Titan ES1000 Oil Tank

Use a dipstick made of a portable steel ruler

-- “I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions which they have delighted in explaining to colleagues, which they have proudly taught to others, and which they have woven, thread by thread, into the fabric of their lives.”

― Leo Tolstoy

Have you asked Titan?

I asked the Gemini AI, and:

"Watchman Sonic / Gauge Settings: For calibrated level gauges (like the included Watchman Sonic), the "sonic height" or true functional internal height used for programming is typically around 1,300 mm to 1,350 mm to account for the double-skinned bund wall and top clearance.

Thomas Prufer

I'm with TNP, measure it yourself, then you know. I do.

Any reason you can’t poke an extending tape measure down to the bottom (or a weighted string)?

Tim