Measuring and testing – Liquid level or depth gauge
Patent
1990-12-04
1991-12-10
Cuchlinski, Jr., William A.
Measuring and testing
Liquid level or depth gauge
367908, 342124, G01F 2328
Patent
active
050707302
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a device for gauging the level of a fluid in a container, above the surface of the fluid there being a gas, the device comprising a first transmitter for transmitting a microwave signal through said gas towards the surface of the fluid, a first receiver for receiving the microwave signal reflected against said surface, an electronic unit arranged to calculate from the propagation time of the emitted and reflected microwave signal a first distance from the first transmitter to the surface of the fluid and thereby its level in the container.
Such devices have increasingly come in use, particularly for petroleum products such as crude oil and products made thereof. By container it is meant in this context very large ones constituting parts of the total loading volume of a tanker as well as even larger, usually circular-cylindrical landbased tanks, with volumes of tens of thousands of cubic meters. Demands for accuracy of measurement have become increasingly greater. In some cases accuracy of measurement of 1 to 2 mm at a 20-meter distance from the transmitter to the surface of the fluid may be required. In order to achieve so high an accuracy of measurement special measures must be taken to eliminate disturbances. One problem frequently occurring in connection with level gauging of the kind mentioned in the introduction, particularly in what regards the container containing the petroleum products or other chemical products, is the condition that the gas above the surface of the fluid, i.e. the "tank atmosphere", shows a partial pressure of vaporized fluid and also usually a partial pressure of air and possibly water vapour. A vaporized petroleum product, i.e. a gaseous hydrocarbon, shows a somewhat lower velocity of translation for microwaves compared with air. This velocity of translation stands in a certain relation to the density of the gas. This means that the velocity of translation of the microwave, or the microwave velocity, depends on the condition in the gas above the fluid in the container. If the container is essentially filled with a petroleum product, the corresponding partial pressure of the hydrocarbons will be essentially equal to the steam pressure when a steady condition has been obtained, i.e. the gas is saturated with gaseous hydrocarbon. The steam pressure varies substantially for different fluids such as hydrocarbon products, from practically zero to atmospherical pressure and thereabove. If, on the other hand, the container is emptied, air flows in from the atmosphere and mixes with the gaseous hydrocarbon so that a mixture of gaseous hydrocarbon and air takes place. In connection with the emptying, the partial pressure of the air will be relatively high, but if the container is left unmoved, more hydrocarbon will evaporate from the surface and the partial pressure of the hydrocarbon will increase. It is evident that the microwave velocity will vary depending on the partial pressure of the hydrocarbon, which in turn means that the gauging result will show corresponding deviations from the correct values of level gauging.
There has long been a need for a simple possibility to correct the gauge values achieved with a device of the kind mentioned in the introduction taking into account the prevailing condition in said gas, but no such device has been provided.
The object of the present invention is to provide a simple and reliable device of the kind mentioned in the introduction that admits correction of the level gauging result achieved therewith. In its preferred embodiment the invention also admits that the correction becomes relatively independent of horizontal density gradient.
According to the invention such a device is characterized by a first means for gauging the sound velocity in said gas and a second means for correcting said first distance to a second distance with consideration to the microwave velocity in the gas through a known relation between the sound velocity and the microwave velocity.
The relation between the sound velocity and the micr
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Bennett G. Bradley
Cuchlinski Jr. William A.
Saab Marine Electronics Aktiebolag
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