# Why textbooks use geometric center/centerline of the pipe when calculating/measuring pressure?

Studying Fluid Mechanics, I started to notice that almost every textbook/website uses a specific point to make calculations about the pressure in a liquid at a given depth (hydrostatic pressure): the geometric center (as shown in the images below), when presenting pressure gauges/manometers/piezometers.

Note: This happens regardless of the field to which the book is directed (I looked in textbooks of Fluid Mechanics for Civil, for Electrical, for Mechanical...).     Images sources: MATHalino/PennState College of Engineering (MNE)/The SensorsGuide/University of Sydney (MDP)/ScienceStruck/Chegg

Sources: Introduction to Fluid Mechanics - Nakayama & Boucher/Mecânica dos Fluidos - Noções e Aplicações - Sylvio R. Bistafa/Chegg

One of the textbooks I looked at even draws attention to this fact, but it doesn't explain the reason for the choice:

Note the origin of the measurement of h, in the center of the tube

Source: Mecânica dos Fluidos - Franco Brunetti

A similar behavior can be identified when textbooks present liquids in motion: they use the centerline of the pipe to make calculations/measurements. Here's an example:

Source: Fluid Mechanics for Civil Engineers - N.B. Webber

So why is the choice of geometric center/centerline of the pipe so common when measuring/calculating pressure? Some hypotheses:

• Maybe all the textbooks/websites are unconsciously copying each other?
• Maybe is this some kind of "convention"?
• Maybe it is because a point in the horizontal plane of the geometric center gives the average pressure of a tank?
• Because the geometric centre of the tank is at the same height as the centreline of the extended pipe, all the points in between are at the same potential, or they all lie on the same equipotential plane. you can choose another point, but don't forget to take the pressure difference into account. – Sam Farjamirad Oct 25 '18 at 16:11
• @Sam But what about the tube B of the first of the 6 images (left part) and the vertical tube of image below "Note the origin...of the tube"? I can't use the "same height" argument in them – Vinicius ACP Oct 25 '18 at 20:20
• you are right my mistake! My explanation fails to describe all the conditions, but here is an out, suppose you choose a point on the far right or left or up or down, then what would be the pressure at those points ? if you can answer this then your are almost there. – Sam Farjamirad Oct 25 '18 at 20:30
• @SamFarjamirad I think I found the answer, but a new question arose from that answer. Can you help me? – Vinicius ACP Oct 31 '18 at 6:29
• yeh fire it ... – Sam Farjamirad Oct 31 '18 at 6:42

I'll divide my answer in two cases. First, I'll talk about liquids in motion (assuming incompressible flow). Then, I'll talk about liquids at rest.

## Liquid Flow:

Reading the comments of this YouTube video about piezometers made by Donald Elger, I found the answer for this case:

Why is it [the pressure measurement with piezometer] taken from the middle of the pipe?

Elger's answer: The pressure variation across a section of a pipe is hydrostatic; thus, the pressure will vary linearly with radius and the pressure at the center of the pipe is the average pressure. If you use this value of pressure in your calculations, this will be give you the most accurate results. Thus, engineers nearly always apply or measure the pressure at the center of the pipe.

The question that came to me as soon as I read this was: "Why using average pressure in calculations gives the most accurate results?".

(I recommend reading my answer to this question before proceeding)

Briefly, in general, the average pressure gives the most accurate results if used in calculations because there are many applications/cases in which the locations with $$P=P_{average}$$ are the best places for experimental data collection.

In the case of a pipe, this location is its centerline. So, I believe that this is why textbooks generally choose this location in case of liquids in motion: the centerline is associated with $$P_{average}$$ that, in its turn, is associated with the best places for experimental data collection for many applications.

## Liquids at rest:

For this case, firstly I would like to quote part of the answer written by David White to my question on Physics.SE "Where is the right place to put the pressure gauge to measure the pressure of a tank?":

The location depends on why you are measuring the pressure. There will be a process reason for the pressure measurement, and that will determine the location of the pressure measuring device.

When textbooks present pressure gauges/manometers/piezometers for the first time, the presentation is usually "application neutral" (i.e., there's no process reason), the diagrams/sketches/figures are only to illustrate the concepts/formulas. Therefore there are no best points as in the liquid flow case, for two reasons:

• There is no process reason that determines the location of measurement;
• Since the liquid is at rest, there are no points that lead to most accurate results, they all provide the same accuracy.

But the authors need to choose a point to do the pressure-related calculations...

After everything I've researched, my hypothesis is that the "point choice" of hydrostatics was imported from hydrodynamics. So, instead of choosing a random point to pressure-related calculations, they choose one that at least has importance/meaning for other areas of Fluid Mechanics.

• If you have a round or cylindrical container then its also easyest to make the hole in the middle. – joojaa Nov 2 '18 at 4:34

Looks you could add it in any part of the vessel but only the heigth matters you put it in the side and people will be confused if it is in the center it is easy to understand.