Page 301 - gas transport in porous media

Chapter 17: Subsurface Flow Measurements

Figure 17.2. Two types of rotary vane anemometers 299

Ultrasonic flow meters use the sound carrying properties of the gas medium to
make minimally invasive measurements of the gas flow (Dane, 1997; Brassier et al.,
2001; Calogirou et al., 2001). These instruments have only recently been developed
and are fast becoming a very popular method for wind speed and direction as well as
gas metering applications. Recent developments in electronic instrumentation have
enabled these instruments to become reasonably priced and their non-invasiveness is a
veryimportantfeature. Ultrasonicflowmetersincurvirtuallynoflowloss. Ultrasound
measurements are made using either a Doppler measurement technique or axial time
of flight. The instruments generally require a laminar flow regime to be accurate and
are therefore most appropriately applied in lower flow applications.
Diaphragm meters are mechanical devices that use gas pressure as the driv-
ing force to alternatively fill and empty volumes within the meter (Nilsson and
Delsing, 1998). The gas pressure forces flow into the meter deflecting a diaphragm,
which controls a valve that allows flow either into or out of the meter. Each time
the chamber is filled and emptied a mechanical register records the event. Generally
the meter requires on the order of 2000 Pa gas pressure to operate. The pressure loss
in these meters is relatively high as a result of friction and other mechanical causes
as well as leakage. Nevertheless, these meters are a cost effective method for mea-
suring gas consumption and could potentially be used for flow monitoring in some
situations.

17.6 MICROMACHINED SENSORS
The future holds much promise for the measurement of subsurface gas flow. Actual
pore velocities may ultimately be measured in situ using technology that is currently
being developed. Nguyen (1997) describes a plethora of micromachined sensors in his
review article that are designed for measuring gas flow in the relevant range for vadose
zoneapplications. Thesesensorsareprimarilybasedonintegratedcircuittechnologies
and materials using thermal properties and responses of these tools although some
cantilever systems using piezoresistive effects are also mentioned. Some problems
of these sensors that are particularly important for subsurface applications that still