debating the need to change bulb in UV

[wooden_reefer]

They say the output declines to about 60%.

But 60% is still very decent if the unit is a bit oversized to start with.

Would it be a good idea to oversize a UV and don't change the bulb for a very long time?

 

[billsreef]

But 60% is still very decent if the unit is a bit oversized to start with.

Won't work that way. It's about flow rate, contact time, and UV output strength. A bigger unit is not stronger in terms of that output strenght, just the contact time...i.e. bigger bulb allows for longer contact time. A 60% reduction in strength on that oversized unit will only get you longer contact time at a weaker strength. If the bulb isn't putting out enough to kill what you want to kill, doesn't matter how long the contact time is, still won't kill.

 

[wooden_reefer]

Won't work that way. It's about flow rate, contact time, and UV output strength. A bigger unit is not stronger in terms of that output strenght, just the contact time...i.e. bigger bulb allows for longer contact time. A 60% reduction in strength on that oversized unit will only get you longer contact time at a weaker strength. If the bulb isn't putting out enough to kill what you want to kill, doesn't matter how long the contact time is, still won't kill.

This is not how the UV works with water. This may be how it works with air.

Water does not flow in a plug until the flowrate is high.

In slow enough flow (Reynolds number 2100 or less), the flow in mostly laminar.

The velocity profile is more like a capital letter V than like a U. Some organisms near the wall where the water travels slowly enough will be killed.

This is why contact time based on plug flow calculation overstates the min size of the UV needed.

Do bulbs stabilize at 60% UV output after a long time until it burns out? I think this is the question.

Forget about contact time calculation; if the flow is laminar, there will be enough contact time for some kill even for a small UV unit. This is why a small UV unit must operate at slow laminar flow.

 

[billsreef]

This is not how the UV works with water. This may be how it works with air.

What I described is certainly how us professional aquaculturists and aquarists understand it work when calculating needs for our systems.

Water does not flow in a plug until the flowrate is high.

Never said or implied that it did

Some organisms near the wall where the water travels slowly enough will be killed.

The question is what organisms are you trying to kill. Algae is easy and certainly will still be killed with reduced efficiencies. Many parasites, especially Crypt. need full bore high kill doses and won't be killed at reduced bulb efficiencies, no matter how slow the flow rate.

Do bulbs stabilize at 60% UV output after a long time until it burns out? I think this is the question.

Fairly standard fluorescence lighting technology in aquatic UV sterilizers, so no it does not stabilize at 60%. It will continually degrade till burn out.

Forget about contact time calculation; if the flow is laminar, there will be enough contact time for some kill even for a small UV unit. This is why a small UV unit must operate at slow laminar flow.

You can't forget about contact time calculations any more than you can forget about bulb strength and degradation without resorting to simply blundering about blindly. With bulbs, high wattage (stronger) ones are larger, hence the unit is bigger and contact time is also longer, or flow rates can be increased while allowing same contact time as the smaller unit at a lower flow rate.

 

[frgrasso]

i change my UV lamps once a year, they run 24/7

 

[wooden_reefer]

What I described is certainly how us professional aquaculturists and aquarists understand it work when calculating needs for our systems.



Never said or implied that it did



The question is what organisms are you trying to kill. Algae is easy and certainly will still be killed with reduced efficiencies. Many parasites, especially Crypt. need full bore high kill doses and won't be killed at reduced bulb efficiencies, no matter how slow the flow rate.



Fairly standard fluorescence lighting technology in aquatic UV sterilizers, so no it does not stabilize at 60%. It will continually degrade till burn out.



You can't forget about contact time calculations any more than you can forget about bulb strength and degradation without resorting to simply blundering about blindly. With bulbs, high wattage (stronger) ones are larger, hence the unit is bigger and contact time is also longer, or flow rates can be increased while allowing same contact time as the smaller unit at a lower flow rate.

I believe something about of how and why the UV works in the aquarium is not understood by even some pros for other aspects of this hobby.

Yes, indeed you can forget about contact time as it is self-adjusting if you operate the unit within the laminate flow rate.

Do you think pathogens have the smart or sense to move to the center with the greatest flowrate? I don't.

When the flow is slow enough, the water flows in layers or sheets without eddies.

For through put UV, contact time is important if you when 100% kill per pass. In a batch operation of the typical setup, the objective is to drastically reduce the pathogen concenration.

You can cut 10% off a 10 foot pipe every hour, you can never have no pipe no matter how many times you cut. The batch operation is similar, except the pathogens are generated at some rate, the pipe does not grow at all. To be effective, the power of the UV, the kill each pass, has to gravitate toward the rate of generation of the pathogen after many many passes.