We purchased a new replacement galvanised iron tank for the house some time ago. After it being delivered and placed on the stand we were advised to fill the tank to a depth of 10 cm or so to ensure it did not blow away in the event of a strong wind event. After we did this it became apparent that there was a slow leak. The advice from the tank maker was to leave it for a while to see if the leak would ‘take up’. After a couple of months it continued to leak.
A couple of days ago the manufacturer called at the farm and after draining and drying the tank he applied more silicone to the lower joins which should mean it no longer leaks. This was the prompt for us to finally connect the two downpipes from the house, which we finished yesterday.
The tank was custom-built to the same dimension as the old one. By so doing it meant that we did not need to alter the tank-stand foundations. We did replace the old cypress pine boards that were between the concrete base and the old tank bottom with new galvanised iron sheeting. This is much better for the tank longevity as it means there is minimal damp or wet contact with the bottom of the tank itself.
Now we will just wait and see how long it takes to fill the tank from normal rainfall. In this context and out of general curiosity I decided to see how much rainfall we will need to fill the tank, assuming no usage or water loss with all water that falls going into the tank. Here are the calculations:
Tank Volume in Litres
The formula for calculating round tank capacity from Page 198 of the publication “Lysaght Referee” 27th Edition published by John Lysaght (Australia) Limited in 1985 ISBN 0 909349 23 1 is:
C = (∏/4) x (Diameter)2 x height.
The diameter of the tank is 2810 mm and the height is 1760 mm.
In order to get a capacity calculation that is in litres one needs to use measurements that are in decametres. A cubic decametre (100 mm x 100 mm x 100mm) equates to 1 litre of volume. To do this, divide the diameter and height measurements by 100 if they are in millimetres. This gives a diameter figure of 28.1 (decametres) and a height of 17.6 (decametres).
Working through the formula with the actual dimensions:
C = ([22/7]/4) x (28.1 x 28.1) x 17.6
C = 10,915 litres
NB: A potentially easier way for those with internet access is to go to the following website and input the diameter and height figures in the appropriate fields: http://www.spry.net.au/?q=node/6.
Roof area of the house
The house is 12.3 metres long by 11.6 metres in width, giving a total roof area of 142.68 square metres.
Millimetres of Rain Required to Fill the Tank
1 millimetre of rain on 1 square metre of area produces 1 litre of water. Thus to calculate millimetres of rain needed to fill the tank we simply divide the tank volume in litres by the roof area in square metres:
Required Rainfall = 10,915 / 142.68 = 76.5 millimetres.
Why We Opted for a Galvanised Iron Tank
The logic is basically:
• We had an existing tank stand in place, needing little repair
• Grenfell has a business that makes galvanised iron tanks. Buying locally supports the local community. Product pricing is fair.
• The concrete tanks keep the water cooler in the summer, but the water tends to be ‘harder’ from these tanks. We already have a concrete tank at the Shearing Shed
• Plastic tanks come in fixed sizes. We needed a custom built tank, removing the need to alter the tank stand.
• There is some research coming through suggesting that some plastic containers (including drink bottles and tanks) ‘leak’ unfavourable chemical residues into the water they hold. Amway, for example, is apparently reverting all of its products away from plastic containers to glass.
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