68
3 Flow Rates and Pressure Measurements
where
• v: Air speed (m s
−1 ),
• P v : Dynamic pressure (Pa), and
• ρ: Air density (kg m
−3 ).
3.4 Pressure Drop Surveys
As air flows through an airway a pressure drop occurs. This pressure loss can be determined by two procedures, namely (Prosser and Loomis 2004): (a) Direct measurement of the differential pressure (gauge-and-tube method or trailing hose); (b) Indirect determination by the absolute pressure difference between two points (barometer
or altimeter method).
In the gauge-and-tube method, the total pressure difference between two locations
connected by a rubber tube is directly read by a manometer (Fig. 3.4). The system
will be displaced along the wind direction from the intake path to the return path.
This method is time-consuming and there is a limitation on the distance between
measuring points given by the length of the hose. However, it is the most accurate
method, so it is preferred when small pressure differences are to be measured.
The indirect method is carried out by simultaneously measuring the absolute static
pressure at two stations with the aid of a barometer. The method can be performed
even at long distances between measurement points but requires the existence of
large-pressure differences to reduce measurement errors. To conduct a differential
pressure survey by the indirect method, there are two alternatives, namely single-base
method and leapfrogging.
In the single-base method, one barometer is located underground while the other
one is located at a fixed base which can be either outside or inside the mine. The base
must be located at a point with minimal temperature and pressure fluctuations that
might be caused by sunlight, fans, regulators, hoisting equipment, etc. Measurements
are recorded or logged at regular time intervals (approx. 5 min). This method requires
correction for atmospheric pressure and air speed changes, as well as elevation.
Upstream
end
Downstream
end
Manometer
Reel
Max. 300 m
Wind
direction
Fig. 3.4 Differential pressure measurement by the gauge-and-tube method. Modified from Prosser
and Loomis (2004)
3 Flow Rates and Pressure Measurements
where
• v: Air speed (m s
−1 ),
• P v : Dynamic pressure (Pa), and
• ρ: Air density (kg m
−3 ).
3.4 Pressure Drop Surveys
As air flows through an airway a pressure drop occurs. This pressure loss can be determined by two procedures, namely (Prosser and Loomis 2004): (a) Direct measurement of the differential pressure (gauge-and-tube method or trailing hose); (b) Indirect determination by the absolute pressure difference between two points (barometer
or altimeter method).
In the gauge-and-tube method, the total pressure difference between two locations
connected by a rubber tube is directly read by a manometer (Fig. 3.4). The system
will be displaced along the wind direction from the intake path to the return path.
This method is time-consuming and there is a limitation on the distance between
measuring points given by the length of the hose. However, it is the most accurate
method, so it is preferred when small pressure differences are to be measured.
The indirect method is carried out by simultaneously measuring the absolute static
pressure at two stations with the aid of a barometer. The method can be performed
even at long distances between measurement points but requires the existence of
large-pressure differences to reduce measurement errors. To conduct a differential
pressure survey by the indirect method, there are two alternatives, namely single-base
method and leapfrogging.
In the single-base method, one barometer is located underground while the other
one is located at a fixed base which can be either outside or inside the mine. The base
must be located at a point with minimal temperature and pressure fluctuations that
might be caused by sunlight, fans, regulators, hoisting equipment, etc. Measurements
are recorded or logged at regular time intervals (approx. 5 min). This method requires
correction for atmospheric pressure and air speed changes, as well as elevation.
Upstream
end
Downstream
end
Manometer
Reel
Max. 300 m
Wind
direction
Fig. 3.4 Differential pressure measurement by the gauge-and-tube method. Modified from Prosser
and Loomis (2004)