Liquid level detector and system

[0002] The present invention relates generally to non-invasive detection systems and, more particularly, to a non-invasive system and method for detecting the level of fluid in a fluid containing storage vessel.

[0003] Conventional storage vessels, such as propane tanks, are beneficial both for their mobility and for their suitability to the energy needs of remote locations where access to other forms of energy is often unavailable or costly. As with any storage vessel, however, propane tanks have finite storage capacity and eventually run dry. Few are equipped with precise level detectors nor is it practicable or cost effective in most contexts to remove them individually from service for recharging. When propane tanks run dry, they can also become a safety hazard, particularly when pilot lights go out and during start-up operations.

[0004] Typical level detectors such as float-type gauges must be retrofitted to the vessels during assembly. Retrofitting usually requires that the vessel be shut down, depressurized and emptied. In operation, the gauge electronically signals an indicator located externally to the vessel; the signal corresponding to the quantity of product remaining in the vessel. Although generally effective, retrofitting these devices has been found impractical and uneconomical, as well as disruptive of operations.

[0005] Another method is level estimation using a computer. In particular, specially designed software is used to monitor usage and predict when refill or recharging is necessary. Previous liquid usage patterns are considered, as well as weather and other data. While useful, computers have been found generally unreliable in predicting runouts.

[0006] U.S. Pat. Ser. No. 5,209,115 (Bond) appears to disclose a liquid detector for thin-walled tanks including temperature sensors A, B and a heater 9. Bond appears to disclose further that activation of the heater 9 causes different rates of increase in the temperatures sensed by sensors A, B depending upon the presence of liquid next to the tank wall 3 at the respective locations of the sensors A, B enabling detection of liquid at the respective locations. Bond, however, does not show or suggest the sensors A, B and the heater 9 contained in a modular unit which would facilitate their attachment-to the tank wall 3. In contrast, separate mounting of each of the sensors A, B and the heater 9 on the tank wall 3 appears to be required in Bond. Additionally, Bond appears to provide a pressurant inlet 22 for the supply of pressurizing gas to the tank 1 in addition to the liquid propellant contained in the tank. Adding pressurizing gas to the liquid propellant in the tank 1 obstructs reaching thermodynamic equilibrium thereby inhibiting vaporization of the liquid propellant. This increases the proportion of the heat applied to the tank wall which increases the temperature of the tank wall rather than vaporizes the propellant. As a result, the temperature increase of the tank wall when liquid is present at the sensors A, B is closer to the increase when liquid is not so present thereby impeding indication of such liquid by the temperature difference. Moreover, the liquid detector of Bond appears to be for use in zero gravity which would inhibit convective circulation.

[0007] It is therefore an object of the present invention to provide a non-invasive system and method for detecting the level of fluid in a fluid storage vessel.

[0008] Another object of the present invention is to provide a durable, reliable and economical system and method for detecting the level of fluid in a fluid storage vessel.

[0009] A further object of the present invention is to provide a non-invasive system and method for detecting the level of fluid in a fluid storage vessel and signaling the same to a user.

[0010] Still another object of the present invention is to improve safety of fluid storage vessels, particularly in remote locations.

[0011] Yet a further object of the present invention is to provide a non-invasive system and method for detecting the level of fluid in a fluid storage vessel, which may be readily fitted to-any vessel without interruption of service or depressurizing and emptying the vessel.

[0012] Still a further object of the present invention is a system and method for detecting the rate of heat dissipation at the surface of a fluid storage vessel.

[0013] According to one aspect of the present invention there is provided a system for detecting the level of fluid in a fluid containing vessel. A device is provided for applying energy to a selected location on the vessel. A sensor in proximity to the location detects the energy level at the location during first and second time intervals. A memory device is provided for storing data representative of the energy detected by the sensor at each time interval. A logic circuit compares the amount of energy of the first time interval to that of the second time interval, and data corresponding to the level of fluid in the vessel is output to a display device. A power supply is also provided for operating the system.

[0014] In accordance with another aspect of the present invention is a system for detecting the level of fluid in a fluid containing vessel. A device is provided for applying energy to a selected location on the vessel. A first sensor in proximity to the energy location detects the energy level at the location at first and second time intervals. A second sensor, positioned at a selected distance from the surface of the fluid, detects the energy level at that position at the same time intervals as the first sensor. A memory device is provided for storing data representative of the energy detected by both sensors at each time interval. A logic circuit compares the amount of energy of the first time interval to that of the second time interval in each respective sensor, and data corresponding to the level of fluid in the vessel is output to a display device. A power supply is also provided for operating the system.

[0015] According to a further aspect of the present invention is a method of detecting the level of fluid in a fluid containing vessel, which comprises the steps of:

[0016] (a) applying energy to a selected location on the vessel;

[0017] (b) detecting the energy level at the location at time intervals using a first sensor in proximity to the energy location;

[0018] (c) detecting the energy level at that position at the same time intervals as the first sensor, using a second sensor positioned at a selected distance from the surface of the fluid;

[0019] (d) storing data representative of the energy detected by both sensors at each time interval in a memory device;

[0020] (e) comparing the amount of energy of the first time interval to that of the second time interval in each respective sensor, using a logic circuit; and

[0021] (f) outputting data corresponding to the level of fluid in the vessel to a display device.

[0022] In accordance with still another aspect of the present invention is a method of detecting the level of fluid in a fluid containing vessel, which comprises the steps of:

[0023] (a) applying energy to a selected location on the vessel;

[0024] (b) detecting the energy level at the location at a first time interval using a sensor in proximity to the energy location;

[0025] (c) storing data representative of the energy detected by the sensor at the first time interval in a memory device;

[0026] (d) detecting the energy level at the location during the second time interval;

[0027] (e) storing data representative of the energy detected by the sensor at the second time interval in the memory device;

[0028] (f) comparing the amount of energy of the first time interval to that of the second time interval using a logic circuit; and

[0029] (g) outputting data corresponding to the level of fluid in the vessel to a display device.

[0030] The above and other features and advantages of the present invention are realized in specific, illustrative embodiments thereof, presented hereinbelow in conjunction with the accompanying drawings, in which:

[0031] FIG. 1 is a perspective view of a device and system for detecting the level of fluid in a fluid storage vessel, according to one aspect of the present invention,

[0032] FIG. 2 is a plan view of the device shown in FIG. 1 with portions cut away to show the circuit board and sensor.

[0033] FIG. 3 is a side view of the device shown in FIG. 2 with portions cut away to show the circuit board and sensor;

[0034] FIG. 4 is an end view of the device shown in FIG. 2 with portions cut away to show the circuit board and sensor; and

[0035] FIG. 5 is a plan view of a device for detecting the level of fluid in a fluid storage vessel, according to another aspect of the present invention with portions cut away to show the circuit board and first sensor.

[0036] The same numerals are used throughout the various figures of the drawings to designate similar parts. Still other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments.

[0037] Generally speaking, in a closed pressurized fluid-containing vessel, a liquid and its vapor are at equilibrium. The equilibrium or vapor pressure P_vaporof the liquid-vapor system in the vessel is determined by the temperature T of the liquid, rather than that of the vapor. Put another way, a closed system's vapor pressure corresponds to the equilibrium pressure of the liquid contained therein and its vapor at a given temperature, and that temperature, under such circumstances, is the temperature of the warmest liquid.

[0038] The boiling or vaporization temperature of a liquid at a given pressure is known as the saturation temperature. This temperature is the boiling point of the liquid, and the condensing point of the vapor. Corresponding to each substance are mathematical relationships between pressure, specific volume or density and temperature known as the equations of state.

[0039] Possible equilibrium states of a substance are described on what is known as a p-v-T surface. A p-v-T surface is a three-dimensional contour graph of volume vs. temperature vs. pressure. The relationship between a liquid and its vapor at a given temperature can be determined readily by plotting its location on the p-v-T surface.

[0040] The p-v-T surface for propane, for example, indicates the temperature of the warmest liquid in a propane tank or vessel, given pressure measurements from the vessel. This surface will also predict the pressure in the vessel where the temperature of the warmest liquid is known.

[0041] In accordance with various aspects of the present invention, when heat is applied to a point or selected volume of liquid in the vessel, the liquid vaporizes, at which time a considerable amount of heat is removed or absorbed from the heat source. As the vapor condenses, the absorbed heat is liberated throughout the system. By this mechanism, large amounts of heat may be removed at a relatively constant rate. It has also been found that the temperature does not rise appreciably as long as the vessel is sufficiently large that the total surface of the vessel can easily dissipate the heat added at the point source. Provided there is liquid adjacent the heat source, the temperature at the source remains relatively constant. Under these conditions, neither the overall temperature of the system nor its pressure rise significantly.

[0042] It has now been discovered that the foregoing principles find particular application to non-invasive detection of the fluid level in a fluid storage vessel. More particularly, the present invention relates to a novel system-and method for detecting the rate of heat dissipation at the vessel surface.

[0043] In general, upon applying heat to a selected location on the vessel exterior and for a relatively short period of time, there are two responses. First, if there is no liquid present on the side of the vessel wall opposite the hot spot, i.e., if the tank is empty at the thirty percent (30%) line or liquid-vapor interface level, the temperature of the vessel wall comprising, e.g., metal or the like, rises generally greater than about ten (10) degrees, up to about forty (40) degrees or more, depending upon the heat density applied and energy being added in watts. This temperature gradient has been found generally necessary to dissipate or conduct away heat, applied at the heat source through the thin dry metal wall of the vessel.

[0044] Second, where there is liquid on the other side of the vessel wall at the same level as the heater, the liquid acts as a refrigerant, lessening the temperature rise to generally within a range of two (2) and five (5) degrees. Since the propane inside is at equilibrium with its vapor, a several degree increase in temperature causes it to boil like a refrigerant, efficiently carrying the heat away. Therefore, the vessel preferably contains only liquid propane and propane vapor because if heat is applied to a location of the vessel wall, where liquid is on the other side, a large proportion of the heat will vaporize the liquid rather than increase the temperature of the vessel wall. Thus, the temperature increase of the vessel wall adjacent to the heater will be limited thereby facilitating indication of the presence of liquid opposite to the location at which heat is applied, i.e., the liquid level being above the location of heat application. Similarly, if the vessel contains substances other than propane, it is preferred that the vessel contain only liquid and vapor having the same chemical composition to facilitate indication of the presence of liquid opposite the heat applied to the vessel since such presence will result in a limited temperature increase of the vessel wall adjacent to the heater.

[0045] Referring now to the drawings and more particularly to FIGS. 1 to 5, there is shown generally a specific, illustrative system 10 for detecting the level of a fluid, e.g., 20a, such as propane, in a fluid containing vessel 30 in accordance with various aspects of the present invention, such as a horizontally disposed propane tank having about a 200 to 1000 gallon capacity. According to one aspect of the present invention, the system 10 comprises an energy modulator 11 for applying energy at a selected location 12 on the vessel 30.

[0046] Using an energy modulator 11 such as an electric heater, a relatively small amount of energy, e.g., 100 watts or less of heat, is applied at relatively high concentration to a selected location 12 on the vessel exterior, e.g., over an area about the size of a nickel, hereinafter referred to as the “hot spot.” Heating is preferably done for a selected period of time, e.g., about 5 minutes. It is also preferred that the location 12 be at a predetermined level line indicative of a minimum desirable fluid level in the vessel 30, e.g., where the vessel is about thirty percent (30%) filled with liquid, as indicated by 20ain FIGS. 2 and 3. The heater 11 is preferably fastened to the vessel 30 using a suitable bonding material 13, such as a conductive epoxy, or other means of connecting the heater to the vessel in a thermo-conductively efficient manner.

[0047] Suitable point source heaters 11 include a 5 watt point resistance heater, e.g., a halogen electric light bulb, mounted to the vessel 30 with a conductive epoxy 13 or a heated aluminum wafer conductively bonded to the vessel. Desirably, the heater 11 is energized periodically, for instance, once or twice a day, or continuously, to test the fluid level, e.g., 20a, within the spirit and scope of the present invention. Alternatively, the heater 11 may be energized continuously.

[0048] Next, a temperature sensing device or sensor 14, such as a thermistor or thermocouple, in proximity to the location 12 or hot spot detects the energy level at the location during first and second time intervals. In some configurations, the optimal separation distance between the sensor 14 and heater 11 has been found to be between one (1) and three (3) inches. However, separation distances of other amounts may also prove optimal. Optimal separation distances result in a large temperature change when the portion of the vessel 30 adjacent to the sensor 14 is dry (i.e., the fluid level indicated by 20bin FIGS. 2 and 3 is below the sensor 14) and a small temperature change when the portion of the vessel 30 adjacent to the sensor 14 is wet (i.e., the fluid level 20cis above the sensor 14). A memory device 15 stores data representative of the energy detected by the sensor 14 at each time interval. A logic circuit 16 then compares the amount of energy of the first time interval to that of the second time interval. The logic circuit 16 is mounted on a substrate, such as an electronic circuit board 22.

[0049] Alternatively or concurrently therewith, a device 17, e.g., an LED or LCD display, connected to the logic circuit 16 displays selected data indicating the fluid level, e.g., 20a, in the vessel 30. A data transmission device 18, e.g., a data transmitter, may also be provided for selectively signaling the data to a remote data receiving device. The information received may then be shown on a data display such as a computer monitor or the like.

[0050] The system 10 is mounted preferably to the outside surface of the vessel 30 at an appropriate location corresponding to a selected liquid level, e.g., 20a, to be signaled as the vessel empties. Desirably, the sensor 14 is located between about two-thirds (⅔) and three-fourths (¾) of the way down from the top to the bottom of the vessel 30, for effective monitoring and warning of impending run-out of the liquid, e.g., 20a. According to one aspect of the present invention, the system 10 is permanently affixed to the vessel 30 using suitable fasteners. Alternatively, the system 10 may be readily detachable for replacement, storage or transfer to another vessel. According to a further embodiment, the system 10 is portable for ready transportation by the user from one vessel 30 to another as part of a regular monitoring program.

[0051] Suitably, the system 10 is housed entirely within a case 31, e.g., constructed of a polymeric material, to thermodynamically isolate the same from the environment. Desirably, insulation materials 32 are used between the case 31 and the system 10 for thermal isolation and insulation. The heater 11 and sensor 14 are preferably housed separately in recessed portions 33 for thermal isolation from one another. For added isolation/insulation, materials 32 may also be wrapped about the case 31, as will be understood by those skilled in the art.

[0052] In one embodiment, the insulation 32 is foam or wool and bonded to the inner surface of the circuit board 22, as shown in FIGS. 3 and 4. The inner surface of the insulation 32 has recessed portions 33 each having a base on which is seated a resilient pad 23, such as a helical spring. The heater 11 is mounted in one of the recessed portions 33 on the associated resilient pad 23. The sensor 14 is mounted in the other of the recessed portions 33 on the corresponding resilient pad 23. Epoxy, polyester, or potting compound is applied to the heater 11 and sensor 14 to solidly encapsulate them in their respective recessed portions 33. The epoxy, polyester, or potting compound, need not necessarily have good insulation properties. During the encapsulation, the respective resilient pads 23 resist insertion of the heater 11 and sensor 14 such that respective, predetermined longitudinal portions of the heater and sensor extend beyond the inner-surface of the insulation 32, as shown in FIGS. 3 and 4.

[0053] The memory device 15 is bonded to the outer surface of the circuit board 22. Also mounted on the circuit board 22 is the logic circuit 16. The logic circuit 16, memory device 15, heater 11 and sensor 14 are electrically connected by wires which extend through the insulation 32 and may also extend through the circuit board 22. The circuit-board 22 is thereby a substrate which supports the heater 11, sensor 14, insulation 32, logic circuit 16 and memory device 15. This detector assembly 24 is then mounted in the case 31 such that the exposed inner surfaces of the heater 11 and sensor 14 are flush with the inner edges of the case. The side inner edges 25 of the case 31 are arcuate.

[0054] The case 31, with the detector assembly 24 mounted in it, may be attached to the outer surface of the vessel 30 by applying, such as by smearing, a conductive epoxy (such as a paste containing silver available from Metech, Inc. of Elverson, Pa.) to the inner surfaces of the heater 11 and sensor 14. Epoxy, or alternatively self-adhesive release paper, is applied to the inner edges of the case 31, which are subsequently placed against the outer surface of the vessel 30. The arcuate edges 25 of the case 31 facilitate mating of the case to the curved outer surface of the vessel 30, as shown in FIG. 3. The flush relation of the inner surfaces of the heater 11 and sensor 14 relative to the inner edges of the case 31 result in heater and sensor mating with the outer surface of the vessel 30, as shown in FIGS. 3 and 4. The epoxy bonds the heater 11 and sensor 14 to the outer surface of the vessel 30, and the conductivity of the epoxy enables heat transfer between the vessel 30, and heater 11 and sensor 14. The epoxy or release paper bonds the case 31 to the outer surface of the vessel 30 and provides a water-tight connection to obstruct rain of the like from reaching the interior of the case, and the heater 11 and sensor 14 housed in it. The insulation 32 provides thermal isolation of the heater 11 and sensor 14 from the atmosphere surrounding the vessel 30;

[0055] A power supply 21, internal or external, such as a battery, electrical outlet, and/or portable generator, may be used to operate the system 10 and its components including the heater 11, sensor(s) 14, memory device 15 and/or logic circuit 16. Preferably, a low voltage heater 11 is used. The power supply 21 may be separated from the vessel 30 by a distance of approximately six (6) feet.

[0056] Where a single heater 11 and sensor 14 are used as shown in FIGS. 1 to 4, the system 10 analyzes the rate of temperature change at the surface of the vessel 30 when the heater 11 is activated. When the rate of change is relatively low, the system 10 signals that the liquid level is still above the thirty percent (30%) full mark, indicated by liquid level 20ashown in FIGS. 2 and 3. At a relatively fast rate of change, a different signal is used to indicate that the vessel 30 is becoming empty.

[0057] Another aspect of the present invention shown in FIGS. 1 to 4 is a method of detecting the fluid level, e.g., 20a, in a fluid containing vessel 30 using one sensor 14. First, energy, e.g., heat, is applied to a selected location 12 on the vessel 30. The energy level at the location 12 is detected at first and second time intervals using a sensor 14 in proximity to the location. The energy level at the location 12 is detected during the first time interval, then during the second time interval. Data representative of the energy detected by the sensor 14 at each time interval is stored in a memory device 15. The amount of energy of the first time interval is compared to that of the second time interval using the logic circuit 16. The information is then output either locally to a display 17 or the like or to a transmitter. 18 for remote display.

[0058] In accordance with another aspect of the present invention, the system 10 includes a device 11 for applying energy to a selected location 12 on vessel 30, as shown in FIG. 5. Parts in FIG. 5 having counterparts in FIGS. 1 to 4 have the same reference numeral, respectively, as in FIGS. 1 to 4. A first sensor 14 in proximity to the energy location 12 detects the energy level at the location at first and second time intervals. A second sensor 19, e.g., a thermistor, positioned a selected distance from the surface of the liquid 20, e.g., about four (4) inches or more below the first sensor 14, detects the energy level at that position and at the same time intervals as the first sensor. The memory device 15 stores data representative of the energy detected by both of the sensors 14, 19 at each time interval. The logic circuit 16 then compares the amount of energy of the first time interval to that of the second time interval in each of the respective sensors 14, 19.

[0059] When the vessel 30 is filled or the liquid level 20cis above the sensor 14, upon activating the heater 11 the temperature at the first sensor 14 is slightly warmer than that of the second sensor 19 below the first sensor. When the liquid level 20bfalls to a selected level, e.g., about two (2) to three (3) inches below the heater 11, the second sensor 19 being:below the liquid level 20b, the temperature differential between the temperature at the first sensor 14 and at the second sensor is relatively high. The first sensor 14 then experiences a rapid rise in temperature until thermal equilibrium is reached in the vessel 30.

[0060] A third condition occurs where the liquid level 20dis below both of the sensors 14, 19. Under these circumstances, the first sensor 14 (near the heat source 11) experiences a relatively rapid rise in temperature, whereas the second sensor 19 indicates a temperature increase that is relatively slower and smaller. In other words, there is a large temperature differential between the two sensors 14, 19 under these conditions but not as large as that experienced when the second sensor 19 only is immersed in the liquid resulting, e.g., from the liquid level 20b.

[0061] Although the present invention has been described in connection with a computer monitor for data display, other devices for signalling or display may be utilized, giving consideration to the purpose for which the present invention is intended. For example, a light, telephone or buzzer could be utilized within the spirit and scope of the present invention. Alternatively, a memory chip, wheatstone bridge or relay could be utilized. In addition, according to the present invention, a different signal could be-produced for each of the following conditions: (i) where the vessel 30 is full (the first and second sensors 14, 19 being below the liquid level 20c), (ii) the vessel 30 is about thirty percent (30%) full the (first sensor being above the liquid level 20band the second sensor being below the liquid level 20b), and (iii) where the vessel is effectively “empty” (i.e., the liquid level 20dis below both of the sensors 14, 19).

[0062] Another aspect of the present invention is a method of detecting the fluid level, e.g., 20a, in a fluid containing vessel 30 using at least two sensors 14, 19. Initially, energy is applied to the selected location 12 on the vessel 30. Next, the energy level is detected at the location 12 at first and second time intervals using the first sensor 14 in proximity to the energy location. Using the second sensor 19, positioned a selected distance from the first sensor 14, the energy level is then detected at that position at the same time intervals as the first sensor 14. Data representative of the energy detected by both of the sensors 14, 19 at each time interval is stored in the memory device 15. Finally, the amount of energy of the first time interval is compared to that of the second time interval in each of the respective sensors 14, 19, using the logic circuit 16, and the output displayed either locally via the display 17 or to a transmitter 18 for remote display.

[0063] Logic circuits can measure a number of different parameters, in accordance with various aspects of the present invention. For example, a logic circuit 16 compares a location thermally remote from the hot spot 12 with a spot closely associated with the hot spot. In this connection, there are two data temperatures to compare. Alternatively or concurrently, the logic circuit 16 compares the starting temperature in proximity to the hot spot 12 of the tank 30 at the beginning of a test cycle with the temperature at the end of the cycle, e.g., about ten (10) minutes later. A large temperature change indicates no liquid, e.g., the fluid level 20bin FIGS. 2 and 3, whereas a small gradient means there is the liquid 20 at or above the hot spot 12, e.g., the fluid level 20cin FIGS. 2 and 3.

[0064] Another embodiment of the present invention is that the logic circuit 16 may measure the rate of change or the length of time until the rate of change settles down to a very low value. For instance, where there is no liquid, e.g., the fluid level 20bin FIGS. 2 and 3, the change in temperature of the heated spot 12 over time is incrementally relatively large from about two (2) to about eight (8) minutes. Where there is the liquid at or above the hot spot 12, e.g., the fluid level 20cin FIGS. 2 and 3, the incremental change over time is relatively low, eventually leveling off near zero within about five (5) to about six (6) minutes.

[0065] In a further embodiment of the invention, the logic circuit 16 measures the length of time until the ΔT becomes minimal. In this case, a relatively long time without leveling of the temperature indicates no liquid inside the vessel 30 at the level tested which is the level of the sensor 14, e.g., the fluid surface 20bin FIGS. 2 and 3. A relatively short time indicates the presence of liquid 20, e.g., the fluid surface 20cin FIGS. 2 and 3.

[0066] In another embodiment of the invention, the heater 11 may be activated continuously. When the liquid level falls below the level of the heater 11, equivalent to the fluid surface 20a, the temperature of the nearby sensor 14 increases. This method of detection is simpler because timing of the tests (e.g., activation of the heater 11 and measuring temperature changes, via the sensor, over time) is not required. Also, this method provides a more rapid indication as soon as the liquid falls below the limit, equivalent to the fluid surface 20a. Preferably, two sensors 14, 19 are employed, as shown in FIG. 5, one sensor 19 that is remote from the heater 11 and another sensor 14 that is fairly close, e.g., one-half (½) inches to two (2) inches. The remote sensor 19 and the nearby sensor 14 will stay fairly congruent within a few degrees until the liquid level falls below the fluid surface 20aat which point the temperature will rapidly increase on the nearby sensor 14. For this method, the size of the heater 11 may be smaller.

[0067] While the present invention has been shown and described for use with propane tanks and materials, its application to other storage vessels and the like is understood, giving consideration to the purpose for which the present invention is intended. For instance, it is understood that explosion-proof materials may be utilized, as required, without departing from the spirit and scope of the present invention.

[0068] Cryogenic systems are also considered within the purview of the invention. For example, the system 10 could use an energy modulator comprising a locally applied cryogenic source to operate the system analogously to operation via a locally applied heat source, such as the heater 11, such that the observed temperature change or the rate of temperature change would be in the opposite direction as compared to that resulting from the heater 11.

[0069] Still a further aspect of the present invention is a system and method of fluid mass detection, alternatively or concurrently with the foregoing, directed to periodic weighing of the vessel 30. Each of four pipe legs (not shown) of the vessel 30 include an SR4 strain gauge or the like for effecting measurement of stress on the legs. A logic circuit 16 computes the corresponding weight of the vessel 30, including its contents. Corresponding signals are sent to the user when the vessel 30 is almost empty.

[0070] Overall, the present invention advantageously provides a system 10 which is safe, reliable, maintenance free, and further provides a method for detection of the liquid level, e.g., 20. The liquid level, e.g., 20a, for a liquid such as propane, or other fluid, especially a thermodynamic fluid at equilibrium with its vapor, may now be readily detected without interference with the service or function of the vessel 30. The system 10 uses relatively little energy but emits a definitive signal detectable using a relatively inexpensive, logic circuit 16 coupled to the sensors 14, 19 each which may comprise a thermistor. Moreover, since operation of the present invention is based upon absolute temperature, it is operational and effective in any weather environment, including extremes of heat, cold, dryness and moisture.

[0071] Since from the foregoing the construction and advantages of the invention may be readily understood, further explanation is believed unnecessary. However, since numerous modifications will readily occur to those skilled in the art after consideration of the foregoing specification and accompanying drawings, it is not intended that the invention be limited to the exact construction shown and described, but all suitable modifications and equivalents may be resorted to which fall within the scope of the appended claims.