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  1. #40
    Join Date
    Feb 2004
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    Quote Originally Posted by heatingman View Post
    I would call that a Venturi effect.


    Sent from my iPhone using Tapatalk
    The same effect. very related. Like a carburetor or that air powered cool/heat suit. The vortex something or other. I usually think of a venturi as a device and vena contracta as an effect. Often something not planned or sometimes intentional like in a diffuser design.
    The name just means a vane ( or jet) contracting on itself.
    A lot of staitc measurements go south because this effect is common. Common in resi ducts when extended plenums are used. No mechanical way to raise pressure after takeoffs and velocity takeoffs on the end of the duct.
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  2. #41
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    Feb 2016
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    Louisburg Kansas
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    Thread Starter
    One reason static pressure measurements in an HVAC system are unreliable is the air is not static. The measured pressures are dynamic and Pascal's law does not apply. Pascal's law that states the pressure is the same in all directions is only true for fluids (air) at rest. In a system where the air is pressurized but is not moving the air molecules bang into each other and fill the entire space until all pressures are the same. That happens almost instantaneously. In a duct since air is compressible the molecules don't reach steady state position because they are moving down the duct and their position changes with the effects of duct friction, elbows and etc. and the air is being compressed and decompresses constantly.
    More later.
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  4. #42
    Join Date
    Mar 2018
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    Because your readings are not purely static pressure - there will be also velocity pressure due to turbulence and/or incorrect use of instrument.

  5. #43
    Join Date
    Aug 2019
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    Central Florida
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    I find the most reliable static pressure readings are obtained by using a pitot tube and pulling it up against the inside of the duct or unit casing . In my opinion the most reliable discharge static is obtained by taking a reading on each side of the duct ( 4 readings ) and averaging them.

  6. #44
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    Feb 2016
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    Thread Starter
    The term static pressure in a dynamic system in my opinion is a contradiction in terms. I have always taken static pressures at the equipment because it is required but I don't have much trust in them. If you can get a good run of duct near the fan inlet that is your most reliable reading.
    No man can be both ignorant and free.
    Thomas Jefferson

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