Method of Calculating the Liquid Level

The present invention relates to a cylinder for pressurised liquefied gas and a method of calculating the liquid level of such a cylinder.

The only known method of measuring the content of a mobile gas vessel filled with pressurised liquefied gas is to weigh the cylinder and to subtract from this measurement the empty weight of the cylinder (known as the Tare weight). From this measurement the weight of the liquefied gas and hence the liquid level can be determined.

It is not possible to measure the remaining content by measuring the pressure as can be done for a permanent gas because the head pressure is more of less constant until the last droplet of liquid phase has vaporised. Once that happens, the pressure quickly drops as gas is released from the cylinder.

Weighing the cylinder is not particularly useful in many cases, particularly where the cylinder is connected up to piping. Disconnecting the cylinder from the piping is time- consuming and causes gas losses. On the other hand, weighing the cylinder whilst it is connected will skew the results as the pressure and tension in the connected pipes will affect the measured weight.

The present invention aims to provide a method and apparatus for determining the liquid level in a cylinder which does not suffer from these problems.

According to a first aspect of the present invention there is provided a method of calculating the liquid level in a cylinder filled with pressurised liquefied gas, the upper portion of the cylinder containing vaporised gas and the lower portion containing vaporisable liquid, the method comprising providing an optical fibre in the cylinder, the fibre having a free end at a known location within the cylinder; radiating a light along the fibre; detecting the light reflected back along the fibre; comparing the detected signal from the fibre with a reference value to determine whether the end of the fibre is within the vaporised gas or the vaporisable liquid; and outputting a signal representative of the depth of the liquid depending on whether the fibre ends is determined to be within the vaporisable liquid. Thus, with a number of low cost components, the level of liquid within the cylinder can be determined to some extent without needing to place the cylinder on scales.

In the broadest sense, there may be a single optical fibre. This may be positioned, for example, towards the bottom of the cylinder to provide an indication to the user that the cylinder is almost empty. However, preferably, there are at least two optical fibres with the free ends of each fibre being positioned at different depths within the cylinder such that the output signal is representative of the depth of the liquid depending on how many fibre ends are determined to be within the vaporisable liquid. This allows more than one liquid level to be detected.

The optical fibres may be attached separately to the cylinder wall. However, preferably, the optical fibres are attached to a common support insertable through the top of the cylinder. The common support may be a dip tube in which the fibres are retained or may be a rod to which they are attached. In the case of the dip tube, the dip tube must, of course, be open to the liquid level within the cylinder.

There may be only two fibres which give different depth indications. However, the more fibres that are present, the more precise a value can be given of the depth. On the other hand, each fibre adds associated expense. Preferably, therefore, there are three or more optical fibres. Preferably the free ends of the optical fibres get closer to one another towards the bottom of the cylinder. This allows a more precise determination of the level content as the cylinder approaches an empty state. There may be an individual light source and detector for each fibre. Preferably, however, a multiplexer is provided to allow the fibres to share the light source and/or detector.

The present invention also extends to a transportable gas supply cylinder for pressurised liquefied gas, wherein there is an optical fibre in the cylinder; a light source arranged to radiate light along the fibre; a detector to detect light reflected back along the fibre; a controller to compare a detected signal from the fibre with a reference value to determine whether the end of the fibre is within the vaporised gas or the vaporisable liquid; and an output device to output a signal representative of the depth of the liquid depending on whether the fibre end is determined to be within the vaporisable liquid. The cylinder preferably also has the various preferred features above. An example of a cylinder in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic cross-section through the cylinder; and

Fig. 2 is a diagrammatic view of the sensor arrangement.

The cylinder 1 has a wall 2 which is able to maintain the pressure of the pressurised liquefied gas to the required level. The pressurised liquefied gas comprises a gas component G and a liquid component L. The gas G maintains a constant pressure as it is withdrawn through an outlet valve 3 assembly because, as the gas escapes, some of the liquid L vaporises to maintain the pressure. The valve assembly 3 is attached to a neck at the top of the cylinder as is known in the art. This provides an entry point for a support 4, which may be a dip tube or rod, which supports a number of optical fibres 5-9. Each optical fibre has a different length. The first optical fibre 5 terminates at an end 10 towards the top of the cylinder 1 . The second optical fibre 6 terminates at an end 1 1 halfway down the cylinder. The third optical fibre 7 terminates at an end 12 30% of the way up the cylinder from the bottom. The fourth optical fibre 8 terminates at an end 13 20% of the way up the cylinder from the bottom. The fifth optical fibre 9 terminates at an end 14 close to the bottom of the cylinder (in this case 10% of the way up the cylinder from the bottom).

The exact number and spacing of the ends of the fibres is a matter of design choice. The five fibres described here provide sufficient information to a user for most purposes to alert them to the liquid level within the cylinder. By grouping the ends more closely together towards the bottom of the cylinder, the user is provided with a more precise indication as the cylinder approaches its empty state.

A light source 15 and receiver 16 are depicted schematically in Fig. 2. The light source is preferably an LED. There may be one LED for each fibre. However, preferably, there is a single LED 15 and a single receiver 16 with a multiplexer 17 to supply a light to and receive signals from the various optical fibres. These components are provided on a controller 18 which is integrated into the valve assembly 3 on the top of the cylinder. The controller is attached to a display 19 which can display the fill level of the cylinder. This may be part of a universal display which displays other characteristics of the cylinder. This arrangement works on the basis that the amount of light reflected back up each fibre will be different depending on whether or not the end of the fibre is immersed in the liquid or is in the gas. There are a number of ways in which the fibre can be designed to selectively totally internally reflect different amounts of light depending on whether or not the end of the fibre is within the vaporised gas or the vaporisable liquid. These can be most conveniently configured to totally internally reflect more light when the fibre end is in the gas, but it is also possible to configure the fibre to achieve greater total internal reflection when in the liquid. These various techniques are well known in the art. This reflected value is compared in the controller 18 with a reference value allowing the controller 18 to determine how many of the ends are within the liquid. In the present case, the liquid level is shown just above the end 1 1 of the second fibre 9. On the other hand, the end 10 of the first fibre 5 is outside of the liquid. Because of this, the display gives a 50% reading indicating that the sensor it at least 50% full. What is actually displayed on the display 19 is of secondary importance. For example, in the present case, it is possible for the display to give a "full" reading until such time as the liquid level passes the end 1 1 . It can then give a 50% reading between the second 1 1 and third 12 ends and so on. Once the level drops below the final end 14, it is preferable for the display to display an empty value. A more precise determination of the depth can, of course, be provided by having additional optical fibre ends at different depths.

In addition to or as an alternative to the display 19, the controller could be configured to emit an alarm or to send a message to an external device such as a control system or a user's smart phone. Alternatively, it may be used to trigger a replacement order.