When you pick up the nozzle to start filling up, the pump selects your fuel from the appropriate underground tank (at most modern sites they have one underground tank per type of fuel). It also sends an electronic signal to a cashier asking them to authorise the sale. The cashier has to visually check that it is safe to dispense fuel and that the person dispensing is over 16.
Once authorised the pump is ready to go. Fuel is pumped through a meter which measures the amount being dispensed, while the flow is maintained by holding open a mechanical valve in the nozzle. Once finished, the nozzle valve shuts when the lever is released. When the nozzle is replaced in the holder on the pump, it sends a signal to the pump controller, and ends the electrical operation of the pump. These details are then sent to the cashier who accepts and closes the authorization, putting it on the till ready for payment.
The pump display shows how much fuel you are dispensing and the charge as you fill up. Even though the pump measures every drop it dispenses, the display only shows to two decimal places (it used to be three when ½ pennies were still legal tender), meaning the amount dispensed maybe slightly higher than that shown on the display. For example if you had dispensed just over one litre the pump will still show 1.00 litres until it clicks over to 1.01 litres – even though the price will have already moved. For example, at a price of £1.25 per litre, 1 pence equates to 8ml of fuel or 0.008 litres, less than the 0.01 litres that the pump can display.
Before any pump can be used on a forecourt, it has to be checked, tested and granted a Certificate of Approval issued by the Government’s National Measurement Office. After calibration, the fuel pumps are sealed to prevent any adjustment and fraud.
Pumps are remotely monitored on a daily basis to ensure that they remain accurate at all times and can be checked at any time by trading standards officers.