From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Description of a petrol gauge's capillary tube and hand pump unit operation.
| Identifier | ExFiles\Box 44\4\ Scan182 | |
| Date | 17th May 1929 guessed | |
| contd :- -2- a fine capilliary tube is inserted between the dashboard unit and the tank unit so that any rapid pressure change in the tank does not cause a rapid change in the unit on the instrument board. The result being that mere surging of the petrol in the tank does not alter the reading of the gauge. It should be noted that when the rear tank is filled up rapidly by means of a petrol pump, the dashboard instrument will not instantaneously record the exact quantity of petrol that has been put in, but will probably shew ¼ gall. or so short for a few seconds after the replenishment operation has been completed. (3) Hand pump unit. In hot countries or when driving fast on Continental roads where the tank gets heated up by the silencer and the petrol is thrown about a great deal in the tank, the air bell gets filled with a mixture of petrol vapour and air. Therefore, though when driving along the road the gauge on the instrument board shews the correct quantity of petrol in the tank, when the car is left standing for any length of time a certain quantity of petrol vapour condenses in the air bell and consequently the reading of the gauge may drop one or two galls. For this reason the hand pump is provided, and it should be used after the car has been left standing in the garage. Its function is to displace petrol which will have risen up in the air bell due to condensation of petrol vapour. One or two strokes of this pump will ensure that the gauge is reading the exact quantity of petrol in the tank no matter how long the car has been contd :- | ||