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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