malawi_man Posted January 13, 2012 Report Share Posted January 13, 2012 Just filling up a 1200x450x450 tank, the stand is nearly flat, except it sags 1mm for about 400mm along the front. I've got 10mm poly under the tank, and it's about half full atm. It's been years since I filled a new tank, will the poly absorb the difference? I tried putting some cardboard in too, but it was too thick. Any help would be much appreciated! Quote Link to comment Share on other sites More sharing options...
ryanjury Posted January 13, 2012 Report Share Posted January 13, 2012 I would be inclined to stick a sheet of ply on top and pack it level or get a completely flat stand.. It's one of those things that you don't want to gamble with really, it might be ok or the tank might crack and flood the place and all your fish die.. Quote Link to comment Share on other sites More sharing options...
livingart Posted January 13, 2012 Report Share Posted January 13, 2012 how thick is base glass? and has base euro bracing internally? is poly high density or low? ie: easy to compress Quote Link to comment Share on other sites More sharing options...
Hamstar99 Posted January 13, 2012 Report Share Posted January 13, 2012 is it a solid base or does it just have some cross members supporting the tank. The poly will absorb the difference if its only 1mm. It would be preferable if the base was fully supported as Ryan suggests with something like ply to help spread the load. Quote Link to comment Share on other sites More sharing options...
malawi_man Posted January 13, 2012 Author Report Share Posted January 13, 2012 It's a solid base, and not full euro bracing but it has a glass strip along the bottom front and back. The poly seems pretty hard, so I guess high density? Quote Link to comment Share on other sites More sharing options...
Artem Posted January 15, 2012 Report Share Posted January 15, 2012 White poly is usually low-density. Especially if it comes off in little round balls. Higher density poly tends to be foam-like in appearance and green or blue. At least that has been my experience with it. Quote Link to comment Share on other sites More sharing options...
spoon Posted January 15, 2012 Report Share Posted January 15, 2012 high density has small beads low has large ones like in a bean bag Quote Link to comment Share on other sites More sharing options...
livingart Posted January 15, 2012 Report Share Posted January 15, 2012 +1 high density is good to carve Quote Link to comment Share on other sites More sharing options...
Warren Posted January 16, 2012 Report Share Posted January 16, 2012 It sounds like the stand is deflecting with the weight of the water in the tank. If this is the case the gap will get bigger as you fill the tank more. The poly will not take up the gap as it does not squash more than a fraction of a millimeter. Poly is intended to only take up point loads like grit or screw heads sticking up a little. The force on the poly is approx (0.001 x height in mm) grams per mm². In your case your tank is 450mm tall so you have .45g/mm² (not allowing for the weight of the tank) - not enough to deform the poly. Another way to look at it is like this - if the force on the poly is enough to take up gaps of a large area then it will end up squashing it nearly flat (like when you stand on poly). Another way to calculate it is: Tank 1200 x 450 x 450 = approx 50kg empty Volume = 243L (approx) so the water weighs 243kg Total = 293kg 293kg / (1200 x 450 for the base) = 0.0005426kg / mm² = 0.54g / mm² (the difference to above is the weight of the glass added in). It will not be a problem if the bottom glass of the tank is strong enough to take the full weight of the water. You will need to strengthen the stand if you want it to stay perfectly flat and take the weight of the water so the bottom glass has little to no weight or deflection. The top of the stand will have to have little to no deflection with the full weight of the tank on it. Any deflection means force on the glass. Alternatively you can share the force on the glass with the top of the stand if you calculate the bottom glass deflection with a full tank and make a stand to deflect half this amount with the full weight of the tank on it. Quote Link to comment Share on other sites More sharing options...
malawi_man Posted January 16, 2012 Author Report Share Posted January 16, 2012 The stand didn't change under the weight of the tank, it was already like that before. I put in an extra support before I started filling. I've packed the 'sagging' area and filled the tank. I'm pretty happy with it now. Quote Link to comment Share on other sites More sharing options...
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