GB2247531A

GB2247531A - Liquid level measuring devices with density compensation

Info

Publication number: GB2247531A
Application number: GB9115449A
Authority: GB
Inventors: Robert Muir Thomson
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-07-18
Filing date: 1991-07-18
Publication date: 1992-03-04
Anticipated expiration: 2011-07-18
Also published as: GB2247531B; GB9115449D0; GB9015787D0

Abstract

A liquid level measuring device comprises a hydrostatic pressure transducer 13 located below the normal lowest liquid level in a tank and a liquid density measuring device comprising a body 14 such as a float and a strain gauge 15 which measures the vertical force on the body 14. An indicator device provides a liquid level output from the output of the pressure transducer modified by the strain gauge output in response to change in liquid density due to changes of temperature or of liquid. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO LIOUID LEVEL MEASURING DEVICES This invention relates to a device for measuring liquid levels. Gauges for measuring liquid levels in tanks or vessels are known in which hydrostatic pressure is measured at a predetermined submerged location in a tank to determine the level of liquid in the tank above the predetermined location. Such gauges can be calibrated to indicate the overall liquid level in the tank or the overall volume of liquid in the tank. Such hydrostatically operated gauges suffer from a serious drawback in that they can only be calibrated to work with a liquid of specified density. If the liquid density changes, the gauge will be inaccurate. Changes in density occur with temperature or if the tank is used for liquids of different densities or different blends of densities.

As a consequence such hydrostatically operated gauges can be so inaccurate as to be useless.

This invention provides a a liquid level measuring device for a tank or vessel comprising a hydrostatic pressure sensor to be mounted in a tank or vessel at a predetermined location which is submerged below the normal range of liquid level in the tank, to provide a first output which varies in accordance with variation in liquid level above the sensor, a density compensation device comprising a body mounted adjacent the hydrostatic sensor and means to detect variations in vertical force on the body due to variations in density of the liquid to provide a second output which varies with the varying force and indicator means to provide an indication of liquid level or volume in the tank computed from the first output and adjusted in accordance with the second output.

Archimedes Principle states that the apparent loss in weight, or upthrust experienced by a submerged body, is equal to the weight of fluid displaced.

Since the volume of the body will remain substantially constant, the decrease in weight or upthrust, experienced by the body, will vary in direct proportion to changes in fluid density. By measuring the apparent weight of the body, (by suitable electronic, mechanical, pneumatic or other force measurement techniques, e.g. strain gauges) a density correction factor can be obtained. This when applied to the hydrostatic level measurement, will provide a true measure of level or contents. By further sensing the fluid temperature, and applying this to the density measurement, a value for specific gravity can be obtained. All the computations can be easily handled by electronic techniques. Local and remote indication, and transmission of level, contents, density and specific gravity can all then be provided if required.The invention provides an accurate measurement of fluid level irrespective of changes in fluid density.

Furthermore by combining the hydrostatic, and density sensors in one transducer unit, installation can be made much easier, and less costly.

The following is a description of the specific embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic view of an above ground tank containing a liquid with the liquid level measuring device mounted on the wall of the tank; Figure 2 is a diagrammatic view of a buried tank with a liquid level measuring device mounted on a rod extending down into the tank; and Figure 3 is a diagrammatic view of a density compensation device for the liquid measuring devices.

Referring firstly to Figure 1 of the drawings there is shown an above ground tank 10 with a sensor unit 11 mounted by a flange 12 on a side wall of the tank. The sensor or transducer unit consists of a pressure transducer 13 for measurement of hydrostatic fluid head and a body 14 of greater density than the liquids to fill the tank for density determination.

The apparent weight of the body 14 when submerged in the liquid is sensed by a suitable transducer 15 such as a strain gauge type load cell.

The pressure transducer and load cell are coupled in an electrical circuit including a display device for giving a readout indicating the liquid level or liquid volume present in the tank. The signal from the pressure transducer is modified by the signal from the strain gauge in accordance with the variation in density of the liquid in the vessel or tank.

Figure 2 shows a similar system applied to a buried tank 20. Here the sensor unit is again flange mounted and is suspended on a post 21 extending from the top of the tank down into the body of liquid contained in the tank. In both of the above examples, strain gauge type sensors can be used for sensing both the hydrostatic pressure and the apparent weight of the body in the liquid.

Figure 3 illustrates a further form of device determining and thereby calibrating the gauge in accordance with variation in density in the liquid.

The device comprises a ceramic differential pressure element 30; a pressure measurement device 31, a float 32 and a mounting flange 33.

The technique used to measure the apparent weight of the displacer could be a strain gauge as indicated above, an LVDT displacement transducer or an optical transducer.

The invention has many applications including the following: General storage tanks. Heating, fuel oil etc.

Process vessels, especially those used for mixing and blending, where stirrers, size and space restrictions often preclude the use of ultrasonics.

Brewing and food industry, hygienic design and ability to withstand CIP (high temps and caustic) essential.

Chemical industry, I.S. often required.

Petro Chem. I.S. usually required.

Drilling mud tanks, I.S. required. Onshore/Offshore.

Portable tank hire operators. Battery operated and rugged construction required. Currently use "cat and mouse" type gauging, costs 1000+ and very unreliable.

It will be appreciated that many modifications may be made to the above described embodiments without departing from the scope of the invention. For example, the body may be in the form of a float which experiences nett upward thrust in the liquid rather than a body more dense than the liquid which experiences a nett downward thrust.

Claims

CLAIMS:

1. A liquid level measuring device for a tank or vessel comprising a hydrostatic pressure sensor to be mounted in a tank or vessel at a predetermined location which is submerged below the normal range of liquid level in the tank, to provide a first output which varies in accordance with variation in liquid level above the sensor, a density compensation device comprising a body mounted adjacent the hydrostatic sensor and means to detect variations in vertical force on the body due to variations in density of the liquid to provide a second output which varies with the varying force and indicator means to provide an indication of liquid level or volume in the tank computed from the first output and adjusted in accordance with the second output.

2. A measuring device as claimed in Claim 1, wherein the body is a displacer or a float to experience a varying vertical force with liquids of differing densities.

3. A measuring device as claimed in Claim 2, wherein the body is arranged to act on a load cell to determine variation in vertical force (i.e. apparent weight) on the body in the liquid to provide said second output.

4. A measuring device as claimed in any of Claims 1 to 3, wherein the pressure sensor and the density compensation device have a common mounting to be located in said tank or vessel.

5. A liquid level measuring device substantially as described with reference to and as illustrated in Figures 1, 2 or 3 of the accompanying drawings.