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Close-top rain-tank -

Discussion in 'Cave Diving' started by seascaper, Aug 22, 2019.

  1. seascaper

    seascaper Marine Scientist

    # of Dives: I just don't log dives
    Location: Montego Bay, Jamaica
    Wise styglobites,
    We've bought a farm house in the hills above Montego Bay, Jamaica, and it's got a farm's rainwater tank: ~20'x30'x 12' deep. It's also got a flat concrete cover, three internal tank/modules each with its own ~18x18" access hatch. Annoyingly, I'm being told that I need to get into it and check the pump intakes, free the drains etc.

    Looks to be proper cave conditions- water is still & crystal with fine sediments to the far/deeper end. I'm really hoping there's nothing more than muck & dead lizards in there...

    Thoughts, particularly re. safety?
    Good, serviced gear, certainly. Ropes to surface-guy at hatch, certainly. Lights, certainly. I'll make sure there's at least 3' of airspace before I get in, though that will complicate getting back out again. I'll never be more than 20" from a/the hatch & 10' from surface.

    Yes, I've checked with my life-insurance guy. Yes, he tells me that my wife has made no changes to our policy. :wink:

  2. PfcAJ

    PfcAJ Contributor

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
    Those drains can absolutely cause enough pressure differential to pin you to the drain and ultimately drown you. Delta P is worth googling. It ain't nothin to play with.
    EireDiver606 likes this.
  3. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, Washington State
    Those drains will kill you if someone opens them or turns on the pumps. Lock them closed and leave the key someone who knows what they are doing.
  4. PfcAJ

    PfcAJ Contributor

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
  5. ofg-1

    ofg-1 Course Director

    Are these your drinking water cisterns? How big are these drains? Are there 3 different pumps? We had multi compartment cisterns in the Bahamas. First, as soon as you go in, the vis will probably go to zero. If these are rainwater charged, everything from bird crap to asteroids that lands on your roof is in there. The granules from the shingles are the worst, and yes, some do come off. Wood shingle roofs will turn the water brown due to tannin's in the shingles.

    We set it up to backflush the drains so we did not have to stick our hands in it. We had a pool vacuum that we ran around in the bottom of the tanks powered off the water pump, got some of the crud and a few of the frogs out. A trash pump works better, you can suck the crud off the bottom, but you loose a lot of water. If the pump has a plastic check valve on the suction end change it.

    If this is not drinking water, just backflush the drains, change the check valves and don't worry about it.
  6. 2airishuman

    2airishuman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Greater Minnesota
    If there's a lot of crap on the bottom, the air in the space above the water might not be breathable after you've stirred things up and released a bunch of methane.

    I would put in a ladder rather than relying on ropes. If a regular ladder won't fit, you can make one out of a 2x8 or whatever with cross pieces.

    I would pump the compartments out one at a time, and set up a blower before climbing in, rather than doing it on scuba.
  7. EireDiver606

    EireDiver606 Public Safety Diver

    What AJ said is correct, you should leave it to the professionals who have steel helmets, comms, are tethered, and hopefully know what they’re doing. It’s not worth trying to be cheap doing it yourself and drown as a result.

    That pressure differential is crazy strong and “when it’s got ya, it’s got ya” - end of Delta P video...

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