Gas installations in which gas is supplied from two gas cylinder banks are known.  These two banks are connected to inlets of a device, comprising also a valve and an outlet.  For clarity we will distinguish the inlets and banks connected thereto as “left” and “right”.  From constructional point of view, however, the left inlet and left bank can be (and in most of known devices: is) identical to the right inlet and the right bank. 
In use, the situation is as follows: At the beginning the gas cylinders in both banks are full and one of the inlets (and so: one of the banks) is exclusively connected to the outlet.  The bank connected to the outlet is referred to as the “primary bank” (also known as the “service bank”), while the other bank is called the “reserve bank”. 
The connection between one of the gas inlets and the gas outlet can be done in the simplest case by manually setting the valve in a corresponding position (e.g. by turning a knob).  More advantageously, an automatic valve is used, which switches from one bank to another when the pressure in the currently connected gas bank drops below the threshold level characteristic for a given device. 
Such automatic devices are well known in the art.  They may easily be realized electronically (with pressure detectors) or mechanically—by a pressure gas regulator.  Devices equipped with such automatic valves are known as automatic cylinder changeover devices (ACO).  Both manually controlled and automatic changeover devices allow for replacement of the gas cylinder exhibiting the lower gas pressure or contents (possible and empty cylinder) with the fresh/full one, while another gas bank (reserve bank) is used to supply gas to the installation.  This way gas may be constantly supplied to the installation (without breaks), if only gas cylinders are replaced timely. 
In order to allow for such timely replacement, especially for gas installations in remote/hardly accessible locations, it is advantageous to monitor the signal indicating occurrence of an automatic changeover of gas banks (this signal is often called a “reserve signal”).  To this end, numerous automatic changeover devices equipped with telemetric means have been proposed. 
An automatically switching valve with remote signaling is known from the document U.S. Pat.  No.  6,763,843 B1. 
Another automatically switching valve with remote signaling is described in the document U.S. Pat.  No.  5,868,162 A. The valve known from this document measures pressure at an outlet line using an electric, pressure-sensitive switch which can be set to close at a predetermined pressure threshold within relatively narrow tolerances such that a pressure regulator can be placed between the pressurized gas supply tanks and the switching valve.  Closing of the pressure-sensitive switch in response to the drop in pressure below the predetermined threshold sends an electric signal to a relay.  In response to the electric signal, the relay causes a different inlet line to be selected, thus changing the particular pressurized gas supply from which the outlet line receives pressurized gas.  When gas is supplied through a first inlet line, an electric signal from the pressure-sensitive switch causes the relay to toggle to supply gas through a second inlet line.  A subsequent signal from the pressure sensitive switch causes the relay to toggle back to again supply gas through the first inlet line.  No manual resetting of the switching valve according to the present invention is required.  The electric signal of the pressure-sensitive switch also triggers a remote signaling device to signal a remote location.  For example, the electric signal can trigger dialing of a predetermined telephone number by an automatic dialer.  When connected to the remote location, the dialer can transmit a predetermined message which uniquely identifies a particular pressurized gas dispensing installation as the installation in which an empty pressurized-gas supply is detected. 
U.S. Pat.  No.  6,223,769 B1 discloses a gas pressure sensor and indicator apparatus for recreational vehicles and the like.  According to U.S. Pat.  No.  6,223,769 B1, when the supply gas container is at or near empty (i.e. the sensed gas pressure is low), the sensor transmits an output signal to an indicator, such as a lighting element, to notify the user that a changeover to a reserve gas container has occurred and that the initially selected supply container requires refilling. 
U.S. Pat.  No.  6,820,647 B1 discloses a valve position monitor that provides simultaneous visual and electrical signals of valve position.  A method of monitoring the position of a valve by utilizing the valve position monitor described herein is also disclosed.  According to U.S. Pat.  No.  6,820,647 B1, the device includes at least two trigger housings.  Additional trigger housings may be utilized.  Each trigger housing contains at least one trigger.  More than one trigger on a trigger housing may be included to allow for additional signals.  Preferably, the body comprises two trigger housings, each with one trigger that is detected by a limit switch.  The triggers may be any inductive, magnetic, mechanical, or electrical trigger which, when detected by a limit switch, is capable of transmitting an electrical signal of valve position.  Preferably, the trigger is a magnet. 
In automatic changeover devices, when the service bank cannot withstand the gas vapour requirement and the ACO detects pressure drop, it automatically opens reserve bank in order to combine both flow to deliver gas at required pressure on the outlet.  That situation typically happens when gas level in service bank is low.  In such circumstances, there may arise remarkable fluctuations of the gas level in the reserve bank, lasting for hours or even days.  Alternatively, such fluctuations may arise if the gas consumption rate (at the outlet of the automatic changeover device) is too high.  In both cases they lead to spontaneous switching back to the primary bank, although the gas in the primary bank has been to large extent consumed, while the gas cylinder(s) in the primary bank has/have been not replaced with fresh/full one(s).  The period of unstable indication and switching is called dithering.  The ACO indicates with colour indicator on the knob (typically red) that the reserve bank is used (i.e. connected to the outlet).  During dithering, the ACO can temporary switch back and forth between service and reserve mode indicated on the knob.  It is impossible to distinguish dithering state from cylinders replacement unless knob position is monitored. 
Dithering is illustrated in FIG.  1, which serves to illustrate the technical problem solved by the present invention.  In FIG.  1, the gas level in two gas cylinders (namely the primary cylinder and the reserve cylinder) are shown as the function of time (refer to the two lines starting in the upper left-hand side corner of the graph).  In addition, a reserve signal (bottom rectangular curve) is shown vs. time.  The reserve signal (in this case) is a binary signal indicating that the reserve cylinder is used at a given moment for supplying gas to the installation.  Several changeovers between the primary and the secondary cylinders can be observed, while the gas amount in each of them decreases systematically over time and none of the cylinders is empty before the time T4.  Moreover, none of them have been replaced with a fresh/full one within the timeframe shown in FIG.  1.  Neither there is a need to replace any of them before the time T4, as both still contain considerable amounts of gas. 
Specifically, with reference to FIG.  1, the blue line (M1) represents cumulative weight of cylinders with LP gas constituting the service bank (left bank, left OY axis), while the red line (M2) represents cumulative weight of cylinders with LP gas constituting the reserve bank (right bank, right OY axis).  The green line/area represents bank selection: if present—only the service bank is used (the left bank in this case); no green area corresponds to the reserve bank (the right bank in this case) also being used.  In FIG.  1, it can be observed that:                a) Before T1 gas is consumed from the service bank (left).         b) Between T1 and T2, the gas consumption is significantly increased and since the service bank (left) is close to empty, the reserve bank (right) is opened to vaporization cater for demand on the output of the ACO.         c) Between T2 and T3, the gas consumption is lower and vaporization from service bank (left) is sufficient, therefore the reserve bank (right) is closed.         d) Between T3 and T4, the service bank (left) is practically empty, the reserve bank (right) is open.         e) Between T4 and T5, the gas consumption is stopped and pressure from the remaining gas in the service bank (left) has closed the reserve bank (right).         f) After T5, the service bank (left) is completely empty, the gas consumption is only from the reserve bank (right).Therefore, it is clear that remote monitoring of the reserve signal, is insufficient for distinguishing between the situation of aforementioned fluctuations (without the replacement of the empty cylinder bank with a fresh/full one, i.e. when the replacement may be still needed or such need should be expected) and the situation in which after the automatic changeover of cylinders, the empty cylinder bank has been replaced with a fresh, full cylinder (and thus the replacement is no longer need or should not be expected soon).  Needless to say such distinction is highly desired for hardly accessible gas installations (e.g. installations used in remote locations).  The exact information to this end is invaluable from the perspective of gas cylinder delivery planning on one hand and continuous exploitation of the gas installation on the other hand.         
Some solutions on how to eliminate the dithering phenomenon are known from commercially available ACO devices.  For example, in one of such devices, a magnet is placed on the membrane in the valve and sticks to the valve when the automatic changeover takes place for the first time.  Once the magnet sticks to the membrane of the valve—the membrane is stabilized and dithering is eliminated. 
None of the aforementioned prior art solutions provides reliable means for detecting whether or not gas cylinder in a depleted gas bank has been replaced or does it still need to be replaced.