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inline devices


5 Mar 2008
call me sad, but reading about using 12/16 reactors with 16/22 filters got me thinking, what is the actual reduction in flow from going from 16mm tubing to 12mm.

i am sure people's practical experience is more valuable, but...

if we assume that all the resistance to flow for the filter comes from the tubing (and none from the filter itself, a simplification i appreciate). flow is proportional to length divided by radius to the power 4.

so assume there is 2000mm of tube, and 8mm radius, and then compare this to 1900mm tube with 8mm radius and 100mm with 6mm radius in the device. i won't bore you with the maths, but this increases overall resistance to flow by about 11%.

obviously there is some resistance to flow from the filter itself, and from the inline device, e.g. bioballs in the reactor cyclinder, therefore the actual increase in total resistance is likely to be less.

does a roughly 10% reduction in flow by adding a 12mm reactor as opposed to a 16mm reactor seem reasonable based on actual experience? or is the resistance from the filter and inline device such that the reduction is acutally much less?


Expert/Global Moderator
11 Jul 2007
Chicago, USA
Yep, that 10% is massive because it's then multiplied by (density)*(g) to get mass flow rate. We are trying to deliver nutrient mass. The mass flow rate carries momentum to the far reaches of the tank. Not only that but minor increases in the fluid velocity has huge effects on the thickness of the boundary layer at the surface of the leaf.

A "boundary layer" for those unfamiliar with the term, is the interface between a solid surface such as a leaf and the fluid in motion across this surface. Because of the friction caused by relative motion of the two, the fluid velocity at this interface is reduced to practically zero, while at a short distance above the layer the velocity increases to the nominal value. It is this boundary layer, referred to as the Prandtl Layer, that has a significant effect on nutrient uptake since the nutrients must first saturate the layer prior to crossing over the cuticle and to continue on across the leaf's surface membrane. The thinner the Prandtl Layer the faster the uptake. The Prandtl layer is a mechanical obstacle to be overcome, so any flow increase, even a minor one has a significant effect.

CO2 molecules also have to cross this boundary, and the speed at which they cross has a direct and significant effect on photosynthesis rates. D.F Westlake's (The River Laboratory, Dorset) 1965 experiments indicate that the difference in photosynthetic rates between plants submerged in water at zero velocity (i.e still water) compared to water movement at 5 mm/sec is a factor of about 3 for some species and under certain lighting conditions. Of course this cannot be unilaterally applied to all species under all conditions but is indicative of the importance of even minor flow increases.

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