A long nededle verus a short one

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Discussion Overview

The discussion revolves around the effects of needle length and diameter on pressure readings during kidney dialysis. Participants explore the underlying physics principles, including the Venturi effect and Poiseuille's law, while considering the implications for fluid dynamics in medical applications.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Stuart Mo notes a difference in arterial and venous pressures when using 1-inch versus 3/5-inch needles during dialysis.
  • Some participants suggest that the internal diameter of the needle may significantly affect the pressure readings, rather than just the length.
  • One participant introduces the Poiseuille effect, indicating that friction between the fluid and needle walls could influence pressure changes.
  • Another participant emphasizes that the pressure drop occurs within the needle due to increased flow rate, particularly in smaller diameter needles.
  • There is a discussion about whether the friction difference is negligible over the short length of the needles, with some arguing that it could still be significant depending on the flow dynamics.
  • One participant questions the application of the Venturi effect, clarifying that the pressure difference is due to resistance to flow rather than the Venturi principle.
  • Concerns are raised about potential cell damage associated with different needle sizes, with a suggestion to consult a biologist for insights.

Areas of Agreement / Disagreement

Participants express differing views on the primary factors affecting pressure changes, with some emphasizing needle diameter and others focusing on length and flow dynamics. The discussion remains unresolved regarding the exact contributions of these factors.

Contextual Notes

Participants note that assumptions about fluid viscosity, flow rates, and needle dimensions are critical to understanding the pressure changes, but these values are not consistently provided or agreed upon.

Who May Find This Useful

This discussion may be of interest to professionals in medical fields, particularly those involved in vascular access and dialysis, as well as students and researchers studying fluid dynamics in biomedical applications.

smott2011
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I'm a vascular access nurse for a Kindey dialysis center my question is this; when we cannulated with a 1 inch needle and run the pump at 400 we get art and venous pressures of 240 to 280. When we use the 3/5 inch needles the pressure drops to 160 to 180 what is the law of physics for this in novice terms
Stuart Mo
 
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I wouldn't say the length has much bearing on this, you'll probably find the internal diameter of the needle is different.

Giving you are pumping at a constant pressure, the smaller the internal diameter of the needle, the faster the flow through it and so the lower the pressure.

It's the venturi effect.
 
smott2011 said:
I'm a vascular access nurse for a Kindey dialysis center my question is this; when we cannulated with a 1 inch needle and run the pump at 400 we get art and venous pressures of 240 to 280. When we use the 3/5 inch needles the pressure drops to 160 to 180 what is the law of physics for this in novice terms
Stuart Mo

I think the Poiseuille effect is what's needed here - there is friction between the fluid and the walls of the needle.

(Pi-Po)=\frac{8\mu L Q}{\pi r^4}

where Pi and Po are the input and output pressures, L is the length of the needle, Q is the volume flow rate, r is the needle radius, and \mu is the viscosity.

If I understand what you are saying, Pi=400, Po=260 for the 1 inch needle, Po=170 for the 0.6 inch needle. So if \mu,Q, and r stayed constant, (Pi-Po)/L should stay constant, but it does not: (400-260)/1 is much smaller than (400-170/0.6. That means that something else is changing, the radius of the needle or the flow rate thru the needle (not the viscosity).

Do you know the radius and the flow rate for the two needles?
 
Rap said:
I think the Poiseuille effect is what's needed here - there is friction between the fluid and the walls of the needle.

(Pi-Po)=\frac{8\mu L Q}{\pi r^4}

where Pi and Po are the input and output pressures, L is the length of the needle, Q is the volume flow rate, r is the needle radius, and \mu is the viscosity.

If I understand what you are saying, Pi=400, Po=260 for the 1 inch needle, Po=170 for the 0.6 inch needle. So if \mu,Q, and r stayed constant, (Pi-Po)/L should stay constant, but it does not (400-260)/1 is much smaller than (400-170/0.6. That means that something else is changing, the radius of the needle or the flow rate thru the needle (not the viscosity).

Do you know the radius and the flow rate for the two needles?

The friction difference over such a small length is negligible. You're talking of 2/5 of an inch.

If all values of your above are constant aside from Po and L, the change in L doesn't make up for the change in Po (a conclusion you draw yourself).

The flow rate will be determined by the diameter of the needle.

So I stand by the needles internal diameter conclusion.
 
jarednjames said:
The friction difference over such a small length is negligible. You're talking of 2/5 of an inch.

3/5 of an inch, but point taken.

Well, we both agree the radius has an effect. I have not checked the pressure drop we might expect from the Poiseuille effect, but if you know it is small, I will take your word for it. But if there is no friction in the needle, then there is no other force on the fluid while it is in the needle, and that means there is no pressure drop across the needle.

That would mean the pressure drop occurs before (or at the entrance to) the needle, yet is caused by the needle. I agree, the Venturi effect covers that, the Venturi effect says

(Pi-Po)=\frac{\rho}{2}(v_i^2-v_o^2)=\frac{\rho Q^2}{2}(1/A_i^2-1/A_o^2)

where Ai is the larger input cross sectional area and Ao is the cross sectional area of the needle, and Q is again the volumetric flow rate.

So if we could get the radii and flow rates for the two needles, we would know a lot more about what's going on. So again I ask Smott2011 if the radii and flow rates are availiable for the two cases.
 
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Rap said:
3/5 of an inch, but point taken.

You have a 1 inch needle (5/5 inch) and a 3/5 inch needle - the difference is 2/5 inch.
Well, we both agree the radius has an effect. I have not checked the pressure drop we might expect from the Poiseuille effect, but if you know it is small, I will take your word for it. But if there is no friction in the needle, then there is no other force on the fluid while it is in the needle, and that means there is no pressure drop across the needle.

That would mean the pressure drop occurs before the needle, yet is caused by the needle. I agree, the Venturi effect covers that, but the Venturi effect says (using Q as volume flow rate again).

So if we could get the radii and flow rates for the two needles, we would know a lot more about what's going on. So again I ask Smott2011 if the radii and flow rates are availiable for the two cases.

We don't need the flow rates, we can calculate them with entry/exit pressures and needle dimensions.

The pressure drop occurs within the needle due to the flow rate increase. The smaller the diameter the greater the flow rate inside and the lower the pressure. That's really all there is to it.
 
jarednjames said:
You have a 1 inch needle (5/5 inch) and a 3/5 inch needle - the difference is 2/5 inch. We don't need the flow rates, we can calculate them with entry/exit pressures and needle dimensions.

Yes, as long as we are sure of our model.

jarednjames said:
The pressure drop occurs within the needle due to the flow rate increase. The smaller the diameter the greater the flow rate inside and the lower the pressure. That's really all there is to it.

For the Venturi effect, I think the pressure drop occurs at the interface where the radii change. Unless there is an opposing force along the length of the needle (i.e. viscous), there will be no pressure gradient along the length of the needle.

PS - sorry for the Venturi equation confusion.
 
jarednjames said:
The friction difference over such a small length is negligible. You're talking of 2/5 of an inch.

That depends on whether viscous or pressure/inertial forces are more important here. That is a difference of 40%, which would be pretty significant if the flow was dominated by viscosity effects. Of course, given the stated data, I would expect that the internal diameter between the two needles is significantly different, which would also need to be taken into account.
 
[OK everything is the process pump speed all the same they only difference is the length.1 Inch versus 3/5 inch. I have been told that if you had a garden hose 50 feet long and one 250 long you would have a different pressures this the venturi effect. The reason i need this info is that I'm doing a article for a magazine. Where would i be able to find articles to support this . One more question would there be any cell damage the 3/5 versus the 1 inch? Human blood flows threw

Thank Yo
 
  • #10
smott2011 said:
I have been told that if you had a garden hose 50 feet long and one 250 long you would have a different pressures this the venturi effect.

No, that is not the venturi effect.

The difference in pressure here is due to resistance to flow. There will be R resistance per foot so a 50 foot hose will have 50 x R and 205 foot will have 250 x R - that gives you the difference in pressures.
The reason i need this info is that I'm doing a article for a magazine. Where would i be able to find articles to support this .

You won't, as above. It's wrong.
One more question would there be any cell damage the 3/5 versus the 1 inch? Human blood flows threw

If you're writing for a magazine, note spelling and grammar = threw should be through.

This is a question to ask a biologist, not a physicist.
 
  • #11
jarednjames said:
The friction difference over such a small length is negligible. You're talking of 2/5 of an inch.

If all values of your above are constant aside from Po and L, the change in L doesn't make up for the change in Po (a conclusion you draw yourself).

The flow rate will be determined by the diameter of the needle.

So I stand by the needles internal diameter conclusion.

Everthing is the same diameter flow the only difference is the length 1 inch versa 3/5. we get the same results on thousands of patints[
 
  • #12
smott2011 said:
Everthing is the same diameter flow the only difference is the length 1 inch versa 3/5. we get the same results on thousands of patints[

In which case, it is down to the viscosity of blood and the length is the controlling factor - assuming you haven't forgotten something.

I didn't realize how much 'thicker' blood is than water, according to the numbers it's about 4 times more viscous. So it could certainly account for it over the length.

You need to look up viscous flow and flow through pipes. Churn some numbers and you should come out with your answers.
 
  • #13
smott2011 said:
Everthing is the same diameter flow the only difference is the length 1 inch versa 3/5. we get the same results on thousands of patints[

Do you know if there is any difference in the rate that blood flows through the needle? Is there any measurements that tell how much blood is flowing through per minute?
 
  • #14
The flow is always the same needle size is the same except for the length 1 inch versus 3/5 inch. so what do you all thing is the physics equations for this?
 
  • #15
smott2011 said:
The flow is always the same needle size is the same except for the length 1 inch versus 3/5 inch. so what do you all thing is the physics equations for this?

Well, there are two effects - Venturi effect and Poiseuille effect. It would really help if you knew the diameter of the two needles (inside and outside). Is it like a hypodermic needle? Do you know what the flow rate is?. Do you have any of these numbers? Do you know how much total blood is being recirculated? Do you know how much time it takes to to do one complete recirculation? Anything will help.
 
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  • #16
Rap said:
Well, there are two effects - Venturi effect and Poiseuille effect. It would really help if you knew the diameter of the two needles (inside and outside). Is it like a hypodermic needle? Do you know what the flow rate is?. Do you have any of these numbers? Do you know how much total blood is being recirculated? Do you know how much time it takes to to do one complete recirculation? Anything will help.

I must concur.

Physics won't say anything without these numbers.

It appears you just want an equation you can show off with for the magazine, but that alone doesn't mean anything. If that's all you want, you might as well just give the stuff related to the venturi effect and be done with it.
 
  • #17
outer diameter 1.81 to1.85mm
 
  • #18
smott2011 said:
outer diameter 1.81 to1.85mm

How about flow rates?
 
  • #19
all are 400
 
  • #20
smott2011 said:
all are 400

That's a pressure not a flow rate isn't it?
 
  • #21
smott2011 said:
all are 400

In order to do this right, we really have to get flow rates, including units. You have to follow 400 by units, like liters per second or something. And yes, 400 was the input pressure, 400 psi, that's not the flow rate.
 
  • #22
OK i will have that for you tomorrow
 
  • #23
400 hundred milliliters per minut
 
  • #24
smott2011 said:
400 hundred milliliters per minut

Lovely, I don't suppose you have the pressures? (I understand you gave the outlet side, what about the inlet?)

I'd also request the internal diameter of the needles (I'm under the impression the one you gave was external?)
 
  • #25
jarednjames said:
Lovely, I don't suppose you have the pressures? (I understand you gave the outlet side, what about the inlet?)

I'd also request the internal diameter of the needles (I'm under the impression the one you gave was external?)

I checked needles, assuming it was OD. The corresponding ID is 1.372 mm. Heres my Poiseuille calculation

\mu=3.5e-3 kg m/sec
L=2.54e-2 m (1 inch)
Q=6.66e-6 m^3/sec (400 ml/min)
r=6.86e-4 m

Using

\Delta P=\frac{8\mu L Q}{\pi r^4}

I get delta P = 6808 pa = 0.99 psi.

As I understood the input pressure was 400 psi, output pressure 260 and 170 psi for the 1 and 0.6 inch needle respectively. This tells me its not Poiseuille effect.

The problem with the Venturi effect is that it is not a function of needle length. Unless there is a third possibility, this tells me that the problem we are solving is not the problem at hand.

Check out http://pubs.acs.org/doi/abs/10.1021/es00141a021 - I do not have a subscription, but I would be interested in what it says.
 
  • #26
Rap said:
As I understood the input pressure was 400 psi, output pressure 260 and 170 psi for the 1 and 0.6 inch needle respectively. This tells me its not Poiseuille effect.

The flow rate is 400, not the pressure (or at least I'm waiting for clarification on that).
The problem with the Venturi effect is that it is not a function of needle length. Unless there is a third possibility, this tells me that the problem we are solving is not the problem at hand.

I'd be interested to know if both lengths of needle have the same internal dimensions.
 
  • #27
smott2011 said:
I'm a vascular access nurse for a Kindey dialysis center my question is this; when we cannulated with a 1 inch needle and run the pump at 400 we get art and venous pressures of 240 to 280. When we use the 3/5 inch needles the pressure drops to 160 to 180 what is the law of physics for this in novice terms
Stuart Mo

jarednjames - good point.

smott2011 - Please state what "240 to 280" means. Is that pressure in psi, mm of mercury?
Same for "160 to 180". Also, what exactly does "run the pump at 400" mean. 400 ml/minute? 400 psi? 400 mm of mercury?
 
  • #28
If you assume the density of blood to be the same as water (1 gram per mL) and use the numbers Rap provided, that gives a Reynolds number of approximately 1800. This suggests you are probably experiencing a pressure drop due to turbulence.

There are at least two other issues you should consider.

The coupling between the needle and the tubing from the pump may cause a noticeable pressure drop. Are the couplings the same style and dimension for both needles?

Blood is not a Newtonian fluid, so its viscosity will change depending on the shear stress. It may not be significantly different between those two pressures though.

Turbulence seems the most likely cause; 400 mL/min is a pretty high flow rate for a needle of that diameter.
 
  • #29
Skrambles said:
If you assume the density of blood to be the same as water (1 gram per mL) and use the numbers Rap provided, that gives a Reynolds number of approximately 1800. This suggests you are probably experiencing a pressure drop due to turbulence.

The dynamic viscosity of blood is approximately 4 to 5 times that of water. Big difference and something that needs to be factored in.

Bloods density is around 1060kg/m3 from what I've seen.
 
  • #30
jarednjames said:
The dynamic viscosity of blood is approximately 4 to 5 times that of water. Big difference and something that needs to be factored in.

Bloods density is around 1060kg/m3 from what I've seen.

The number I used for viscosity was about 3.5 times that of water, but it will vary from person to person and can change due to temperature and pressure.

The density you gave is almost exactly the same as water.

The Reynolds number also depends on the diameter of the pipe, in this case the needle. A small change in this variable can have a huge effect on turbidity. Just because the needles are specified at the same inner diameter doesn't mean they are actually the same. Unfortunately that dimension is extremely hard to measure.