Calibration of Bourdon Gauge

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CE-271 FLUID MECHANICS

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EXPERIMENT NO 05- CALIBRATION OF BOURDON GAUGE

Objectives To study and sketch the construction of a typical Bourdon Gauge and to describe how it works. To establish the calibration curve of the Bourdon Gauge

INTRODUCTION

The pressure intensity at any point in static or moving fluid can be measured using various types of  pressure measuring instrument. One of these devices devices is the Bourdon tube pressure gage. Bourdon-tube Bourdon-tube  pressure gages are most widely used now-a-days now-a-days because of their reliability, reliability, compactness, compactness, low cost and ease of use. It consists of a curved tube of elliptical cross-section bent into a circular arc as shown in Fig. 1. When pressure is applied to the tube, it tends to straighten out, and the deflection of the end of the tube is communicated through a system of levers to a recording pointer. This gauge is widely used for steam and compressed gases. The pressure indicated is the difference between the system pressure and to the external (ambient) pressure, and is usually referred to as the gauge pressure

Fig. 1: Schematic of a bourdon-tube pressure gage

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As the Bourdon tube pressure gage is used extensively, the stiffness of the internal components change from factory setup and therefore calibration is necessary to give correct pressure readings. Calibration means checking the pressure gage readings against a very accurate device. One of the calibration devices that is available in our lab is the so-called “Dead Weight Tester”

APPARATUS

This dead weight pressure gauge calibrator consists of a precision machined piston and cylinder assembly mounted on levelling screws. A Bourdon gauge is supplied for calibration. The weights supplied are added to the upper end of the piston rod which is rotated to minimise friction effects. The gauge is thus subject to known pressures which may be compared with the gauge readings and an error curve drawn.  The TecQuipment Bourdon Gauge and dead weight tester   Set of Test weights  Laboratory Scales

THE BOURDON GAUGE AND DEAD WEIGHT TESTER

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BACKGROUND

The bourdon gauge is the most popular pressure measuring device for both liquids and gasses. It can be connected to any source of pressure such as a pipe or vessel containing a pressurized fluid. The connection can either be direct or via a small tube called a capillary tube. This means that it can be mounted at any convenient location. It is also very versatile in that it can be designed to operate over virtually any range of pressures. The Bourdon gauge normally measures so called Gauge Pressure, which is the difference  between the pressure in the pressure source and the current atmospheric pressure. It can however be modified to measure difference in pressure between two sources of pressure (i.e. pressure difference or differential pressure). The Bourdon gauge is an indirect measuring device which depends for its operation on the tendency of an internally applied pressure to cause an initially bent tube (called a bourdon tube) to straighten. Because the measurement is indirect and it is necessary to calibrate the gauge before it can be use. The calibration consists of applying a known pressure to the gauge and noting the position of the gauge needle on the scale. The gauge can be calibrated in a wide variety of units to suit the particular application provided that there is a linear relationship between actual pressure and the unit of calibration.

THE BOURDON GAUGE

The TecQuipment Bourdon Gauge is fitted with a transparent dial, which lets you see the internal workings of the gauge. The gauge consists of a thin walled closed ended tube which is oval in cross section. This tube is bent through an angle of about 270  along its long axis. The open end of the tube is welded to a hollow mounting block which allows the pressurized fluid to reach the tube. This causes the  pressure from the source to be transmitted directly to the inside of the bourdon tube. The applied pressure causes the oval tube to become rounder (since a round cross section has the maximum area for a given circumference). As it becomes more rounder, the bourdon tube tends to uncurl which causes its free end to move. A system of linkages and levers transmits this motion to the gauge needle which moves over the scale. The distance moved by the needle is directly proportional to the pressure applied over the working range of the gauge. The sensitivity of the gauge depends on the material and thickness of the  bourdon tube and the quality of the instruments mechanism.

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THE TECQUIPMENT DEAD WEIGHT TESTER

To obtain very accurate pressure measurements, it is essential to regularly re-calibrate the gauge. This is because the tube tends to become weaker with extended use. The usual procedure is to apply a known pressure to the gauge using a device called a Dead Weight Tester. The normal calibration procedure is to load the gauge for known pressures, using a dead weight tester including a liquid of known specific gravity (use water as the liquid). This dead weight tester uses a simple piston and cylinder arrangement to provide a source of pressurized liquid (in the experiment water will produced a better result than oil) which is transmitted to the gauge. Since the true pressure of the liquid can be easily calculated, the value can be compared directly to the reading on the gauge over the complete scale range. (The scale range is the range of pressures from zero to the full-scale deflection value). The dead weight tester consists of a cylindrical piston which is free to move vertically in a close fitting cylinder. A  Platen is attached to the piston which can be loaded with a series of accurate weights. The pressure developed in the cylinder is transmitted via a transparent tube to the gauge under test.

PROCEDURE  The

weight of the Piston, and its cross sectional area should be noted.

 The

piston should be removed, and water is poured into the cylinder until it is full to the overflow level.

Any air trapped in the tube may be cleared by tilting and gently tapping the apparatus. A small amount of air left in the system will not affect the experiment, however, a large amount of air would cause the piston to bottom on the base of the cylinder.  The

piston is then re-placed in the cylinder and allowed to settle. A spirit level placed on the platform at

the top of the piston may be used to ensure that that the cylinder is vertically upright.  Weights

are now added in convenient increments, and at each increment, the pressure gauge reading is

observed.  A

similar set of results is then taken with decreasing weights.

 To

guard against the piston sticking in the cylinder, it is advisable to rotate the piston gently while the

 pressure gauged is being read.

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The following table shows the ideal experimental result given at A First Course in Hydraulics by Professor E. Markland.

Above Table present a typical set of results. The weights are converted from units of kilogram force to  Newton by multiplying by factor 9.81, and the true pressures by dividing force by the piston area.

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DISCUSSION

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