Hello,

 

Greetings.

 

Maybe I can answer a question before it's even asked.

There is no such thing as suction, there are only pressure differentials. So what we call suction is due to ambient atmospheric pressure pushing its way into a space with lower pressure (ie. vacuum). For low pressure differentials, a pump can be on either end of a pipe. A vacuum on the high end allows atmospheric pressure (about 15 psi at sea level) to push the liquid uphill. Since a column of water exerts about 0.5psi per foot, no pump can suck water up more than about 30 feet. If you need to pump higher than that, the pump MUST be at the bottom where the water is. Then the height you can pump to is limited only by the mechanicals of the pump.

 

Hello,
i pressed tab to indent my first paragrap and it doesnt tab, it goes to the buttons below then it just posted. I thought this thread was then locked

Anywho....

I am curious about the physics behind a pump's inlet and outlet pressures. Say you had a parastaltic pump above a bucket with both inlet and outlet of equal length. they were submerged in water. Would the inlet pressure be equal to the outlet pressure? The pressure to raise a fluid inside of a container would be related to the pressure outside pushing the fluid up, but would the fluid leaving the other side then require the same pressure in order to overcome the water head?

Maybe I can answer a question before it's even asked.

There is no such thing as suction, there are only pressure differentials. So what we call suction is due to ambient atmospheric pressure pushing its way into a space with lower pressure (ie. vacuum). For low pressure differentials, a pump can be on either end of a pipe. A vacuum on the high end allows atmospheric pressure (about 15 psi at sea level) to push the liquid uphill. Since a column of water exerts about 0.5psi per foot, no pump can suck water up more than about 30 feet. If you need to pump higher than that, the pump MUST be at the bottom where the water is. Then the height you can pump to is limited only by the mechanicals of the pump.

yup, 10.3 meters or the like is the max height. Thanks for the Help. sorry for the incomplete post, please see above for complete version

 

I am curious about the physics behind a pump's inlet and outlet pressures. Say you had a parastaltic pump above a bucket with both inlet and outlet of equal length. they were submerged in water. Would the inlet pressure be equal to the outlet pressure?

At zero to low speed pumping, yes, both sides would be at roughly the same absolute pressure, which would be below ambient. As the flow rate increases, a dynamic delta-P will develop across both hoses. This will move the two pressures in opposite directions; reducing the input and increasing the output pressure. Eventually you could reach the maximum flow and pressure differential that the pump is capable of. Or blow a hose.

 

At zero to low speed pumping, yes, both sides would be at roughly the same absolute pressure, which would be below ambient. As the flow rate increases, a dynamic delta-P will develop across both hoses. This will move the two pressures in opposite directions; reducing the input and increasing the output pressure. Eventually you could reach the maximum flow and pressure differential that the pump is capable of. Or blow a hose.

Thanks very much!
This info is what i am after. The premise for this question was in relation to dialysis catheters. I am curious why the arterial side of a pumping catheter seems to have more (or less depending on the way you look at it) pressure than the venous side. When i say "it depends on the way you look at it" i mean;
would it be recognized as pressure if you have a negative reading such as a vacuum? For example: if you place a gauge on the inlet of a pump and it read -100mmHg, could this be interpreted as a pressure of 100mmHg on the opposite side of the gauge? Would it be considered pressure relative to your reading.
If it is, then why id a pumps inlet pressure not similar to its outlet pressure?

Greetings.

hello! sorry i didn't respond earllier. Nice to meet you. Sorry for the incomplete post!

Hello,
Once again its nice to see friendly people on here that just pop up to say hello!

Nice to meet you.

 

I am curious why the arterial side of a pumping catheter seems to have more (or less depending on the way you look at it) pressure than the venous side.

This is outside my wheelhouse. In this case you have the issue of arterial pressure and the pump might be operating as more of a flow-control regulator than as a pump. I believe it would be possible to have higher inlet pressure than outlet pressure with a peristaltic pump, and it would be limiting the flow instead of causing the flow. That's one reason we use peristaltic (positive displacement) pumps; they produce a reasonably steady flow rate independent of the differential pressure at the inlet and outlet. I don't really know about how a dialysis pump might be operating and don't want to speculate further.

...would it be recognized as pressure if you have a negative reading such as a vacuum? For example: if you place a gauge on the inlet of a pump and it read -100mmHg, could this be interpreted as a pressure of 100mmHg on the opposite side of the gauge? Would it be considered pressure relative to your reading.
If it is, then why id a pumps inlet pressure not similar to its outlet pressure?

There are two commonly used pressure scales, absolute and relative, or "gauge" pressure. If you stick to absolute pressure, it's always a positive number because the reference pressure is total vacuum. This scale is widely used for people working at low pressures. A piece of vacuum equipment for instance will be rated by how few millibars of pressure it can achieve. It would be cumbersome to compare -14.6669 psi to -14.7 psi and have to keep correcting for altitude and such. For many other applications such as measuring and reporting the pressure in your tires, it's far more sensible to use gauge pressure.

The pump and bucket scenario can be modeled and predicted fairly well. When you introduce another pump into the system - the person's heart - it becomes more complicated. Since it's a closed system (right?), I suppose ambient pressure is not an important factor in determining the pressures seen at the pump.

 

. That's one reason we use peristaltic (positive displacement) pumps; they produce a reasonably steady flow rate independent of the differential pressure at the inlet and outlet. I don't really know about how a dialysis pump might be operating and don't want to speculate further.

My wife does dialysis, luckily so far she just does the peritoneal type. When we both had to go through the training for it, we had to watch the machine/pump style too. They or at least the center she goes to, do use peristaltic pumps, they are on the outside of the dialysis machine, under a clear cover. This is so the plastic pump tubing can be change with each patient.

 

Here's a similar question that was presented in a book "Thinking Physics".

If a lawn sprinkler spins in a given direction when water is forced through it, which direction would it spin if it were connected to a vacuum pump?

 

If you stick to absolute pressure, it's always a positive number because the reference pressure is total vacuum

Hello Wayneh,
this does help to click the idea into my mind. I was constantly thinking of the pressure from a gauge stand point but thinking of it in therms of absolute zero does make sense. Thank you for this insight.

I believe it would be possible to have higher inlet pressure than outlet pressure with a peristaltic pump, and it would be limiting the flow instead of causing the flow.

This also makes sense to a point, but then is strikes another question. Say the pump was limiting the flow, would the pressure against the inlet side be then transferred (in a friction-less whiteboard world of course) to the mechanism on the output side? Im thinking of a waterwheel in my mind. But as i type this it does seem reasonable that since a peristaltic pump just basically pinches the tube and moves the water that there could easily be a pressure differential there.
The system is also a closed system as you predicted.

My wife does dialysis, luckily so far she just does the peritoneal type.

Hello shortbus,
i am very sorry to hear of your wife's misfortune. I would be curious to get a patients actual data on the venous/arterial pressure as well as a pumping flow rate if you were comfortable supplying this information?

 

Hello shortbus,
i am very sorry to hear of your wife's misfortune. I would be curious to get a patients actual data on the venous/arterial pressure as well as a pumping flow rate if you were comfortable supplying this information?

I'm sorry but don't have access to that type of information. My wife does the peritoneal type of dialysis at home while she sleeps. It is done by filling the peritoneal cavity with dialysis fluid, basically just Ringers Lactate, and then draining it out after a 'dwell period', several times during the night. While the machine has a pump to both fill and drain, I have know way of knowing the pressure used. But it is a very low pressure.

The venial type dialysis only has to over come the blood pressure, I think. And is probably done with some type of pressure feedback from the input side of the machine. If the pressure was too much higher than the blood pressure an artery/vein could get damaged. My wife does have a fistula implanted in case she need to go on a venial type machine at a later date. They implanted both the fistula and peritoneal tube at the same time. She lost a kidney in 1980, and only had to start dialysis a few years ago.