In functional terms, hydrocarbon dispensers essentially comprise a pump to draw hydrocarbon from a storage tank and to force it to the dispenser nozzle, a measuring unit to measure the volume of hydrocarbon actually dispensed to the user, and a calculating unit to convert this measurement and to display the indication provided by the measuring unit in the form both of a volume and of price for the hydrocarbon dispensed.
In almost all hydrocarbon dispensers, there is only one principle at present which allows fluid to be metered with an error which does not exceed 10.sup.-3 of the amount measured over a flow range of not less than 1 to 20. Thus all fuel dispensers today are fitted with positive-displacement meters with a sealed piston, operating on the principle of a steam engine. The reciprocating displacement of a piston in a cylinder under drive from the pressure of the fluid, and as effected by automatic and synchronous opening of inlet and outlet valves, is converted into continuous rotary movement, and makes it possible to ensure the desired accuracy of measurement, by virtue of:
correctly setting the opening and closing of the valves; PA1 perfect internal sealing; PA1 complete filling and emptying in each cycle: PA1 accurate incremental measurement of the angle of rotation.
The positive-displacement meter so formed is in general of large size and involves a non-negligible loss of head in order to overcome the internal friction between the moving pistons and the walls of the cylinders. As a consequence, this type of positive-displacement meter is equally a genuine hydraulic motor which can with no problem provide enough power also to drive the complex mechanisms of the volume/price calculators generally in use.
However, mechanical calculator units are now generally being replaced by electronic calculators, formed by a contact-free electronic detector of the rotation of the measuring unit. The need for hydraulic power which used to be required to drive the mechanical calculator no longer exists.
In addition it is known that, in the case of a hydrocarbon dispenser, the mechanical power which is needed to drive the pump or the electrical power is wasted.
In the light of this situation, an object of the present invention is to transform a simple positive-displacement meter system of low performance but high economy into a device giving the same accuracy as prior art meters, while being less sensitive to the nature of the fluid, which is of smaller dimensions and lower cost, and which exhibits smaller loss of head.