The delivery intervals between fuel tanker visits and the quantity delivered at each visit is one way.
Given the shape of the tank, monitoring the fall in level over time is another way. One could use an instrument as simple as a dipstick or as complex as a BMS-based multi-tank inventory monitoring system.
A third way is to look it up in the manufacturer's instructions for the equipment, either in the paperwork or the website.
Another way is to mount the tank on scales, and measure the falling weight over time. Those scales could be something as simple as a spring-based instrument or as complex as a BMS-based multi-tank load-cell application.
A fifth way would be to telepone the manufacturer for information.
Any of those suitable? And if not, why not?
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Umm Alex, what is call the generator manufacturer and speak to tech support or an engineer for this information if it is not contained in or you do not have the generator documents?
(Sorry - Jeopardy was on this morning, just couldn't help myself)
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