Puzzle

[gennisis]

Probably the wrong forum but...

I have a 50 gallon fish tank 42 " long with a 4" partition at one end. The smaller partition is sealed from the larger partition with a 1" dia pipe linking the two. There is a foam filter at the inlet in the big section.,which is porous and kept clean.

The tank is open to the air.

Water is pumped from the big section to the small section where it is filtered and returned to the big section by an overhead spray pipe.

Now... since atmospheric pressure is equal on both water surfaces the water levels in both tanks SHOULD be the same,since the pipe links the two.

But it isnt !!

With or without the spray pump working the water level in the smaller section is always 1" lower than in the bigger section.

My science education many years ago tells me that the water levels should be the same.

So...why is'nt it the case here.

Another example of 'AmazingThailand ???

[Shot]

Tide?

[gennisis]

Tide?

Had'nt thought of that !!

[Dakling]

Could be some sort of head resistance in the filtered inlet that prevents things from leveling out but that should equalize quickly once the pump is turned off. Sounds pretty wonky unless the 4" section is not open to atmosphere. Are there any other return systems for the water? It is easy to see why the levels might be different while a pump is in operation but otherwise they should level out. Is there a valve in the pipe that closes when the pump is off?

[planemad]

As long as the pump is working, there will always be a difference in water level. The pump is drawing water from a far smaller "reservoir" than from the main body of water in the tank itself and the return is restricted by the filter media. You might also notice that if you turn the pump off, when the levels equalise, the level in the main tank may be below the level of the overflow into the filter compartment.You can use the water level in the pump compartment as an indicator of when to "top up". However, if you undertake weekly partial water changes (10 to 15% for instance) that should not be necessary. Either way, it is quite normal and you can spend more time watching the fish and less time watching the water level . Just enjoy the terrific array of fish available here.

PM

[jpinx]

there's probably a non-return valve in the pump

[lovelomsak]

You say you have a one inch pipe connecting the two tanks yes. You also say water is pumped from the large tank to the small tank. So I have a question. Does the pump push the water through the one inch connecting pipe. By this I am asking does the water from the large tank get pumped to the small tank through the one inch pipe and if so does the water going from the large tank that goes through the pipe first have to go through the water pump. If there is no direct connection between the tanks,then when the pump is shut off the tanks simply maintain the level they were at when the pump was stopped. The pump acts as a barrier between the levels, allowing them to maintain different levels.

.

[krading]

Coriolis effect.

[lovelomsak]

Coriolis effect.

Nice word and nice try but his question doesnot deal with motion so Coriolis effect isnot relavent.

[gennisis]

You say you have a one inch pipe connecting the two tanks yes. You also say water is pumped from the large tank to the small tank. So I have a question. Does the pump push the water through the one inch connecting pipe. By this I am asking does the water from the large tank get pumped to the small tank through the one inch pipe and if so does the water going from the large tank that goes through the pipe first have to go through the water pump. If there is no direct connection between the tanks,then when the pump is shut off the tanks simply maintain the level they were at when the pump was stopped. The pump acts as a barrier between the levels, allowing them to maintain different levels.

.

If I call the small section 'A' and the larger section 'B',then the setup is Section 'A' contains the water pump and ceramic filter bits. The pump sucks in free water from this section. Water enters the section via a 1" tube at tank base level which goes into section 'B.....no valves or restrictions.

Water pumped from 'A' is sprayed back into 'B'.

So water from one section is equaly returned to the other section.

If this were not the case the water level in 'A' would decrease and in 'B' increase.

Obviously water travels for each section if the pump is on and also if the pump is not on,since atmospheric pressure is the same on both sections and...Should give the same water level in both.... but it does'nt...a 1" difference in levels.

I just do not have any reasons as to why this is. As far as I am concerned the water levels should be the same. I ignore the ...say half a liter in the spray pipe when the pump is on. The pump cannot act as a barrier since the water from 'B' flows throught the 1" pipe unobstructed to 'A'

It is crazy...can only be 'Amazing Thailand".

[CharlieH]

Amount of water in the pipe and in the pump. I bet if the pump was turned off and the pipes/pump emptied they would be the same. OR prime the pipe and the pump then fill the shortage and it should then stay balanced.

Edited April 9, 2012 by CharlieH

[Semper]

Does it really matter?

[gennisis]

Does it really matter?

When you have an inquiring simple mind like me.....YES

[lovelomsak]

You say you have a one inch pipe connecting the two tanks yes. You also say water is pumped from the large tank to the small tank. So I have a question. Does the pump push the water through the one inch connecting pipe. By this I am asking does the water from the large tank get pumped to the small tank through the one inch pipe and if so does the water going from the large tank that goes through the pipe first have to go through the water pump. If there is no direct connection between the tanks,then when the pump is shut off the tanks simply maintain the level they were at when the pump was stopped. The pump acts as a barrier between the levels, allowing them to maintain different levels.

.

If I call the small section 'A' and the larger section 'B',then the setup is Section 'A' contains the water pump and ceramic filter bits. The pump sucks in free water from this section. Water enters the section via a 1" tube at tank base level which goes into section 'B.....no valves or restrictions.

Water pumped from 'A' is sprayed back into 'B'.

So water from one section is equaly returned to the other section.

If this were not the case the water level in 'A' would decrease and in 'B' increase.

Obviously water travels for each section if the pump is on and also if the pump is not on,since atmospheric pressure is the same on both sections and...Should give the same water level in both.... but it does'nt...a 1" difference in levels.

I just do not have any reasons as to why this is. As far as I am concerned the water levels should be the same. I ignore the ...say half a liter in the spray pipe when the pump is on. The pump cannot act as a barrier since the water from 'B' flows throught the 1" pipe unobstructed to 'A'

It is crazy...can only be 'Amazing Thailand".

The water is pumped from tank A to tank B via the 1" tube at tank base right. Is the connection between the pump and the tube sealed so as no other water can enter the tube except water from the pump? If so then there isnot a direct connection between tanks A and B.

The levels of the tanks will remain constant as related to original level, each independent of the other.If the pump flow is the same as the spray flow then levels will remain near a constant also, keeping in mind each level is independent and not related.

A good way to test this is remove the pump after stopping pumping and allow the 2 tanks A and B to be connected via the 1" tube at base level,directly.. This should allow the tanks to equalize. Give it a try and get back to us.

[Dakling]

When the pump is on you have a head differential between the 2 tanks so it is common for them to be at different levels. Once the pump is turned off they should return to the same level provided there is no valve arrangement that closes once the pump is turned off. have you seen a difference in level once the pump is off or is the different level only observed while pump is operating?

[loong]

You say you have a one inch pipe connecting the two tanks yes. You also say water is pumped from the large tank to the small tank. So I have a question. Does the pump push the water through the one inch connecting pipe. By this I am asking does the water from the large tank get pumped to the small tank through the one inch pipe and if so does the water going from the large tank that goes through the pipe first have to go through the water pump. If there is no direct connection between the tanks,then when the pump is shut off the tanks simply maintain the level they were at when the pump was stopped. The pump acts as a barrier between the levels, allowing them to maintain different levels.

.

If I call the small section 'A' and the larger section 'B',then the setup is Section 'A' contains the water pump and ceramic filter bits. The pump sucks in free water from this section. Water enters the section via a 1" tube at tank base level which goes into section 'B.....no valves or restrictions.

Water pumped from 'A' is sprayed back into 'B'.

So water from one section is equaly returned to the other section.

If this were not the case the water level in 'A' would decrease and in 'B' increase.

Obviously water travels for each section if the pump is on and also if the pump is not on,since atmospheric pressure is the same on both sections and...Should give the same water level in both.... but it does'nt...a 1" difference in levels.

I just do not have any reasons as to why this is. As far as I am concerned the water levels should be the same. I ignore the ...say half a liter in the spray pipe when the pump is on. The pump cannot act as a barrier since the water from 'B' flows throught the 1" pipe unobstructed to 'A'

It is crazy...can only be 'Amazing Thailand".

The pump that takes the water from A has a non return valve and whilst pumping creates a negative (less than atmospheric) pressure in the pipe between the tank and the pump. When the pump is turned off, this negative pressure will remain in the pipe. Because the negative pressure will suck the water from the tank and atmospheric pressure will push down on the water's surface, the water level will be lower than in the other section that only has atmospheric pressure acting on the surface

[lovelomsak]

You say you have a one inch pipe connecting the two tanks yes. You also say water is pumped from the large tank to the small tank. So I have a question. Does the pump push the water through the one inch connecting pipe. By this I am asking does the water from the large tank get pumped to the small tank through the one inch pipe and if so does the water going from the large tank that goes through the pipe first have to go through the water pump. If there is no direct connection between the tanks,then when the pump is shut off the tanks simply maintain the level they were at when the pump was stopped. The pump acts as a barrier between the levels, allowing them to maintain different levels.

.

If I call the small section 'A' and the larger section 'B',then the setup is Section 'A' contains the water pump and ceramic filter bits. The pump sucks in free water from this section. Water enters the section via a 1" tube at tank base level which goes into section 'B.....no valves or restrictions.

Water pumped from 'A' is sprayed back into 'B'.

So water from one section is equaly returned to the other section.

If this were not the case the water level in 'A' would decrease and in 'B' increase.

Obviously water travels for each section if the pump is on and also if the pump is not on,since atmospheric pressure is the same on both sections and...Should give the same water level in both.... but it does'nt...a 1" difference in levels.

I just do not have any reasons as to why this is. As far as I am concerned the water levels should be the same. I ignore the ...say half a liter in the spray pipe when the pump is on. The pump cannot act as a barrier since the water from 'B' flows throught the 1" pipe unobstructed to 'A'

It is crazy...can only be 'Amazing Thailand".

The pump that takes the water from A has a non return valve and whilst pumping creates a negative (less than atmospheric) pressure in the pipe between the tank and the pump. When the pump is turned off, this negative pressure will remain in the pipe. Because the negative pressure will suck the water from the tank and atmospheric pressure will push down on the water's surface, the water level will be lower than in the other section that only has atmospheric pressure acting on the surface

I think I understand what you said. I was wondering if he lowered the water level some more in tank B by simply syphoning water out, so that it is 2 inches lower, than tank A if the 2 inch level would remain constant. If it did remain 2 inchs lower then would that be an indication of non related levels.

[loong]

If he lowered the level in tank B, atmospheric pressure would attempt to make the levels equal, but the negative pressure in the pipe will continue to cause the level in tank A to be lower.

[Puschl]

Here's a small ascii-art sketch of the system as OP has described it.

                               Pump
                         +======XX======+
                         | +====XX====+ |
                         | |          | |
|                          F   |     | | |  |
|                              |     | | |  |
|~Water~Level~B~~~~~~~~~~~~~~~~|     |~|G|~~|Water Level A
|                              |     | | |  |
|                              |     |  E   |
|                              |     |      |
|          B                   =======      |
|                              C     D      |
|                              =======      |
|                              |     |      |
|                              |     |  A   |
|                              |     |      |
|                              |     +------+
+------------------------------+

The big basin B connects to the smaller basin A via a pipe (C-D). A pump sucks water from the small basin and sprays it into the big one (E-F).

If there truly is no obstruction in the pipe C-D, and the pump is switched off, then the water levels will be the same. At the surface, the pressure is atmospheric, and increases linearly with depth. If the pump has a valve, a water column will stay inside the pump tubing, with sub-atmospheric pressures above G (by creating complete vacuum, you can suck water from an atmospheric pool only about 10 meters high). If there is no valve, the tubing will be filled with air and the water level inside the tubing only goes up to G. If the pump is running, water will be sucked from E to F, leading to higher water levels in B and lower water levels in A. The pressure difference will lead to water flowing from C to D. If there is a flow resistor at C (as mentioned in the initial description), the flow will be slowed down and the water level in B will rise significantly, before eventually the flow C->D will reach equilibrium with the flow E->F.

When the pump is switched off, the flow E->F stops. The flow C->D continues, but slows down as the water level in B sinks and the level in A rises, until they eventually are the same. This may take some time.

If water levels remain different, then it's time to challenge the level metering. In the sketch above, basin B is 11 characters deep, and basin A only 10 characters deep. The surface level is the same, though.

Another possible explanation: the water in basin B has a different density (e.g. higher salinity) than the water in basin A. Again, strong indication of some obstruction in the C-D tube.