The present invention relates to apparatus for indicating quantity of liquified gas contained in a storage container for supplying gas to a consuming appliance. The invention is primarily concerned with an apparatus which indicates the quantity by weight or volume of liquified butane of propane remaining in a container. Since containers for liquified butaneor propane are normally made of metal, they are non-transparent and therefore it is not possible to see how much liquified gas is present in the container.

Heretofore, the most common determination of the quantity of liquified gas in such a container has been achieved by weighing the container and substracting from that reading the weight of the empty container. This method is inconvenient since it requires the operative to know the weight of the empty container prior to it being emptied and also requires the operative to lift the container in order to weigh it. This can be very difficult partic- ularly bearing in mind that a full container can be very heavy for a person to lift.

There is therefore a need to provide an apparatus which can indicate in a convenient manner, the quantity by weight or volume of liquified gas in the container without the need to actually weigh the container.

According to one aspect of the present invention, there is provided apparatus, for indicating quantity of liquified gas contained in a storage container for supplying gas to a consuming appliance, the apparatus including a pressure sensing means for sensing gas pressure within the container and being arranged to operate an indicator means which gives a desired indic¬ ation, the indicator means being calibrated to indicate

the quantity by weight or volume of liquified gas con¬ tained in said container.

According to another aspect of the present invention there is provided a method of determining the quantity, by a weight or volume, of a liquified gas in a container comprising the steps of determining the gas pressure within the container during use, and using the determined gas pressure to indicate. the quantity of liquified gas in the container. Reference is now made to the accompanying drawings, in which:-

Figure 1 is a graph showing the relationship between gas pressure and weight of liquified gas within a containe during controlled release of gas from the container; Figure 2 is a schematic view of a system including an apparatus according to the present invention;

Figure 3 is a front:view of a gauge calibrated in accordance with the present invention;

Figure 3a is a similar view to Figure 3 showing a modification; and

Figure 4 is a schematic view of a further embodiment according to the present invention.

Referring initially to Figure 1 the vertical axis of the graph indicates gas pressure within the container in units of pounds per square inch (p.s.i.) and the horizontal axis indicates the weight in pounds of the liquified gas in the container. When no gas is being released from the container, the pressure of the gas remains at a constant pressure at a constant ambient temperature. This is referred to as a static system. Thus measurement of pressure in a static system gives no indication of the quantity of liquified gas in the container since the pressure will remain unchanged (ignoring temperature effects) whilst there is liquified gas present.

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However, it has been surprisingly discovered that when gas is released from the container in- a controlled manner (for instance as when the pressurised gas is supplied to the consuming appliance via a pressure regulator) then there is a pressure variation within the container. This is referred to as a dynamic system.

The graph of Figure 1 relates to a dynamic system and is a plot of pressure drop during usage. It has been found that the shape of the curve A is the same for any liquified gas/vapour system contained within a constant volume and in which the vapour or gas is released in a controlled manner.

Accordingly it has been discovered that by calibrat¬ ing indicator means which are responsive to a pressure sensing means it is possible to indicate the weight or volume of liquified gas contained within a container whilst gas is being released.

It has also been discovered that once the release of gas is stopped, then as expected the gas pressure rises to that of the vapour pressure of the liquid. However once the gas is released again, the pressure rapidly falls to the value indicated by the plot of Figure 1. Thus the relationship between weight of liquified gas and gas pressure in the dynamic system holds good even with intermittent release of gas.

As indicated in Figure 1, the plot A has three dis¬ tinct regions; viz. a first region B, a second region C, and a third region D. The third region D indicates a rapid drop in pressure with weight of liquified gas and when calibrating the indicator means, it is this region which is particularly useful. The accuracy in region D is high due to the large change in pressure in relation to a small change in weight and additionally since it has been found that moderate changes in ambient

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temperature have little or no effect on the pressure/ weight relationship in range D.

By way of example, Figure 2 shows a container 10 of liquified gas which supplies gas to an appliance 12. A combined pressure sensing means and indicating means 14 in the form of a gauge 15 is positioned so as to monitor pressure within the container.

As shown in Figure 3 the gauge 15 has a diagram (not shown) which operates a pointer 18 which moves over a dial 19 which is calibrated to show the three regions B, C and D as designated ^' by divisions 20, 21 and 22 respectively. Thus by viewing the gauge 15 during use of appliance 12 it is possible to determine what quantity of liquified gas is present in the container. In order to compensate for ambient temperature changes it is envisaged to make the calibrated" scale or dial 19 movable relative to the pointer 18. In this respect Figure 3a shows a modification of the gauge 15 wherein the scale is mounted upon the glass of the gsuge and the outer casing 15.a holding the glass is rotatably mounted on the remainder of the gauge. A marker 19a. on the glass is superimposed over a scale 19b on the face of the dial behind the pointer so as to give a reference of the extent of rotation of the glass (and hence the scale 19) relative to the remainder of the gauge 15.

The scale 19b is preferably graduated so as to give ambien temperatures so that marker 19a_ is aligned with the corresponding ambient temperature graduation before a reading is taken from the gauge. A further example is illustrated in Figure 4 wherein the pressure sensing means 20 is in the form of a pressure sensitive reostat 21. The ^' reostat 21 forms part of an electrical circuit 30 which also includes a voltage source 31 and an indication means in the form

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of voltmeter 32. The voltmeter 32 is calibrated to indicate weight or volume of liquid present. Thus, in use, pressure changes in the container operate the diaphragm 34 of the pressure sensing means 20 which in turn moves the wiper 'contact 35 of the reostat 21 to change its resistance value. This change in resistance causes the voltmeter to indicate a change in measured voltage.

A modification is also included in the embodiment of Figure 4 wherein a temperature compensator 50 is included which serves to alter the resistance of circuit 30 in order to compensate the readings on the voltmeter for temeperature variations. This is possible since in both the static anddynamic systems the vapour pressure varies in direct proportion to temperature change. Accordingly the compensator 50 is positioned near the container so that changes in ambient temperature have little or no effect on the readings given by voltmeter 32. In the illustrated embodiment the compensator 50 comprises a thermister -52 which has a positive co¬ efficient (i.e. a thermister whose resistance increases with increase in temperature) and this is balanced with a resistor 53 in order to give the desired characteristics. This embodiment is particularly advantageous as it provides an indication means which is remote from the container and the readings of which' are compensated to accomodate for temperature variation's around the container.

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