New customers sometimes have a problem with Ariba Oil requiring a whistle on their tank. Often they say I have lived here for X amount of years and never had a problem with a delivery. That may be true- until some oil driver comes and overfills the tank.
A driver outside your home has no idea what is going on inside until he may or may not see oil coming out a vent pipe. Think of it this way, without knowledge of what is going on inside your home the driver can only hope oil is going in the tank and not spilling in the basement. A whistle lets the driver know oil is going into a contained tank. Otherwise it could be spilling inside the basement. The gauge could have blown off the top of the tank- it has happened this February- and then oil is spilling all over the basement. With the whistle my driver detected the problem and we stopped before any oil spilled out.
The whistle tells the driver that there is room in the tank to accept more oil. Once the whistle stops the tank is full.
If a driver goes to a tank and it does not whistle either the tank is full or the tank has a issue. Either wide open or improperly installed. In any event a tank needs to be properly functioning to avoid spills and a disaster.