Hydrostatic Pressure
Hydrostatic Pressure
Experiment 2: An Introduction to Hydrostatic Pressure
Introduction
Part I: Barometer Measurements
Theory
Barometers are used to determine the atmospheric pressure at their given location. These
devices typically consist of a glass tube filled with mercury (or some other fluid), with one end
closed and the other open to the atmosphere. Above the fluid, an excellent vacuum is needed.
The surface height at which the fluid rests above the fluid reservoir indicates atmospheric
pressure according to the scale on the tube.
Apparatus
The setup of this experiment consists of the world’s tallest barometer.
Procedure
1. Record the measurements indicated by the lab (mercury) barometer.
2. Guided Discovery: To make efficient use of lab time, the group will go through a guided
discovery process to see the various unusual features of the barometer.
3. Record the measurement indicated by the Bennett Barometer.
The purpose of this experiment is to explore the concept of hydrostatic pressure. The lab is
separated into two parts. First students will explore how a barometer functions using the
concept of hydrostatic pressure. Next they will use a Mariotte Bottle to understand how the
surrounding environment of a fluid affects the hydrostatic pressure it imparts on other objects.
2
Part II: Mariotte Bottle
Theory
The Marriotte Bottle, invented around 1670, is a device capable of providing a stream of water
with constant pressure and flow rate using the hydrostatic pressure developed between the air
vent, and the fluid outlet. This deceptively simple apparatus combines water head pressure and
atmospheric pressure into a practical and instructive device. Note that the orifice flow rate
depends on the pressure head, not the height of the free water surface.
Apparatus
Students will use a modified Mariotte Bottle with multiple openings and the
central air tube and external scale acting as a manometer to investigate the
transient behavior of the device.
Procedure
1. Guided Discovery: To make efficient use of lab time, the group will go
through a guided discovery process to see the various unusual features
of the Mariotte Bottle.
2. Next break up into small groups. Make predictions of how the Mariotte
Bottle will behave with regards to the worksheet questions among your
lab partners.
3. Repeat the guided discovery using a meter stick or attached scale to
answer the worksheet questions.
Figure 1. Mariotte
Bottle Apparatus
Fluid Mechanics Laboratory
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Lab 2A Worksheet
Please write answers to the following questions in the space below. Attach any figures or
calculations to the end. Turn in the completed packet to your lab TA one week following the
experiment at the start of lab class.
Mariotte Bottle
1. Complete the attached Table 1 (or prepare your own and attach it) noting what happens
when the stopper above, below, and at the same height of the vent tube is removed. Use
the manometer measurements to clearly explain this behavior.
2. Complete the attached Table 2 (or prepare your own and attach it) to calculate the flow
rate when a stopper below the vent tube is removed. Is it constant? Perform at least three
trials. How would the theoretical flow rate be determined (see your text and describe using
the features of the Mariotte bottle)?
Fluid Mechanics Laboratory
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3. Complete the attached Table 3 (or prepare your own and attach it) to illustrate what
happens when you move the vent tube up or down. What trend do you observe? Explain
why this occurs.
4. Explain how the Mariotte Bottle works making use of the following terms: Atmospheric
Pressure, Absolute Pressure, Partial Vacuum, and Water Head Pressure.
5. What are the height limitations of the Mariotte Bottle? How is this affected by elevation?
6. Explain why the Mariotte Bottle is used to fertilize agricultural crops and with the Double
Ring Soil Infiltrometer.
Fluid Mechanics Laboratory
Barometer Measurements
7. Using the measurements of both barometers and the known density of mercury, calculate
the density of the fluid in the Bennett Barometer.
8. Draw a figure that illustrates the pressure distribution (gauge pressure) within the Bennett
Barometer. Clearly indicate the pressure at the top and bottom of the tube and formulate
an equation that describes the pressure as a function of height above the bottom of the
tube. What is the hydrostatic pressure (roughly) just at the bottom of the tube?
Fluid Mechanics Laboratory
W2A 4/4
Table 1: Effect of air vent height relative to stopper height in a Mariotte Bottle.
Position of
Vent Tube
Fluid Outlet
Height (cm)
Air Vent
Height (cm)
Water Behavior
Observations
Manometer
Observations
Above
stopper
hole
At stopper
hole
Below
stopper
hole
Table 2: Flow rate of water from Mariotte Bottle with constant pressure head.
Trial Volume of Water
(mL)
Time
(s)
Flow Rate
(mL/s)
T
