Question about fluid flow and pressure in pipes

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Discussion Overview

The discussion revolves around the measurement of pressure in pipes, particularly in scenarios where pipes may not be completely filled with fluid. Participants explore the implications of pipe diameter changes and the conditions under which pressure readings can be accurately obtained.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a pipe must be completely filled with fluid to measure pressure, particularly in a scenario where a smaller pipe diverges into a larger one that may not be fully filled.
  • Another participant suggests that if a pipe is not filled, flow is influenced by gravity, which may affect measurable pressure, but emphasizes that the pressure gauge's location is also crucial.
  • A participant expresses concern about accurately measuring pressure when transitioning from a small diameter tube (1/8") to a larger fitting (3/4") and whether this setup would yield reliable readings.
  • It is noted that in a closed system, the presence of air is not necessary for pressure measurement.
  • One participant clarifies that if the system is closed and primed without air pockets, significant pressure changes should not be expected due to changes in pipe diameter.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the importance of system conditions (e.g., being closed and primed) for accurate pressure measurement, but there remains uncertainty regarding the effects of pipe diameter changes and the implications of partially filled pipes.

Contextual Notes

Participants mention various assumptions about system setup, such as whether the system is open to the atmosphere or if it contains air pockets, which may affect pressure readings.

Who May Find This Useful

This discussion may be useful for individuals involved in fluid dynamics, plumbing, or engineering applications where pressure measurement in varying pipe conditions is relevant.

djdoug99
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Does a pipe have to be completely filled with a fluid to measure its pressure? For instance if I have a small diameter pipe that diverges to a bigger pipe. The small pipe will be completely filled with water, but will the bigger pipe be? If the bigger pipe is only half full of flowing water, will a pressure gauge still return a reading even though there isn't direct contact between the gauge and the water?
 
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If I pipe isn't filled, flow is dependent on gravity (like, say, sewer gravity mains). You will not see any measurable pressure (i.e. any head pressure) due to flow, but that has little at all to do with the location of the pressure gauge.

However, with that said, it is important to understand your setup. Simply going from small bore to larger diameter does not necessarily mean you will have a partially filled pipe. It all depends on the setup (i.e. open to atmosphere? Is the small bore force fed? etc)
 
Thanks for your response Travis. What I'm trying to do is measure the pressure in a 1/8" tube. The place that I want to measure the pressure won't be open to the atmosphere. I'm concerned that If I break the the 1/8" tubing and put inline a bigger pvc fitting (3/4") with an attached gauge then I won't get accurate readings.
 
If the system is closed, there need-not be any air in it.
 
If I understand your meaning, you want to break a 1/8" line and install a 3/4" fitting that supports the gauge?

If the system is not open to atmosphere and there are no air pockets (i.e. the system is primed) then you will not notice a significant change in the pressure.

In a flat system (i.e. no elevation changes) the static pressure in the pipe will remain constant regardless of the pipe diameter.

This is a nice explanation http://www.irrigationtutorials.com/advice/using-a-smaller-pipe-to-increase-water-pressure/.
 

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