upon different walls, exactly the same 
vertical elevation may be obtained by positioning the sensing reservoir to 
obtain a zero reading on the gauge. To obtain a different vertical 
elevation, the sensing reservoir is positioned to obtain a precisely known 
different reading on the gauge which positions the sensing reservoir a 
predetermined distance above or below the reference reservoir. 
In summary, the instrument of this invention for assuring that plural 
vertical points are at the same level or at predetermined different levels 
comprises an entirely closed liquid system containing, as the only fluid, 
only substantially non-compressive liquid. The liquid system comprising a 
closed reference reservoir having a flexible bag containing only liquid, 
said liquid incompletely filling the bag to define a first configuration 
such that additional liquid may flow into the bag to define a second 
greater volume configuration and flow out of the bag to effect, with 
negligible resistance, a change in the volume of liquid in the bag, a 
sensing reservoir, and a flexible conduit having a first end and a second 
end for connecting said reservoirs in liquid communication with each 
other. The instrument includes means connecting the reference reservoir at 
the first end of and in fluid communication with the flexible conduit, 
means connecting the sensing reservoir at the second end of and in fluid 
communication with the flexible conduit, the flexible conduit, reference 
reservoir and sensing reservoir containing only said liquid and being in 
fluid communication with one another and the reference reservoir and 
conduit being constructed and configured to define a closed 
liquid-containing system. A closed fluid-pressure gauge having a visible 
pressure indicator and indicia for giving a visual quantitative indication 
of the pressure applied to the gauge and means connecting the 
fluid-pressure gauge with the sensing reservoir for communicating the 
pressure in the sensing reservoir to the gauge are a part of the 
invention. The fluid-pressure gauge and liquid-containing system are so 
connected, constructed and configured that the pressure measured by the 
pressure gauge is proportional to the vertical distance of the sensing 
reservoir above or below the reference reservoir. The means connecting the 
fluid-pressure gauge with the sensing reservoir for communicating the 
pressure in the sensing reservoir to the gauge may comprise a 
flexible-walled gas container in the sensing reservoir and means 
connecting the fluid-pressure gauge in fluid communication with 
flexible-walled gas container; the fluid-pressure gauge, flexible-walled 
gas container and liquid-containing system being so connected, constructed 
and configured that the pressure measured by the pressure gauge is the 
pressure of the gas in the gas container which is proportional to the 
vertical distance of the sensing reservoir above or below the reference 
reservoir. The invention is characterized in that the liquid system is 
entirely closed, i.e. the liquid cannot be spilled, and the system 
accuracy is dependent only upon the accuracy of the pressure gauge per se 
and not upon stretched membranes, bellows, diaphragms or like structures 
in which the extent of movement is a function of the resiliency of the 
material of construction and/or the degree of stretching, etc. 
Many variations from that described as exemplary may be accomplished 
without departing from the spirit and scope of the invention. 
INDUSTRIAL APPLICATION 
This invention is useful in the building trades.