Is head literal

[jameson_uk]

If the head rating for a pump literal or theoretical?
If a small pump is rated at 85cm head and the tank is 50cm high would this mean it could only be at most 30cm below the tank (allowing for 5cm for the pipework) before it would stop being able to return water to the tank?

Or is it 85cm below the intake? Does it make a difference if the outlet is at pressure under water?

 

[Zeus.]

Good question. I would of thought it's how high above the level of the water. So if tank is 30cm high and pump has 85cm head it would rise the water 85cm above the level of the water in the tank, assuming the impeller is clean and no detritus in the pump ie its clean.
So if the pump was 5m below the tank it would still rise the water the same 85cm above. But the less piping/tubing the less resistance so better flow but same head height as at max head height there is no flow so no resistance.

Sent from Mountolympus via neural interface

 

[sparkyweasel]

That's right, The water can go down to the pump and then back up to the water level in the tank even without the pump working, The head is how high the pump has to lift the water above the level in the tank.

 

[zozo]

I guess it is theoretical, since it is not specified and only assumed what kind of tubing you use. But in low numbers (small pumps) there wont be huge differences.
In diagram is looks like this.

That's static head, as given on the lable.. But there is also Dynamic head, to calculate this you need a bunch of other data. friction, diameters, material roughnes and so on. Not realy intersting for the aquarium hobby..

Does it make a difference if the outlet is at pressure under water?

No actualy not, that's atmospheric pressure. it has little influence could be neglected..It doesn't change the head capacity, but it doesn't mean that it will jet verticaly up out of the tank to reach it's head in metre water column as given on the lable ofcourse. The water will disperse in all directions the moment it's past the outlet nozzle. So if you would connect a tube it will push 85cm water column out. But if you don't connect a hose it wont likely spray 85cm high.