Head of water

[DeltaT2]

Been in the trade nearly 40 years & I'm still getting conflicting information regarding this;
Is the head of water from a header tank calculated from the water level in the tank or the base of the water tank?

http://www.tradeplumbing.co.uk/assets/images/abode/techwaterpressure.pdf

 

[monoxide62]

Head is measured @ water level line , imagine a cistern to be 10m tall , if head were to be measured from base of cistern head pressure would be 10m light.

Hey Tommyplumb , noticed you've added to the 'gas safe peeps' thread.

 

[acmedone]

Been in the trade nearly 40 years & I'm still getting conflicting information regarding this;
Is the head of water from a header tank calculated from the water level in the tank or the base of the water tank?

http://www.tradeplumbing.co.uk/assets/images/abode/techwaterpressure.pdf[/QUOTE]

Exactly my question in my "Shower pump head" thread. The physics says it must be the water level. If a manufacturer says "from the bottom of the tank" for a deliberate reason, they are not talking about water pressure.

I found this in my searches:-
http://www.bristan.com/WebRoot/BristanDB/Shops/Bristan/Products/INSTRUC. PDF-1064590-D2.PDF

In the text they say one thing and the pictures show the other. It suggests incompetence rather more than a deliberate reason.

 

[DeltaT2]

Head is measured @ water level line , imagine a cistern to be 10m tall , if head were to be measured from base of cistern head pressure would be 10m light..............................No mate, there would not be 1Bar pressure at the base of your tank - so your theory is mince...............

 

[doitall]

Been in the trade nearly 40 years & I'm still getting conflicting information regarding this;
Is the head of water from a header tank calculated from the water level in the tank or the base of the water tank?

http://www.tradeplumbing.co.uk/assets/images/abode/techwaterpressure.pdf[/QUOTE]

Neither Dickie,

For calculation purposes take the center line of the water level. The water level drops dependent on recovery rate

 

[ChrisR]

No mate, there would not be 1Bar pressure at the base of your tank - so your theory is mince

Oh yes there would, so oh no it isn't.

The only level which makes much sense to use is the initial water level because that's the maximum head you're going to get, when there's zero flow, ie the Static head.
As soon as you draw water, not only does the pressure drop as the level drops, but there's all the pressure drops due to pipe resistances. So the dynamic/working pressure will always be some lower figure.

 

[DeltaT2]

Been in the trade nearly 40 years & I'm still getting conflicting information regarding this;
Is the head of water from a header tank calculated from the water level in the tank or the base of the water tank?

http://www.tradeplumbing.co.uk/assets/images/abode/techwaterpressure.pdf[/QUOTE]

Neither Dickie,

For calculation purposes take the center line of the water level. The water level drops dependent on recovery rate

Thanks doitall, yet another take on it. As I say there has always been a conflict.

 

[DeltaT2]

No mate, there would not be 1Bar pressure at the base of your tank - so your theory is mince

Oh yes there would

Oh no there wouldn't!! Do you think there would be .2Bar 2M down from the static water level??

 

[ChrisR]

there would be .2Bar 2M down from the static water level

Yes that's correct.

A column of water 10m high ( 9.81 m actually) produces a pressure at the bottom 1bar higher than atmospheric, and pro rata.

 

[monoxide62]

No mate, there would not be 1Bar pressure at the base of your tank - so your theory is mince

Oh yes there would

Oh no there wouldn't!! Do you think there would be .2Bar 2M down from the static water level??

Would also be 10mm static head 10mm below WATER LINE.......along with 5m static head 5m below water line.

You sure you've been in the game 40 years?

 

[DeltaT2]

No mate, there would not be 1Bar pressure at the base of your tank - so your theory is mince

Oh yes there would, so oh no it isn't.

The only level which makes much sense to use is the initial water level because that's the maximum head you're going to get, when there's zero flow, ie the Static head.
As soon as you draw water, not only does the pressure drop as the level drops, but there's all the pressure drops due to pipe resistances. So the dynamic/working pressure will always be some lower figure.

Have you ever tried this Chris, taking a static pressure reading on a tank of this nature. So my theory is some manufacturers will only state the static head as from the base of the tank. That's why the conflict!

 

[doitall]

Read Monoxides post again Dickie it's a trick.

 

[monoxide62]

So my theory is some manufacturers will only state the static head as from the base of the tank. That's why the conflict!

Ok , lets make this easy.......................You have a cistern that is 300mm high , the water level is 150mm from the bottom of cistern , static head would be 150mm from base of cistern to water level.................say you have a cistern 20 metres tall with the water level 10mm from the base of cistern , static head would be 10mm from base of cistern to water line , this is irespective of how many gallons/litres are stored within that cistern. jeez........

 

[xr4x4]

Ok, can you increase the water pressure by increasing the size of the tank or having more water in the tank?

Simply put...

 

[cantaloup63]

Ok, can you increase the water pressure by increasing the size of the tank or having more water in the tank?

Simply put...

use an accumulator

On the question of head, would it not be more sensible to use the centre of gravity of the body of water?

Another question that springs to mind is the shape of the tank - would an inverted cone shape produce a different rate of change of head compared to a cone the other way up?