Putting the ballast system on a analogue control isn't recommended because you don't need that much variability. A three position switch is ideal - up is surface, middle is hold, and down is dive.
I use timers to control the bilge pumps on my combat ships, so it is fairly easy to do. Under logic switches, create a timer function that is on for your 21 seconds. Then set a brief off period. During the off period you need to deactivate the switch (move to middle). Now add a mix that uses the logic switch name to activate your pump - not the physical switch.
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