# Question about fluid flow and pressure in pipes

• djdoug99
In summary: Summary In summary, if the smaller pipe is only half full of water, the pressure gauge will read the same even though there is no direct contact between the gauge and the water.
djdoug99
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?

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/.

## 1. How does fluid flow in a pipe affect pressure?

Fluid flow in a pipe can affect pressure by creating a pressure gradient. As the fluid flows through the pipe, it experiences friction with the walls of the pipe, causing a decrease in pressure along the direction of flow.

## 2. What factors influence the rate of fluid flow in a pipe?

The rate of fluid flow in a pipe is influenced by several factors, including the diameter of the pipe, the viscosity of the fluid, the length of the pipe, and the pressure difference between the two ends of the pipe.

## 3. How is Bernoulli's principle related to fluid flow in pipes?

Bernoulli's principle states that as the velocity of a fluid increases, the pressure decreases. In the context of fluid flow in pipes, this means that as the fluid flows through a narrower section of the pipe, its velocity increases and the pressure decreases.

## 4. How does the shape of a pipe affect fluid flow and pressure?

The shape of a pipe can affect fluid flow and pressure in several ways. For example, a wider pipe will result in a lower pressure gradient and therefore a lower pressure drop, while a narrower pipe will result in a higher pressure gradient and a higher pressure drop. The shape of the pipe can also affect the flow pattern of the fluid, which can in turn affect pressure.

## 5. How can fluid flow and pressure in pipes be calculated or measured?

Fluid flow and pressure in pipes can be calculated using equations such as the Bernoulli equation and the Darcy-Weisbach equation. These calculations take into account factors such as pipe diameter, fluid velocity, and fluid density. Pressure can also be measured directly using pressure gauges or indirectly using flow meters.

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