NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Calibration of a Pressure Gage
Purpose
The objective of this experiment is to verify the accuracy of a pressure gage with a dead weight
gage tester.
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Apparatus
Dead Weight pressure gage
Weights (masses)
Scale
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The figure below is a schematic of a dead weight tester. There are three primary components of
this device: a fluid that transmits the pressure, a weight and piston used to apply the pressure,
and an attachment point for the gage to be calibrated. The weight applies a force over a precisely
known area, thereby applying a known pressure to the fluid. The fluid is water that is essentially
incompressible. Since a dead weight tester is relatively compact the effect of elevation changes
on the pressure are negligible. The pressure at the piston face, therefore, is equal to the pressure
throughout the water in the tester.
Figure: Pictures of the two types of pressure gages in the Fluids Lab
Note: It is important to ensure that no bubbles are in the tubing prior to taking the gage reading.
The adjusting piston is used to make sure that the vertical piston is freely floating on the water.
Theory: Pressure Measurement (See chapter 3 of Fluids text)
One of the fundamental quantities of interest in hydraulics is pressure. Because of its
importance, many methods have been developed to measure pressure. Pressure may be expressed
with reference to any arbitrary datum. The usual data (plural of datum) are absolute zero and
local atmospheric pressure. When a pressure is expressed as a difference between its value and
the local atmospheric pressure, it is called gage pressure. Gage pressure can be negative but
absolute pressures are always positive.
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NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Figure: Example Gage and Absolute Pressure Scales
Theory: Measurement of Absolute Pressure
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To determine absolute pressure, a barometer is needed. The two common forms of
barometers are mercury barometers and aneroid barometer. A mercury barometer consists of a
glass tube closed at one end, filled with mercury, and inverted so that the open end is submerged
in mercury. The height of the column, H, can be determined by suitable graduate scales. The
room above the mercury contains vapor of mercury. If the pressure of the mercury vapor, hv, is
given in millimeters of mercury and H is measured in the same units, the pressure at A may be
expressed as
hv + H = HA (in mm Hg)
Where HA is pressure at A. The value, hv is a function of
temperature but it is very small at normal room
temperatures. The barometer reading is also affected by
surface tension properties at the fluid’s surface.
The aneroid barometer contains an evacuated
metal chamber. As the outside pressure changes the
chamber walls deflect, pulling on a mechanical linkage
to which is attached to a pointer causing the pointers to
move.
Figure: Schematic of a Mercury
Barometer
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NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Theory: Measurement of Gage Pressure
To measure gage pressure there are many types of devices. The standard device for
measuring gage pressure or any pressure differential is the U-tube manometer. Manometers are
devices which use liquid columns for determining differences in pressure. For measuring small
positive gage pressures the most elementary manometer, usually called piezometer, can be used.
To measure negative gage pressures or high positive pressures, U-tube manometers are used.
While a manometer is quite simple, it is often awkward to use.
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Another device for measuring gage pressures is the Bourdon tube gage. A Bourdon gauge
uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm
connected to the tube. Below are two images that will help you understand the operation. The
pressure element in this device is a hollow, curved, flat metallic tube closed at one end. The other
end is connected to the pressure to be measured.
H
H
Figure: Bourdon Tube Gage
When the internal pressure is increased, the tube tends to straighten, pulling on a linkage
to which is attached a pointer and causing the pointer to move. The dial reads zero when the
inside and the outside of the tube are at the same pressure. Because of its inherent construction,
the gage measure pressure relative to the pressure of the medium surrounding the tube, which is
the local atmosphere. It is a cheap, rugged and reliable tool.
For increased accuracy and speed of response, an electro-magnetic pressure transducer
can be used. In this device a magnetic circuit is established through a flexible metal membrane.
As the membrane is deflected, the characteristics of the circuit are altered. This information is
demodulated and displayed on a voltmeter.
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NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Procedure
For this experiment, a pressure gage system is filled with water and you will add weights on a
piston. As a group before you conduct your experiment, make a hypothesis as to what will
happen with the pressure gage system if you add weights to the system.
Weights added
Pressure Gage
Our Hypothesis is:
Listen and pay attention to the instructions explained to your group for the procedure. For the
procedure, your team will write the procedure you use in class. Do not forget to take a picture of
your pressure gage apparatus and your team conducting the experiment (3 - 5 photos should be
enough). Make sure someone from the team is recording what your team does for the procedure.
You will be graded on the quality of your procedure.
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NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Results and your Lab Report: Write a scientific lab report
(DO NOT COPY TEXT DIRECTLY FROM THE LAB EXPERIMENT WRITE-UP)
Introduction: A description of the experiment and purpose in your own words
Background: Provide a description explaining the theory associated with the experiment.
Hypothesis: Your group’s hypothesis for the experiment
Data Collected: A clear presentation of the data collected in the experiment
Data Calculations:
1) We know P = F/A. This will provide you with the dead weight pressure inside of the
cylinder and piston.
2) For each gage tested, draw two curves like those in Graph 2.2 and Graph 2.3 on page 6.
Graph 2.2 is a plot of the pressure indicated on the gage versus the pressure of the oil in
the dead weight tester. Graph 2.3 is the error in the pressure gage (assuming the dead
weight tester is perfectly accurate).
Data Analysis: Identify Input and Output. Perform calculations needed for the experiment.
Provide equations and provide sample calculations for at least one set of data. In a professional
format (eg. Table, graph, etc), present your final data output and calculations.
Statistical Analysis: Experiments are typically run in triplicate so you can calculate the
average vale with standard deviation (always perform this calculation for each data set
collected). Summarize sources of error in your experiment and if data does not appear to be
consistent with what you should obtain from a theoretical calculation, explain why you feel the
error occurred.
Accuracy/Precision: Identify sources of accuracy and precision.
Results and Discussion (tips for things to include in your discussion)
1. Briefly explain the principle involved in the deadweight gage tester. How is the pressure
generated? How is it transmitted to the gage? How is the pressure level controlled?
2. What is the maximum error to be expected when this gage is used to measure pressure? Is
this error more likely to happen at low or high pressures? Is there a range of pressures for
which the gage gives significantly more (or less) accurate readings?
3. Is there any difference in the calibration errors between the data taken in order of increasing
pressure, and the data taken in order of decreasing pressure? If so, give a plausible
explanation for this error.
4. Could you use this apparatus to calibrate a vacuum gage? How?
THIS IS THE REPORT FORMAT YOU WILL USE FOR ALL SUBSEQUENT REPORTS
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NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Page 6 of 8
NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
Data Collection
PRESSURE GAGE AND DEAD WEIGHT EXPERIMENT
Piston mass = ________________
Piston Diameter = _________________
Gravitational Acceleration = __________________
Piston Area = _____________________
Gage Readings
Mass of blocks added (kg)
Initial Pressure (Kpa)
Block 1 _________(kg)
Block 2 _________(kg)
Block 3 _________(kg)
Block 4 _________(kg)
Block 5 _________(kg)
Block 6 _________(kg)
Final Pressure (Kpa)
Corresponding
Pressure as blocks
are added (KPa)
Trial 1
Corresponding
Pressure as blocks
are added (KPa)
Trial 2
Corresponding
Pressure as blocks
are added (KPa)
Trial 3
Avg. gage
Pressure
(Kpa)
G
Actual dead
weight pressure
(Kpa)
P
Absolute gage
Error (Kpa)
E
% gage Error
= E/P * 100
NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITY
CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING
CAEE 363
Engineering Fluid Mechanics & Hydraulics Lab
PRESSURE GAGE AND DEAD WEIGHT EXPERIMENT
Gage Readings
Mass of blocks removed
(kg)
Corresponding
Pressure as blocks
are added (KPa)
Trial 1
Corresponding
Pressure as blocks
are added (KPa)
Trial 2
Corresponding
Pressure as blocks
are added (KPa)
Trial 3
Avg. gage
Pressure
(Kpa)
G
Actual dead
weight pressure
(Kpa)
P
Absolute gage
Error (Kpa)
E
% gage Error
= E/P * 100
Initial Pressure (Kpa)
Block 1 _________(kg)
Block 2 _________(kg)
Block 3 _________(kg)
Block 4 _________(kg)
Block 5 _________(kg)
Block 6 _________(kg)
Final Pressure (Kpa)
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