Medical IV's and flow

Andy Resnick

The "three reservoir problem" is exactly what you describe: two reservoirs are the two bags, and the third is the vein:

http://excelcalculations.blogspot.co...r-problem.html

Depending on the resistances of the various tubes, it's possible to have one bag drain faster than the other, or have both drain at the same rate.

wolf!

OK, I understand that:
the top bag yields the highest pressure in the line
the bottom bag yields the medium pressure in the line
the vein has the lowest pressure in the line.
the highest pressure will dominate the flow to the lowest pressure.
the high pressure will not backflow into the medium because it has a lower pressure than that to release to.
what i do not understand is: the top bag has less weight than the bottom one does, so there is less weight of water pressing on the bag outlet than the lower bag.
how does the top bag have higher pressure in the tube?

mfb

The mass in the bags does not matter, just the height of the water column is relevant. This is a bit unintuitive, but can be calculated easily. In terms of a big bag with a small hole at the bottom: Most of the weight of the water is kept by the surface of the bag, which is connected to the pole.

wolf!

I tried that online calculator, and it is indead cool, and does show an output of higher pressure. from the get go I knew the outcome was higher pressure. What I'm trying to find out is the logic why?

wolf!

I'm beginning to think the further the distance to fall creates increased velocity, which increases pressure?

Studiot

Careful with Andy's calculator. (But thanks to Andy for posting it)

Your mainline and piggyback lines contain flow restrictors which alter and complicate the pressure relationships.

There are two types of flow restrictor

The (connection) nozzles which introduce fixed pressure drops and the roller clamps which introduce variable ones.

Neither of these appear in the spreadsheet.

wolf!

r the sake of answering the question, we'll assume both have identical variables, except one is higher than the other, and absolutely nothing else is different.
I'm beginning to think the answer has not yet been found, and it is considered an accepted truth, without logic, because this thread in going in 10 different directions, except the question.

Studiot

What's the problem?

The piggyback bag produces a slighty higher pressure at the connection point due to its slighty higher elevation.

This will make the fluid flow faster in the auxiliary line.

However the fluid in either the mainline or the auxiliary can be slowed by the use of the roller clamps.

wolf!

yes, I understand that, what it does. my question is why.
Why does The piggyback bag produce a slighty higher pressure at the connection point due to its slighty higher elevation.

Studiot

It is well known that water tries to flow downhill.

So it is down hill from the upper bag to anywhere in the lower bag

The underlying driving force is gravity and we measure this tendency as pressure head which is the difference of elevation.
Downhill is the direction that makes this difference positive.

wolf!

so the reason for the higher bag draining first is not known at preset?
all that is known is that it happens?

mfb

The reason is known and was explained ~5 times here in this thread.
The higher bag leads to a higher potential pressure at the connection, this leads to a higher outflow.

Egregious

Hi Wolf!, I'm a nurse, too. From reading this thread, I think I understand the answer. Think of it this way: Imagine a really long tube extending vertically that contains an entire liter of fluid. Now, imagine a shorter tube (same diameter) extending vertically that contains only 100mL of fluid. For all practical purposes, you may ignore the volume of the bags that the tubes are connected to... Think about the very tall column of water in the 1L tube with its higher weight, pressure, and 'desire' :-) to flow first. The fluid (column) in the lower, and thus 'less tall' tube will simply have to wait...

...does that help?