Measuring and testing – Liquid level or depth gauge – Thermal type
Reexamination Certificate
2002-11-06
2004-07-13
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Thermal type
C073S29000R
Reexamination Certificate
active
06761066
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to level indicators, and particularly relates to external level indicators for determining the level of the interface between a mass of flowable material and the void volume above the mass of flowable material within a container.
BACKGROUND OF THE INVENTION
Liquid level measuring devices have been known for many years. Their purpose is to locate the level of a flowable material, or to indicate the amount of flowable material remaining in a container.
On many occasions, monitoring the amount of flowable material in a container is required. However, direct observation of the flowable material level is not always possible or practical. Measurement of the material in such containers as pressurized cylinders, sealed containers, cryogenic flasks, and opaque vessels is often difficult. Such measurements are even more troublesome when the material within the container is corrosive or potentially toxic or flammable.
Sight glasses and weight scales are some examples of liquid level measuring devices which are commonly employed. Both of these devices suffer from a number of disadvantages. Sight glasses are expensive, and they can crack and break easily. On such occasions where the container is placed outdoors, ultraviolet light can cause the glass to haze. Weight scales are also expensive, and in many instances, measurements provided by weight scales are inexact.
A simple, economical external liquid level gauge which permits a direct reading of the level of a flowable material has been provided by the present inventor in Canadian Pat. No. 1,177,281 issued on Nov. 6, 1984. The liquid level measuring device taught therein employs one thermochromatic material which is coated onto a base layer. The base layer is magnetically mounted to the outside surface of the outside wall of the container, and thus the external liquid level gauge can be repeatedly removed and replaced or relocated when necessary.
The theory is that the rate of heat transfer is different between a mass of flowable material and the void volume above it such that for any container with a modest heat conducting capability, the container wall experiences a temperature gradient which is most pronounced at the interface of the contents with the void volume above the contents, and of course below that interface. That is to say, the rate of heat transfer through the wall of a container will be greater where there is a mass of flowable material located in the container than where there is a void volume above the flowable material. In other words, the rate of heat transfer through the container wall changes most abruptly at the level of the interface, and below. Thus, with the use of a thermochromatic material, a vivid color change occurring at the interface, and below, will permit an observer to obtain a direct reading of the level of the flowable material within a container by discerning where the interface is located.
In a co-pending U.S. patent application, Ser. No. 10/077,971 filed Feb. 20, 2002, for “External Liquid Level Gauge,” the present inventor teaches an external liquid level gauge which is adapted to be affixed vertically to the outside wall of a container. This co-pending U.S. Patent Application claims priority from Canadian Patent Application No. 2,338,407 filed Feb. 26, 2001. The external liquid level gauge as taught therein is in the form of an elongated strip and it comprises a layer of base material and a layer of thermochromatic materials. Furthermore, the thermochromatic layer comprises a light absorbing background and at least two regions of thermochromatic materials which are arranged upon the light absorbing background. The regions of at least two thermochromatic materials are disposed in arrays thereof and are arranged entirely along the length of the external liquid level gauge. Moreover, each of the thermochromatic materials responds chromatically within a different operating temperature range.
Several other prior art thermochromatic external liquid level gauges are now described. They include GILMOUR U.S. Pat. No. 3,696,675 issued Oct. 10, 1972, which teaches an external liquid level gauge adapted to be permanently affixed to the outside wall of a container for determining the liquid-gas interface within the container. The external liquid level gauge described therein consists of a uniform thermochromatic liquid crystalline material which coats the entire base layer of the gauge such that it is at right angles to the liquid-gas interface. The uniform thermochromatic material covers the entire temperature range to which the container is subjected within an overall range of −20° C. to 250° C. Depending upon the thermochromatic material selected, color changes over a gradient from violet to red can occur in a range as small as 2° C. to one as broad as 150° C. Since the temperature differential across the liquid-gas interface is generally small, on the order of less than 2° C., the change in color is slight across the interface. This is particularly the case when the container is placed outdoors and a large temperature range needs to be covered. As a result, it is difficult to visually locate the liquid-gas interface.
In U.S. Pat. No. 5,323,652 issued Jun. 28, 1994 to PARKER, the inventor teaches a thermochromatic level indicator for determining the level of a material inside a container. The thermochromatic level indicator includes at least two thermochromic materials of different opacities and transition temperature. Prior to the attachment of the thermochromatic level indicator to the outside surface of the outside wall of the container, the thermochromic materials are applied to a transparent film by silk screening, other printing and coating methods, or methods which employ the use of microencapsulated thermochromic materials. The thermochromatic level indicator may be permanently adhered to the container wall or it may be adhered to a magnetic strip which can be temporarily affixed to the container wall.
In another U.S. Pat. No. 5,707,590, issued Jan. 13, 1998, the inventor THOMAS et al. has provided a detergent container with a thermochromatic level indicator. In one embodiment of the invention, the thermochromatic substance is added to the container's plastic material during the molding process. In another embodiment of the invention, the level indicator or strip comprises a base material, such as Mylar, which is coated or imbedded with a thermochromatic substance by such methods as painting, stripping, or screen printing.
The foregoing prior art level gauges are useful for indicating the amount of materials remaining inside a container, but they are not specifically useful as overfill indicators. Indeed, in many instances, only an approximation of the level of materials inside the container is provided. Since the thermochromatic materials present in the prior art level gauges are not in direct contact with the outside surface of the outside wall of the container, the chromatic response of these thermochromatic materials may be delayed. Furthermore, these prior art level gauges may not provide a prominent color change at the level of the interface, and thus a reading of the level of materials may be inexact.
However, under certain circumstances, it is critical to determine the precise level of the materials inside a container, such as in the case of liquefied propane in pressurized cylinder. In warm weather conditions particularly, hydrostatic pressure exerted by liquefied propane inside the pressurized cylinder may cause the cylinder to explode if the cylinder is overfilled. Thus, in order to prevent undesirable gas venting from overfilled cylinders, the United States National Fire Protection Association (NFPA) has recently mandated a safe-fill level of propane in pressurized cylinders to be at a level which is 80% of the volume of the propane cylinders such that a 20% volume head space is maintained when the pressurized cylinders are full of propane. Due to the new NFPA regulations, float valves are being applied to cylinders so as to preven
(Marks & Clerk)
Frank Rodney
Williams Hezron
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