Notices
Results 1 to 5 of 5

Thread: Fluid pressure measuring questions..please help

  1. #1 Fluid pressure measuring questions..please help 
    Forum Freshman
    Join Date
    Mar 2010
    Posts
    6
    Im in health care, I got a question about measuring blood pressure.

    When we use sphygmometer (the cuff wrap around the arms) to measure blood pressure. are we measuring static or stagnation pressure? I know it's not dynamic pressure since dynamic pressure involving measuring flow speed.

    We also use a thing called arterial line, it's a cathether inserted into blood vessel. It has only one hole open directed against blood flow. It can read the pressure and change it into a pressure waveform. What type of pressure does it measure?

    Both sphygmometer and arterial line give the same result. Please help me understand this. Thank you


    Reply With Quote  
     

  2.  
     

  3. #2 Re: Fluid pressure measuring questions..please help 
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by speedpump
    Im in health care, I got a question about measuring blood pressure.

    When we use sphygmometer (the cuff wrap around the arms) to measure blood pressure. are we measuring static or stagnation pressure? I know it's not dynamic pressure since dynamic pressure involving measuring flow speed.

    We also use a thing called arterial line, it's a cathether inserted into blood vessel. It has only one hole open directed against blood flow. It can read the pressure and change it into a pressure waveform. What type of pressure does it measure?

    Both sphygmometer and arterial line give the same result. Please help me understand this. Thank you
    The difference between static and stagnation pressure is basically the difference in pressure that would occur if the kinetic energy of the fluid were converted to heat and the temperature of the fluid raised. This is sometimes an important distinction in high velocity (like supersonic) flow of gasses, but in your application is not important. With your cuff you have effectively stopped flow in the vessel that you are measuring, so there is no flow and the measurement is basically static pressure in the vessel.

    Your arterial line is probably measuring stagnation pressure, if I understand the geometry correctly. You are inserting a probe into the flow stream, and bringing the sample to a stop, relative to the laboratory. But again the effect is insignificant.


    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Mar 2010
    Posts
    6
    ahh, great, thank you so much. I was reading stuff online but someone says stagnation pressure is a constant and it doesnt change, i was confused. so since my arterial line measures stagnation pressure and there's change in the waveform, that means stagnation pressure cant be constant in my application right?
    Reply With Quote  
     

  5. #4  
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by speedpump
    ahh, great, thank you so much. I was reading stuff online but someone says stagnation pressure is a constant and it doesnt change, i was confused. so since my arterial line measures stagnation pressure and there's change in the waveform, that means stagnation pressure cant be constant in my application right?
    Stagnation pressure is independent of velocity, for an isentropic process.

    Constant here does not mean constant in time. If your blook pressure is constant in time you are in serious trouble.

    For your application don't worry about the differences. They are not important.

    There is also a difference between static temperature and stagnation temperature. It is very important in rocketry. It does not amount to a hill of beans in medicine -- it matters more whether you use an IR probe in the child's ear or stick a mercury thermometer up the kid's butt.
    Reply With Quote  
     

  6. #5  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    Stagnation pressure is the sum of static pressure and velocity pressure and is what a Pitot tube reads. Your arterial line sounds like a Pitot tube. The person who told you stagnation pressure is constant was thinking of Bernoulli's equation which applies to steady flow, not pulsing flow.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •