Hydrometer for a liquid cryogen

Abstract: A sensor for sensing buoyancy force within a liquid mixture (4) stored within a storage tank (2) and an interlock employing the sensor (10) to prevent a respirable cryogenic mixture from being dispensed from the storage tank (2) with an unsafe oxygen content. The sensor (10) has a float (12) adapted to be submerged in the liquid mixture (4), thereby to exert a buoyancy force referable to the density. The buoyancy force is sensed by a load cell(26) connected to an elongate base element (18) cantilevered from the outlet (3) of the tank (2) by a bracket (38). The mounting of such sensor (10) ensures that the buoyancy force and therefore, the density of the liquid as dispensed will be measured as opposed to liquid density at some other location of the tank (2). Such sensor (10) can serve in an interlock in which a controller (32) responsive to the load cell (26) and a temperature sensor (34), also located within the outlet (3), controls a valve (36) to close the outlet (3) when the mixture has an unsafe oxygen level.

Description: The present invention relates to a hydrometer for measuring the density of a liquid mixture, more particularly to a buoyancy force sensor mounted within an outlet of a storage tank used in storing the liquid mixture. The invention also relates to such a sensor employed in an interlock for the storage tank to prevent dispensing of a respirable liquid cryogen when the liquid contains unsafe levels of oxygen. The use of hydrometers for determining the density of a mixture is well known in the art. In known hydrometers, a float is provided that exerts a buoyancy force that is dependent upon the density of a mixture. Commonly, the height of the float is observed within a column of a liquid sample to determine the density. In mixtures that by and large contain two components, each of different density, the density measurement will define the component concentration of the mixture. Component concentration of a mixture is important to quantify in order to properly conduct a variety of commercial processes. If density is used for such quantification, the measurement becomes difficult because density can vary within any bulk of mixture to be utilised. As can be appreciated, where the mixture will come into human contact, component concentration can become especially critical. For instance, cryogenic oxygen and nitrogen mixtures, or liquid air for that matter, are used in a variety of cryogenic processes to ensure that such processes are conducted in a safe, respirable environment. At one extreme, if not enough oxygen is present, the evolved vapours will act as a suffocant. If too much oxygen is present, the resultant atmosphere will be produce highly flammable conditions in the environment. It is a relatively simple matter to ensure that the oxygen concentration is sufficient to support life at the time a container is filled with the respirable mixture. However, since nitrogen is more volatile than oxygen, heat leakage into the container will cause oxygen enrichment of the liquid due to a preferential boil off of nitrogen. Therefore, over time, it becomes difficult to ensure that the liquid being dispensed has not become so enriched in oxygen as to present a fire hazard. GB-A-2 169 414 discloses a density measuring apparatus with a measuring vessel filled with a fluid to be measured, and in which a float is connected at or adjacent a free, flexible end of a spring which is secured at or adjacent its other end, the positional changes of the float in the fluid to be measured being recorded and displayable as a measure for the density of the fluid, wherein the flexure of the spring is recorded by at least one strain gauge the electrical signal of which is converted and displayed in a measuring apparatus. Accordingly, the present invention provides a sensor for sensing buoyancy force within a liquid mixture stored within a storage tank having an outlet, the sensor comprising a float adapted to be submerged in the liquid mixture, thereby to exert the buoyancy force, an elongate base element, an elastic member connected to the float and the base element so that the elastic member strains in proportion to the buoyancy force, the elastic member having a load cell attached thereto to register the strain of the elastic member and therefore the buoyancy force, and a bracket connected to the elongate base element and configured to wedge the base element into the container outlet so as to be cantilevered therefrom, the bracket having an open cross-section to allow the liquid mixture to pass through the outlet. Such an arrangement provides a sensor to sense buoyancy force that ensures density will be measured in the liquid as dispensed. Such sensor is particularly useful for service as a sensor for an interlock to prevent the dispensing of a respirable liquid-cryogenic mixture if such liquid has an unsafe oxygen level. The sensor for sensing buoyancy force within a liquid mixture stored within a storage tank having an outlet in accordance with the invention comprises a float adapted to be submerged in said liquid mixture, thereby to exert a buoyancy force referable to the density of said liquid mixture. An elongate base element is provided and an elastic member is connected to the float and the base element so that the elastic member strains in proportion to said buoyancy force. The elastic- member has a load cell attached thereto to register the strain of the elastic- member and therefore the buoyancy force. A bracket is connected to the elongate base element and is also configured to wedge the base element into the outlet of the container so as to be cantilevered therefrom. The bracket has an open cross-section to allow the liquid mixture to pass through the outlet. Due to the mounting of the sensor, buoyancy force and therefore density measurements are obtained at the outlet of the storage tank. Therefore any density measurement of the liquid will be the density of the liquid as dispensed. In another aspect, the invention also provides an interlock to prevent the dispensing of a respirable, liquid cryogenic mixture from an outlet of a storage tank if the respirable, liquid cryogenic mixture has an unsafe oxygen content. In this regard, an unsafe oxygen content will be an oxygen concentration by volume above about 22%. Moreover, the "respirable, liquid cryogenic mixture" can be either a binary mixture of oxygen and nitrogen or liquid air containing additional components such as argon, carbon dioxide and etc. The interlock has a float adapted to be submerged in the liquid mixture, thereby to exert a buoyancy force referable to the density of the respirable, liquid cryogenic mixture. An elongate base element is provided and an elastic member is connected to the float and the base element so that the elastic member strains in proportion to the buoyancy force. The elastic member has a toad cell attached thereto to register the strain of the elastic member and therefore the buoyancy force. A means is provided for mounting the elongated base element adjacent the outlet and a temperature sensor is located within the outlet for sensing the temperature of the liquid. A remotely actuated blocking valve is provided having a closed position for closing the outlet and thereby preventing the dispensing of a liquid with an unsafe oxygen content. A controller responsive to the load cell and the temperature sensor is provided to actuate the valve into its closed position when the density of the respirable cryogenic mixture as determined from the buoyancy force and/or the temperature is indicative that the respirable cryogenic mixture has an unsafe oxygen content. As can be appreciated, the interlock as described above can be employed as a retrofit with the minimum of modification to a storage tank. The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic, cross-sectional view of an apparatus in accordance with the present invention installed within a container; and Figure 2 is a sectional view of Figure 1 taken along line 2-2 of Figure 1. Figure 1 shows an apparatus 1 in accordance with the present invention located within a container 2 having an outlet 3, the container being filled with a liquid mixture 4 such as a respirable mixture of oxygen and nitrogen or liquefied air. Apparatus 1 includes a sensor 10 provided with a float 12. When float 12 is submerged within liquid 4, it exerts a buoyancy force F that is proportional to the density of liquid mixture 4 at a specific temperature thereof. Float 12 can be a sealed metal or plastic vessel, the material being selected to be compatible with liquid 4. Float 12 is connected to an arm 14 which is in turn connected to pivot 16. Pivot 16 is connected to an elongate base element 18. An elastic member 20, connected between elongated base element 18 and arm 14, can be a metallic strip. Elastic member 20 in the specific illustrated embodiment is connected between a lug 22 of base element 18 and a lug 24 of arm 14. The foregoing arrangement adds leverage to help deflect elastic member 20. Alternatively, arm 14 is not used, so that elastic member 20 has a free, unconstrained end. In such an embodiment, float 12 is connected to the free end of elastic member 20 to make it flex. The deflection of elastic member 20 is proportional to the buoyancy force F and can be sensed by a strain gauge 26 or other load cell. Strain gauge 26 is connected by a conductor 28, passing through a feed through 30, to a controller 32. The density of liquid mixture 4 can be expressed as a family of linear curves in which each curve is referable to the density of liquid mixture 4 at a specific temperature. In the case of a respirable, liquid cryogenic mixture which, by and large contains oxygen and nitrogen, specification of the density of the mixture will also specify its make-up and therefore the oxygen concentration. In apparatus 1, a temperature sensor 34 is provided to produce a temperature indication of the liquid mixture 4. Temperature sensor 34 is connected to controller 32 by a conductor 35 that also passes through a feed through 30. Controller 32 can be any analogue or digital device responsive to strain gauge 26 and temperature sensor 34. In the case of a digital device, such as a programmable logic controller, controller 32 is programmed with data representing the above-mentioned family of curves. Since apparatus 1 is designed to function as an interlock for a container of a respirable, cryogenic mixture, controller 32 is also programmed to control a remotely activated valve 36 so that it assumes a closed position to close off outlet 3 when the oxygen content of the liquid is above 22% by volume. As can be appreciated, apparatus 1 could easily be modified to serve as a hydrometer by utilising a programmable logic computer having an output of density. In such case, the resulting apparatus might not be used as an interlock in the manner described above and thus, would not include provision for control of a remotely activated valve such as remotely activated valve 36. Although sensor 10 could be connected to the wall of storage tank 2 near outlet 3, sensor 10 is preferably cantilevered from outlet 3 by provision of a bracket element 38. Bracket element 38 has an open cross-section to allow liquid to pass freely out of outlet 3. This cross-section is preferably a triangular-cross section as illustrated, since such a triangular cross-section is both easily manufactured and also easily wedged into pipes of differing sizes by means of simple spacers (cf Figure 2). Bracket element 38 is designed with some flex so that elongate base element 18 is wedged within outlet 3. The foregoing arrangement is advantageous because density measurements are constrained to be made at the outlet of storage tank 2. Also, the foregoing arrangement is particularly advantageous from the standpoint of retrofitting storage tanks because its use does not involve modification of storage tanks, instead the apparatus is simply bunged in the outlet 3.

Claims: A sensor for sensing buoyancy force within a liquid mixture stored within a storage tank having an outlet, the sensor comprising a float adapted to be submerged in the liquid mixture, thereby to exert the buoyancy force, an elongate base element, an elastic member connected to the float and the base element so that the elastic member strains in proportion to the buoyancy force, the elastic member having a load cell attached thereto to register the strain of the elastic member and therefore the buoyancy force, and a bracket connected to the elongate base element and configured to wedge the base element into the container outlet so as to be cantilevered therefrom, the bracket having an open cross-section to allow the liquid mixture to pass through the outlet. Apparatus as claimed in Claim 1 comprising a pivot connected to the elongate base element, and an arm connecting the float to the pivot so that the buoyancy force tends to rotate the arm about the pivot, and, the elastic member being of elongate configuration and connected at opposite ends to the base element and the arm so that deflection of the arm, deflects the elastic member. Apparatus as claimed in Claim 1 or Claim 2, wherein the bracket has an open triangular cross-section. An interlock to prevent the dispensing of a respirable, liquid cryogenic mixture from the outlet of a storage tank if the cryogenic mixture has an unsafe oxygen content, the interlock comprising a sensor as claimed in any preceding Claim, a temperature sensor located within the outlet for sensing the temperature of the, liquid cryogenic mixture, a remotely actuated valve · having a closed position for closing the outlet and thereby preventing the dispensing of liquid with an unsafe oxygen content, and a controller responsive to the load cell and the temperature sensor to actuate the remotely actuated valve into the closed position when the density of the cryogenic mixture, as determined from the buoyancy force and the temperature, is indicative of the cryogenic mixture having an unsafe oxygen content.

Inventors: SAHM MICHAEL K, WARDLE DAVID G, SAHM, MICHAEL K., WARDLE, DAVID G.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1998
Main classification: G01F23
IPC codes: G01N9