These float gauges are for oil or water storage tanks. The gauges give readings of 'Empty', '1/4', '1/2', '3/4' and 'Full'.

They are also available in 2ft. 6", 3ft, 3ft. 6", 5ft and 6ft options 

It is encouraged that you have a gauge on your oil tank for the person delivering your oil to have a visible read on the amount in your tank. This gauge is suitable for any storage tanks with a 1 1/2 inch BSP female thread. If there is no suitable thread insert on your tank, you can still fit as follows;

Drill a 1.1/2" diameter hole in the top of the tank (make sure that it is near to an access point where you can get your hand inside).  Insert the male thread of the gauge and secure with the 1.1/2" back-nut which is provided.