Dwyer A-420 - Manual

20596_ins_Dwyer_A420
Installation & Operation Instructions
Outdoor Air Static Pressure Pickup Port
Dwyer Instruments Inc., 102 Indiana Hwy. 212, Michigan City, IN 46361 USA
Tel: 800/872-9141 or 219/879-8000 • Fax: 219/872-9057 • E-mail: [email protected] • Web: www.dwyer-inst.com
4/23/08 rev.
1
Specifications subject to change without notice.
How It Works
Mounting
Figure 1: Outdoor Air Static
Building static pressure is the pressure difference between the inside and the outside of a building. The outdoor static
pressure is simply the atmospheric pressure at the building site. The building may have positive, neutral or negative
pressure with respect to the outside atmosphere. Differences in pressure are due to powered supply or exhaust fans and
are usually less than 0.1 inches of water.
A complication in measuring the building static pressure is the dynamic action of the wind. A gentle breeze of 10 MPH
provides a pressure of 0.048 inches of water, a gale of 40 MPH is 0.772 inches and a hurricane of 75 MPH is 2.716 inches.
Clearly, the wind’s pressure may be more than the desired building static pressure. Measuring the wind’s pressure instead
of the true outdoor static pressure will radically alter the actual static pressure reading.
Mounting the unit with the plate parallel to the earth’s surface, as shown in Figure 2 allows the predominantly horizontal
flow of the wind an omni-directional entry between the plate and the housing. The plate forces the wind to move perpendicu-
larly to the pressure orifice in the housing, irrespective of the wind’s direction, greatly minimizing the dynamic air pressure.
Mounting the unit vertically, as on the side of a rooftop air-conditioning unit (RTU), will only provide a true static pressure
reading if the wind is blowing within 15 to 20 degrees of the direction of the opening. If the wind is blowing at a vertically
mounted Outdoor Air Static Pressure Pickup Port it will “pile up” providing a higher than correct static pressure reading. If
the wind is blowing away from the sensor it will be in the wake of the RTU providing a lower than correct static pressure
reading.
The Outdoor Air Static Pressure Pickup Port should be mounted at least 12 inches above surrounding obstacles so that
you get the true static pressure reading. Also supplied is a brass bulkhead fitting to get your pressure signal inside the
building envelope, the user supplies all necessary pressure tubing and the 1/2 inch EMT conduit mounting support.
Mount the unit so that other equipment on the building cannot physically or mechanically obstruct it. Do not mount under
awnings or other projections within fifteen feet. If the unit cannot be roof mounted with an unobstructed view of the horizon,
then do not mount near economizers, intake or exhaust fans, or barometric dampers. Do not mount near building corners.
Mount near the center of the building away from balconies and opening windows. Mount at least 24 inches from the wall.
Pressure Pickup Port