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We believe the following to be sources of error.:
- Human error: Despite our best attempt at being consistent (by measuring and marking heights on the test tube, while also holding a ruler on the test tube wall), there will always be human error in observing the bed expansion visually. The next expansion experiment should use a camera so there is record of the heights at each flow rate, and also tape a ruler to the filtration bed wall, rather than holding the ruler or drawing it on.
- Wall Friction: We can attribute the increase in error as flow rate increased due to the increase in wall friction on the test vial. We can minimize the wall and tube friction by increasing the size of our bench scale experiments.
- Sand Properties & Parameters: We might have used an incorrect D60 and porosity for the filter bed in our equations. For the next experiment those parameters should be found out for the sand or material used by using a sieve.
- Preferential flow: Despite our best attempt to keep the test tube as level as possible, we might have introduced preferential flow in our experiment causing an unbalanced backwash flow. In the future, two levels could be clamped on the sides of the filter walls to ensure it is level. A further precaution would be to use two clamps, rather than one.
- Expansion Headloss: The accuracy of our model also required the headloss occurring through the expanded bed to be more or less constant, which we did not have time to test (but is part of Recommended Future Research below). Testing it would involve putting a pressure sensor into the system, connected on either end of the filter.
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