| 21. | _Watch your pressure and depth gauges closely.
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| 22. | The planesman was provided with a prominent depth gauge to monitor this, usually a circular Bourdon gauge.
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| 23. | Use a penny as a tread-depth gauge.
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| 24. | A second string of " ash cans " knocked out her depth gauge and caused other minor damage.
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| 25. | But it does, and slowly, very slowly, the digital depth gauge registers an upward movement in the sub.
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| 26. | Depth changes are'apparent'because depth gauges are calibrated for sea water, and the hydrolevel is filled with fresh water.
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| 27. | Careful filing of the depth gauges, making a radius down to the leading edge, reduces the risk somewhat.
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| 28. | The depth gauge wavers for a few seconds, then starts upward, faster and faster like an express elevator.
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| 29. | "You're watching the depth gauge, and it hits the 70-meter ( 230-foot ) mark, and the tail just stops.
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| 30. | As the depth gauge readout appears in one corner of the screen, the dolphin's tail moves up and down.
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