Introduction: Tidal Geocache

About: Just your average non engineer in beautiful New Zealand, solving my seemingly unique problems because I cant find any one else that has.

If you use it - credit to Alligatorsnz please:-)

I have been trying to prefect a simple, cheap and easy to build geo cache containers which beats the constant problems of condensation and weather conditions...but along the way i discovered a design that somewhat ironically uses water to keep the cache dry....!! ( still looking for the land based design though..)

This Instructable is on building the smallest successful demonstration of the design i have managed;

I You will need;

  • 1 x wide mouthed /screw top - tall bottle ( i used a 500ml medicine bottle)
  • Length of electrical conduit at least the height of the large bottle
  • 1 x small screw top bottle( the smaller the better for anti condensation) that is approx 1/4 the large bottle height

(The best kind will fit snugly in side your electrical conduit to make mounting easy, i use 20ml / 40ml pill bottle)

  • Hot glue gun
  • 5 mm twist drill bit
  • 3 mm twist drill bit
  • Drill
  • Fishing line
  • Hack saw
  • Vice
  • Two x 4 mm x 200mm cable ties
  • Black duct tape ( camo is good too all though i find it doesnt stick as well

Step 1: Camo...

Effective camo is not about the colour's, but about breaking up the outline of the object with irregular shapes - the less straight lines the better...( i have been amazed colour combinations i thought would scream "look at me".. can actually hide a container!)

Normally i would address camo., once the cache was complete, but in this version it needs to be done first.

So far in my tests, simple black duct tape works the best ( perhaps it depends on the brand for camo tape but this tended to drop off at the first hint of water..) I have not tried any of my hand painted containers as yet.

  1. Take the duct tape and it around the entire container - i find that the first couple are across the base, then i wrap it around the body making it nice and smooth.
    • While just the first wrap is sufficient, i do sometimes then add a layer of shapes by stretching and squeezing the tape as i un roll it to create "terrain" and help break up the outline and colour.
    • I Don't put any camo on the lid, as it will soon fall off in the wet and with hands operating it.)
  2. Once the container is wrapped then using the 5 mm drill bit, drill the following holes;
    • The 4 attachment holes. For this design to work these holes must be not be any greater distance then half the volume of the bottle - preferably quarter the volume form the lid - and arranged in equal rows wide enough for the proposed mount.. (diagram 1 -a) Once you have worked this out go ahead and drill all four holes in the pattern shown.
    • The Fill/ Drain holes. These go right around the top of the container top just below the lid level, ideally your bottle will have a shoulder parallel to the top of the lid so your drilling parallel to the neck, but if not then put them evenly spaced around the top edge of the bottle. (diagram 1-b)

Step 2: The Outer Container Construction

Once the container is wrapped then we add the various holes. This cache type is designed requires that it be firmly attached to and object so that the lid is always facing down.

The 4 attachment holes: (using the 5mm drill bit - or one to match your cable ties)

  • With the container upright the 4 attachment holes must be in the top halve of the container, preferable the top quarter if its big enough. (So when the container is inverted, the top halve of the container will be completely air tight.)
  • Arranged the holes in the square pattern shown, make sure there is enough plastic between each hole to prevent the tie "tearing" (diagram 1 -a)

The Fill/ Drain holes:

  • These go right around the top of the container top just below the lid level, ideally your bottle will have a shoulder parallel to the top of the lid so your drilling parallel to the neck, but if not then put them evenly spaced around the top edge of the bottle. (diagram 1-b)

Step 3: The Lid

  1. Take the length of electrical conduit and place it on the inside of the lid, and draw around the base.(Diagram 2)
  2. Then using the 3 mm drill bit, drill approx 6 holes round the circle drawn, using the base circle line as the hole center (Diagram 3)
  3. Measure the thickness of the bottle thread (Diagram 4-a) and then compare this to the lid.(Diagram 4-b) The distance from the thread on the lid to the edge of the 3 mm circles should be enough to drill a 5 mm hole with out interfering with either structure.
  4. Coast is clear, then drill 3 - 6 5 mm holes through the lid inside.(Diagram 4-c)
  5. Take the length of electrical conduit and drill two holes through both sides of the conduit ( close to the end) so that each set of holes are at right angles to the others. (Diagram 5)

Step 4: The Cache Container

Choosing a log container;
My research and observations so far tend to suggest that the smaller the air space in the log container (ie: small container stuffed full of rolled log) the drier the logs stay. I also us waterproof paper i get from a local surveying supplier, and then use a laser printer to add our logo.

Fitting the log container;

  1. Take the smaller container that fits inside the electrical conduit - squirt hot glue around it and push it straight into one end of the conduit as far as possible - taking care not to get glue on the lid thread (Diagram 6)
  2. Take the large bottle and use a ruler, to measure the distance from the top of the lid thread to the inside of the base.(Diagram 7)
  3. Take 5 to 10 mm off this depth of the bottle, mark and cut the open end of the conduit to this new length. Test the clearance by placing the conduit/ small bottle into the big bottle, it should sit the 5 to 10 mm you calculated below the rim of the big bottle thread.

Step 5: Attach to Outer Container

  1. Take the fishing line and thread it through one of the 3 mm holes in the lid form the out side, and alternating through the holes in the end of the conduit and back out through the lid(Diagram 8)
  2. Squirt hot glue all around the conduit end and immediately "pull" the fishing line tight to pull the conduit tightly into the inside of the lid - and tie off across the outside of the lid.
  3. Plonk a blob of hot glue over the outside of the lid to seal the knot - taking care not to block the larger holes in the outer lid

Step 6: Installation

Be prepared;

  • First I always pre thread the cable ties, and put a bit of tape over them to stop them getting out ( most of my installation is done from my sea kayak at high tide...)
  • I attach my cutting tool ( for cutting off the ends of the cable ties) to a line... amazing how they always find the gap between the kayak and the bush... in the deep...

Attachment locations;

  • So far I have only installed these caches on Mangrove Trees either to the branches and trunk at full tide or to the root structures at low tide. The container needs to be installed vertically, with the lid an drain holes at the bottom.
  • Now I am working on installing them in structures which I take with me.. the biggest problem currently is the delivery technique, as these placements will need to be heavy enough / anchored sufficiently to handle the sea..
  • Remember to leave enough clearance below the lid that the container can be withdrawn.

High tide installation;

  • I do this for caches that will be placed in branches, to ensure that that I place them at a level that they will be covered by the every day tides regularly.
  • The only thing to remember is when placing the cache, orientated it correctly before you put it under water, and maintain that orientation while you fix it.

Step 7: How It Works

The observations that lead to the design... some call it learning as you live?

The idea came to me from compressing air... it tends to take a bit of work to compress the stuff, and generated heat or sweat depending on how its done - which tends to indicate its hard work...( seem to remember learning something about compressing air in my medical training all those years ago.. laws or something that made no sense other than sleep was a better option... in those stuffy classrooms?)

So using this idea i went ahead and built a cache to let the water compress the air in the dry zone.. and making sure i installed them as deep as possible, have found so far that while i am busy running around replacing wet land based cache logs.. i have yet to be required to do any of these ones..?

Equally reading about condensation and observing my land based caches i found that there was only ever a small amount of water in the cache - just enough to soak the log but not enough to create a puddle.. something else i read indicated that the smaller the volume of air within a container the harder it is for condensation to form... so i wondered if a small log container stuffed with plenty of log would achieve this end ( i have just installed this idea on 22 of my land based caches that so far due to their location in the bush just seem to be water makers.. we shall see!)

Then i thought i would share the idea here.. so as a matter of interest i would go ask the big ugly text books what they thought..... turns out they kinda agree;

Apparently at sea level the "weight " of pressure is approx 101 kPa (14 lbs/ sq inch), at 10m (33 feet) it increases by a further 101 kPa (14 lbs/sq inch) or doubles in pressure.

Boyles Law is the technical version.. but as i understand it, a drop by 10m below sea level and the volume of air will be squeezed in to half the volume....:-) all the while busy pushing back at the water twice as hard..(any divers in the room?)

I found a pressure calculator and was able to establish the pressure at approx 1m below sea level would be 111 kPa(16.1psi) and just possibly that would mean the air would be compressed by a factor of 1.15 .. ...

so in theory the dry zone of the cache would be able to fight back enough to keep itself dry.. the more the water on top the better it would be able to... and equally any warm air trapped inside that cooled to produce condensation (the biggest problem on land i face!) is free to drip down and rejoin its kin...and be drained at low tide...

So....the easy version;

When the tide comes in it, it enters the large container and as its level rises in the large container the air above halve way is unable to escape, at some point as the water covers the container the water pressure starts to compress the air remaining - meaning at some point their will be some push back by the air remaining that will ensure water stays away from the small inner container containing the log - and the small log container prevents condensation damage?

Or is it all down to the fact that the location means the caches are just not opened regularly enough to create a condensation issue?

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