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Experiment number 02

Calibration of Inclined Tube Manometer

 

Objective:

To calibrate the inclined tube manometer used in the mine ventilation lab against the standard Askania minimeter.

 

Introduction:

Most pressure surveys in the mine are required to measure pressure between two points, so that the pressure loss over a certain section of an airway between those points, & subsequently the airway resistance can be computed. Manometer is a device for measuring the pressure differential by balancing the pressure against the hydrostatic head of a column of liquid of known density (typically water). The basic equation therefore, to calculate pressure difference P in Pascal is:

P = r * g * h where

 

r is the density of the manometer fluid in kg/m3

h is the level difference in the fluid between the limbs (m)

g is the acceleration due to gravity (m/s2)

Manometers are commonly of three types- the vertical U-tube manometer, the inclined gage manometer, & the inclined tube manometer. A manometer needs to be calibrated against a standard and precise pressure-measuring device such as an Askania Minimeter. Calibration is needed because the inclined tube & bore may not be regular & uniform. Or the inclination may not be as precise as it is considered to be. The calibration charts obtained from experimentation can be subsequently helpful in getting accurate pressure estimates from using the inclined tube manometer.

 

Instruments:

Inclined tube manometer, Askania minimeter, & calibration arrangement consisting of a large glass bottle & connections.

Among the manometers available, the inclined tube manometer is commonly used owing to its greater sensitivity in comparison to the vertical U-tube manometer. The inclined tube manometer comprises of an inclined (one vertical to 10 inclined scale graduations) glass tube of 6mm bore serving ads one limb of the U-tube manometer. The other limb is essentially a large diameter vessel. A device for varying the inclination of the tube may also be fitted with the instrument. Owing to the large diameter ratio between the two limbs, the change in the liquid level in the vessel is negligible in comparison to the inclined tube. The pressure difference is thus directly given by the change in the liquid level in the inclined tube. The inclined tube observations are, however, corrected to the vertical scale readings by multiplying with a suitable factor (For example 0.1 for the 1 in 10 inclination).

 

Procedure:

(1)   Place the Askania minimeter & the inclined tube manometer on a firm surface and level them. Take the initial readings of these instruments.

(2)   Blow air into the glass bottle through the tube provided & tighten the stop cork.

(3)   Take observations of the pressure from the Askania as well as the inclined tube.

(4)   By releasing the pressure a little from the glass bottle take another set of observations.

(5)   Repeat the process in step (4) to get about 12- 15 sets of pressure values.

(6)   Plot the calibration graph as the Askania minimeter observations as function of the inclined tube manometer values without adjusting for the vertical units. Fit a straight line passing through these points.

(7)   Calibrate in this fashion the four inclined tube manometers used in the ventilation laboratory for subsequent experiments.

 

Remarks:

(1)   The changes in the ambient temperature conditions may affect the pressure readings. The jacket of air surrounding the glass bottle may help in alleviating the problem to some extent.

(2)   The high-pressure connection from the glass bottle is attached to the fixed vessel of the Askania minimeter.

 

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