Liquids of the liquefied petroleum gas type, such as propane, butane, and the like (often referred to generally as LPG), are commonly used for purposes such as residential or industrial heating, or for powering internal combustion engines on industrial vehicles such as lift trucks (i.e, fork lifts). The LPG is typically stored as a liquid under pressure in a tank or cylinder. A liquid level gauge may be provided on the tank for measuring the level of the liquid in the tank. In some applications such as lift trucks, the LPG cylinders are oriented horizontally while in use on the vehicle but are stored vertically when removed for refueling. In such applications, a liquid level gauge which can measure liquid levels in both orientations is desirable.
In other applications, for example, when an LPG storage tank is used to supply LP gas for the heating and/or energy needs of a house, mobile home, or business, the LPG tank is typically installed in a permanent location. In many regions, the dimensions of LPG tanks used in these stationary applications have become standardized in the form of cylinders having a diameter (measured perpendicular to their elongate axis) of 24 inches, 30 inches, 31.5 inches, 37 inches, 40.5 inches or 41 inches. In other regions, tank sizes approximating the standard sizes are used having a range of diameters from about 24 inches to about 42 inches. The tanks are usually installed on their side with their elongate axis horizontal. It is, of course, desirable to have a liquid level gauge which can indicate the level of LPG within such a stationary tank. In the case of a stationary tank, however, there is no need for the liquid level gauge to indicate the LPG level when the tank is in more than one orientation, as in the case of gauges for use with lift-truck cylinders. Instead, in the case of a stationary cylinder, it is desirable that the liquid level gauge accurately measure the level of LPG within the tank, especially when the LPG level is low, i.e., when the level in the tank equates to approximately 5% full, and when the LPG level is high, i.e., when the level in the tank equates to approximately 80% full. Further, it is desirable that the measured level of LPG be indicated with high angular resolution on the gauge dial. Accurate low level measurements are especially important to help the user avoid running out of LPG, while accurate high level measurements are especially important to help the user determine the level of LPG in a near-full tank. A high resolution indication of the measurement, i.e., where the angular distance between the 5% full mark and the 80% full mark on the gauge dial is at least 180.degree., allows the user to accurately determine the quantity of LPG remaining in the tank and/or the LPG consumption over time, thereby allowing the user to better estimate how soon the tank will need to be refilled.
Liquid level gauges for measuring the level of a liquid such as LPG inside a tank are disclosed in U.S. Pat. Nos. 2,992,560 and 3,688,735. These patents disclose float-type liquid level gauges that utilize a pivoting float arm having a position which is responsive to the liquid level inside the tank. The float arm is connected to a rotatable shaft by means of a geared mechanism, and the shaft is magnetically coupled to an external liquid level indicator. A change in liquid level causes the float arm to rotate the shaft, and the magnetic coupling then rotates the external indicator without requiring a direct physical connection. Thus, the possibility of volatile liquid or vapor leaking through the mechanism is eliminated.
While useful, the previously disclosed float-type gauges have several drawbacks. First, due to the low density of LPG, the heavy-walled hollow float used to resist the pressure in the tank has insufficient buoyancy to float without a counterweight to balance the float arm. Such counterweights are typically discrete components which add to the manufacture and assembly expense of the gauge, and their size often increases the difficulty in installing the gauge through the narrow opening of the tank or cylinder. For example, on many lift truck cylinders and tanks for recreational vehicles (RVs), the in-tank parts of the level gauge must fit through a 3/4 inch opening. A need therefore exists, for a liquid level gauge which does not require a counterweight, or where the counterweight is a small, integral part of another component.
In stationary tank applications, a threaded pressure fitting having an opening about 1.13 inches in diameter is commonly provided on the upper surface of the tank for installation of a liquid level gauge. A magnetically-coupled float-type gauge is typically used for this purpose. While this pressure-fitting opening is somewhat larger than the 3/4 inch diameter opening typically found on a lift truck cylinder, it still requires the that the float, float arm, support arm and other in-tank components of the gauge be configured to fit through an opening of that size. Furthermore, the in-tank components of the gauge must be configured to have clearance with the inner walls of the tank at all times. This includes clearance with the bottom of the tank when the float arm is hanging at the lowest, i.e., empty, position as well as clearance along the sides of the tank as the float arm pivots upwards. The side clearance is important not just during normal use of the gauge, but also as the gauge is being installed by screwing it into the threaded pressure fitting on the tank. It has been discovered that installers frequently spin the gauges at a rapid rate as they are being installed. This can cause the pivoting components of the gauge to swing outward to a significant angle with respect to vertically downward under centrifugal force. Unless the components are sized to ensure proper clearances, then the pivoting components may strike the inner walls of the tank (which can damage the gauge) during installation.
Simply providing clearance between the walls of a tank and the in-tank components of a liquid level gauge does not, however, ensure that the gauge can accurately measure the level of LPG in the tank. It is also necessary that the length of the gauge's support arm and the geometry of the gauge's float arm be properly selected for the dimensions of the tank. For example, if the support arm is too short (i.e., the pivot point is placed too high with respect to the center of the tank), a float arm with a length selected to permit the float to measure low LPG levels (near the 5% full level) will also allow the float to easily strike the inner wall of the cylinder if the gauge is spun during installation. On the other hand, if the support arm is too long (i.e., the pivot point is placed too low with respect to the center of the tank), a float arm length cannot be selected to permit the float to measure both low LPG levels (near the 5% full level) and high LPG levels (near the 80% full level) without causing excessive accuracy error.
To meet the dual requirements of internal clearance during installation and measurement accuracy at high and low LPG levels, most magnetically coupled liquid level gauges for use in stationary LPG tanks are single-size gauges, i.e., gauges constructed or assembled with a specific support arm working length (i.e., the distance from support arm top to the float arm pivot point) for each different size or diameter of LPG tank. The use of such single-size gauges, however, requires manufacturers, distributors, and suppliers of LPG gauges to manufacture and/or warehouse a large inventory of different gauge configurations and gauge components, a situation which leads to inconvenience and increased cost. A need therefore exists for a liquid level gauge that can be used in LPG service, that is suitable for screw-in installation through a hole having a diameter of about 1.13 inches, that provides a high-resolution magnetically-coupled indication of the LPG level, that provides good measurement accuracy at high and low LPG levels and that minimizes the changes required for use in LPG tanks having standard diameters ranging from about 24 inches to about 42 inches.
It is known to use liquid level gauges having an adjustably positionable pivot point on the support arm to reduce the number of different gauge components that must be manufactured and/or stocked to serve a range of different size tanks. For example, U.S. Pat. Nos. 4,671,121, 4,928,526 and 5,152,170 disclose liquid level gauges having float arm pivot assemblies that are adjustably positionable along a fixed-length support arm. However, the disclosed gauges still require float arms of different lengths for use in tanks of different heights. Further, the disclosed gauges do not provide a mechanical indication of the liquid level like magnetically coupled LPG gauges do. Instead, electrically powered devices are used inside tank, posing a potential safety and regulatory code problem if used in the pressurized LP gas environment inside an LPG tank. The adjusting mechanisms of the disclosed gauges also add to the overall number and complexity of components in each device, therefore further reducing the advantage of adjustability. Finally, the disclosed gauges are not designed for installation in a threaded opening such as is commonly used for LPG tanks.
It is also known to use a magnetically-coupled float-type gauge having an adjustable length support arm to fit a variety of LPG tank sizes. An adjustable LPG gauge, Model No. 49S, is produced by Rochester Gauges, Inc., Assignee of the current application. The gauge utilizes a square-brass centershaft telescoping inside a square-aluminum pinionshaft to hold the coupling magnet. Both of these shafts are housed in an adjustable length support arm comprising telescoping aluminum support tubes. An aluminum locknut compresses an aluminum locking sleeve to secure the tubes at the required support arm length. Even though it has an adjustable length support arm, the Model No. 49S gauge still requires assembly with float arms of different lengths for use in tanks of different heights. Further, the telescoping tubular construction of the Model No. 49S gauge is complex and expensive to assemble and requires metallic components to meet the desired strength and size parameters.
It is also been suggested to use a liquid level gauge having an adjustable length float arm to fit a variety of LPG tank sizes. U.S. Pat. No. 5,072,618, assigned to Rochester Gauges, Inc., Assignee of the current application, discloses a magnetically coupled LPG level gauge having a conventional gauge head and gear assembly mounted on opposite ends of a tubular support shaft. An "L"-shaped float arm is disclosed for attachment to the gear assembly, and it is further disclosed that the length of the float arm can be adjusted to adapt the gauge for use in a range of tank sizes from 30 inches to 41 inches. A hollow plastic float is disclosed which allows the gauge to function in the low density LP gas environment. In practice, however, the performance of gauges produced according to the disclosure of U.S. Pat. No. 5,072,618 has not met expectations. The high performance hollow plastic float costs more to produce than conventional rubber foam floats and the "L"-shaped float arm provides insufficient float arm travel for good measurement accuracy. In addition, the conventional construction of the gauge with a tubular metallic support arm and numerous small components is expensive to manufacture and assemble. Finally, a single gauge of the disclosed design cannot be used for a range of LPG tanks ranging from 24 inches to 42 inches in diameter. Thus, using prior art designs requires that several sizes of gauge must be stocked to accommodate the entire range of commonly encountered tank sizes.
Previously disclosed liquid level gauges utilize components formed primarily of steel, aluminum, and other metallic materials fabricated primarily by machining, stamping, welding, casting and other metal-working processes. This is because the size of the in-tank components of the gauge is always limited by the size of the pressure fitting passage through which it must be installed. For example, while the support arm and float arm of a typical LPG gauge might extend several feet into the tank, the width of the support arm must be less than the width of the fitting passage, typically about 1.13 inches wide. It was previously believed that only metallic components possessed sufficient strength, stiffness and chemical resistance to avoid unacceptable deformations in such applications. The cost of fabricating and assembling metallic components is relatively high, however, compared to other materials such as molded plastics. A need therefore exists, for a liquid level gauge which can have principle components such as the support arm and gear assembly formed of non-metallic materials.
Further, the design of previously disclosed float type gauges required the use of a large number of small discrete secondary components, such as gears, axles, bearings, and fasteners, in addition to the primary components, such as gauge head, support arm, drive shaft, magnet, float arm, and float. These discrete secondary components greatly increase the complexity of previously disclosed float type gauges and the expenses associated with the design, production and stocking of these typically small components increased the cost of the finished gauge. A need therefore exists, for a liquid level gauge which does not require a significant number of discrete secondary components.
Similarly, the complex design of previously disclosed float type gauges necessitates the use of skilled workers for their assembly. For example, the previously disclosed tubular support arms required that the magnet drive shaft be inserted through the end of the support arm and held in position by a discrete bearing or end caps which were themselves installed within or onto the support arm as one or more separate assembly operations. The large number of separate assembly operations and the need for skilled workers to perform these operations increases the cost of the finished gauge. A need therefore exists, for a liquid level gauge which does not require skilled workers for assembly and minimizes the number of separate assembly operations.