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Studying the basic concepts of Fluid Mechanics, applied to pressure gauges, and looking at schematics in many places, a question came into my mind: Where is the right place to put the pressure gauge to measure the pressure of a tank?

                          

The first case would be if the tank contains a gas. In this situation, Çengel's Fluid Mechanics book clarified it to me:

Since the gravitational effects of gases are negligible, the pressure anywhere in the tank and at position 1 has the same value.

                                            

Thus, I can put it anywhere in the tank if it contains a gas.

The second case would be if the tank contains a liquid, especially when the tank is large. In this situation, the decision that seems more logical to me is to put the pressure gauge in the bottom of the tank. However, in all the places that I looked, the point "A" was the chosen one to measure pressure (as shown in the images below in points M, N, A and B), which I believe that gives the average pressure of the tank because the point is located at height of its geometric center:
$$p_average=frac1H cdotint_0^Hgamma h ,dh=fracgamma H2=p_A$$

    _{Images sources: MATHalino/PennState College of Engineering (MNE)/The SensorsGuide/University of Sydney (MDP)/ScienceStruck/Chegg}

So, where is the right place to put it to measure pressure of a tank? Why the points M/N/A/B were chosen instead of the botton of their tanks to calculate the pressure in the images above?

In general, there is no "pressure" to measure, because pressure is a field with one pressure at each place. You can measure the pressure anywhere you like, and that will be a correct measurement of the pressure at that location. Often there is only one system variable (head if the tank is open to the atmosphere, system pressure if it is compressed), which means that given one pressure measurement the pressure anywhere else can be solved for. Of course there are some engineering motivations for choosing some spots over others.

If you don't put it at the bottom, your gauge will quit working whenever the tank level drops below wherever you mounted it. In real engineering situations, when the bottom of the tank has a complicated shape (due to submerged equipment for example) it can simplify things to put the gauge above the "complicated" volume so that the fluid above the gauge always has a simple shape.

Unlike a sensor, a gauge has to be where you can see it. Often you dont want the process liquid to get in the gauge, so the gauge is on top of the liquid level.

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. Ex: it's possible to infer tank level from pressure. In that case, you probably want the pressure measurement at the bottom of the tank. On the other hand, if you are worried about something in the vapor space of the tank, you want the pressure measurement on the top of the tank.