| Since absolute pressure transducers sensitive enough to
produce reasonable measurements are extremely expensive, differential pressure sensors
must be used, requiring measurements relative to some other pressure. Establishing a
stable reference pressure is critical to producing reliable pressure measurements. Ideally
this reference pressure should be atmospheric barometric pressure. However there are
difficulties associated with using the ambient pressure as the reference. Two reference
pressure systems have been explored in detail for this study. One of the systems is much
like that currently in place at Texas Tech. With this system all of the pressure sensors
are referenced to the pressure in a hole in the ground. This pressure is not monitored,
but assumed to be atmospheric. The second system that has been looked at in detail
involves running all of the reference pressure lines to a sealed tank. The pressure in the
tank is monitored with a high accuracy absolute transducer. Efforts are made to maintain a
constant temperature and pressure in the tank. Additionally, the pressure in the tank is
continuously compared to barometric pressure. If there is any significant deviation
between the two, the tank is vented to the atmosphere. The preliminary system includes approximately a dozen differential
pressure transducers. All of the transducers are concentrated in the southeast 2nd
floor corner of the structure. Sensym manufactures the differential units being used. The
transducers have adjustable baselines and can be set to measure positive pressures,
negative pressures, or both. Each transducer is custom configured to optimize its dynamic
range in the given location. Eight of the twelve pressure transducers in the initial
system have been placed on the walls, the remainder on the roof. Pressure taps are drilled
through the exterior of the house and all pressure sensors are mounted inside. A method
using a combination of rubber strain reliefs and flexible tubing (3/16" ID,
5/16"OD) has been developed to connect the transducers to the tap locations. The
setup for the wall transducers includes a three-way solenoid valve between the tap and the
sensor. This solenoid allows for the shunt calibrations to be performed. Each roof sensor
setup includes provisions for rainfall collection. The tubing is dropped straight down
from the tap location. The tubing then splits. The rainfall collection tube continues
straight down. A two way normally closed solenoid valve is at the bottom of the rain tube
and will be activated to allow for drainage. The line branches off to connect to a
three-way solenoid and the pressure transducer. |
|
Pressure taps along south wall and
eave of structure. Taps are also
located on the roof and east wall
of the structure.
Reference pressure tank and absolute pressure
transducer. All of the differential pressure sensors are referenced to the pressure in the
yellow tank.
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