NORTH CAROLINA AGRICULTURAL AND TECHNICAL STATE UNIVERSITYCIVIL,ARCHITECTURAL AND ENVIRONMENTAL ENGINEERINGCAEE 363 Engineering Fluid Mechanics & Hydraulics Lab Page 1of 8Calibration of a Pressure Gage U Purpose The objective of this experiment is to verify the accuracy of a pressure gage with a dead weight gage tester. U Apparatus Dead Weight pressure gage Weights (masses) Scale 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. U Theory: Pressure Measurement (See chapter 3 of Fluids text) One of the fundamental qu antities 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 zeroand local atmospheric pressure. When a pressure is expressed as a difference between its value and the local atmospheric pressure, it is calledgage pressure. Gage pressure can be negative but absolute pressures are always positive.
The objective of this experiment is to verify the accuracy of a pressure gage with a dead weight
gage tester.
UApparatus
Dead Weight pressure gage
Weights (masses)
Scale
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 changeson the pressure are negligible. The pressure at the piston face, therefore, is equal to the pressure
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 submergedin 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, isgiven in millimeters of mercury and H is measured in the same units the pressure at A may be
Theory: Measurement of Gage PressureTo 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.
Another device for measuring gage pressures is the Bourdon tube gage. A Bourdon gaugeuses a coiled tube which as it expands due to pressure increase causes a HrotationH of an armconnected 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
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 willhappen with the pressure gage system if you add weights to the system.
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 thecylinder 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 inthe 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 beconsistent with what you should obtain from a theoretical calculation, explain why you feel the
error occurred.
Accuracy/Precision: Identify sources of accuracy and precision.
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