Basic Documentation

Product or company names mentioned herein may be the trademarks of their respective owners. © 2009 Siemens Industry, Inc.
Siemens Industry, Inc. Printed in the USA
Building Technologies Division Document No. 149-975
1000 Deerfield Parkway Page 8 of 8
Buffalo Grove, IL 60089-4513
USA
desired level of control accuracy, response time and
stability.
Experience indicates that the response time for side
wall sensing is seconds longer than for sash position
sensing. Suppliers of side wall sensing control
systems do not dispute this; rather they claim that
the response time will still be within generally
acceptable safe limits. However, given the choice, it
would certainly seem preferable to opt for faster
control response and better control stability.
Suppliers of side wall sensing systems state that
only side wall sensing has the ability to properly
adjust fume hood airflow in relationship to a person
in front of the fume hood. The claim is that a person
in front of a fume hood obstructs some of the fume
hood open sash area and thus the incoming makeup
air must be reduced to maintain the proper average
face velocity. However, a person in front of the fume
hood does not really obstruct a significant portion of
the sash opening since to do this they would have to
mostly be within the plane of the sash opening.
Fume hood designs use a recessed sash that keeps
the sash plane several inches (typically at least 6
inches) in front of the person at the fume hood.
Thus, except for perhaps the protrusion of their
arms, the sash opening does not noticeably change
with a person in front of the hood.
Conclusion
Throughout this paper, credible test results have
been referenced to support the conclusion that sash
position sensing provides better accuracy, faster
response, and greater stability for fume hood face
velocity control. The following statements are a
direct summation quotation from the Laboratory
Ventilation Seminar Presentation "Laboratory Hood
Performance: Commissioning VAV Control Systems
- Sash Position Sensor Versus Through the Wall
Velocity Sensor" during the 1998 ASHRAE Winter
Meeting in San Francisco, California.
 "Hood containment is dependent on proper
response and ability to maintain stable
exhaust airflow."
 "Side Wall Sensors appear to be affected by
more variables than Sash Position
Sensors."
 "Results of field tests indicate that sash
position sensors provide more reliable flow
control."
 "Ensuring satisfactory hood performance
requires proper installation, tuning and
commissioning of VAV control systems."