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Red Bull has suffered another weekend of grid penalties as result of refuelling errors in China, with Mark Webber dropped to the back of the grid after stopping out on the circuit during qualifying.

This drama comes just five races after the same issue befell Sebastian Vettel in Abu Dhabi last year. In both cases the issue was with the refuelling bowsers used in the pit garage failing to deliver the correct dose of fuel.

In order to qualify with the lightest possible car, teams fuel to the very minimum, leaving no margin for error. The process and equipment used to refuel the car is absolutely critical to this.

Fundamental to the whole process is the fuel bowser. This is a trolley that is effectively a mobile filling station. The trolley houses the fuel for each car within an internal tank and can fill or drain the cars' fuel tank.

Although teams will use a lot of resources belonging to their fuel and oil supplier, the bowser is not one of them. The bowsers are made by the teams themselves, albeit with electronics and fittings bought in from external sources.

Usually situated to the side of the pit garage, the bowser is lifted off its castors and mounted on static feet, to ensure its accuracy.

The bowser has two pipes emerging from its side; one is used for filling the car, the other for draining. These connect to the car's fuel filler plate. The plate will also have other connectors for venting the tank.

Craig Scarborough's AUTOSPORT technical blog: Chinese GP updates

In Formula 1, fuel is measured by weight and not volume, thus the teams calculate in kilogrammes, rather than litres, when fuelling the car. Volume isn't as the measure used because the fuel volume will change with temperature, so rather than the confusing combined effect of volume and temperature, the fuel's mass is used.

To reflect this, the bowser works on weight, so inside there is a set of scales that weigh the fuel within its tank. Pumps will drain the excess fuel from the main tank inside the car, in order to 'zero' the fuel mass remaining in the car.

Despite this, there will always be fuel left inside the fuel tank, as it's hard to drain every last drop from the main tank. The fuel collector and pipework will also contain one to two litres of fuel.

As fuel in these latter areas should be constant, the teams need only worry about the draining the main tank. The drained fuel is weighed and recorded, in order to be cross-checked with the mass the team calculated to be remaining in the tank.

Once zeroed, the bowser will be programmed to deliver a set mass of fuel. Again the pumps inside the bowser will transfer the fuel until the correct weight is achieved, the internal scales being used to measure the fuel accurately.

Cross checking the fuel delivered into the car is hard it is once inside the fuel tank. The tank's complex shape and the lack of time before the car exits the pits mean that neither fuel level sensors nor load cells in the car's suspension can accurately confirm the weight of fuel.

Thus, as Red Bull has demonstrated, any errors in the bowser's scales and pumps will go unnoticed until the main fuel tank fails to deliver fresh fuel to the collector tank.

This is what happened to Webber in China during qualifying. The bowser short-delivered fuel to the tune of 3kg. Telemetry from the car flagged up low fuel pressure going into the collector and the team asked him to stop to the car prevent damage to the fuel lump or engine.

Quite how a simple pump and scales can fail to give an accurate delivery twice in such a short period is puzzling. No doubt Red Bull will again need to look into its processes and the hardware inside its bowsers, to prevent another embarrassing short-fill at future races.

AUTOSPORT Plus subscribers can read Craig Scarborough's technical insight features throughout the season