I want to display he volume of a spherical tank, but is there a function that can do that?

Try the broken line function.
That function comes with the UT52A, UT55A, UT75A, UP55A, and UM33A.

Advanced Application UT75A

Temperature Controller with increased monitoring and operating capabilities

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Mid-level UT55A/UT52A

Increase monitoring and operating capabilities

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UM33A

Digital Indicator with 9 alarms outputs and input correction function

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UP55A

Program controller with 30 patterns, simultaneous monitoring, and ladder sequence function

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