|
Specifying Sensors
Like any element of a
control design, selecting the right sensors, and making sure that what
is specified is being provided, helps assure a quality control
installation.
|
Paul Ehrlich,
Ira
Goldschmidt
& Angela Lewis
Building
Intelligence Group
As
published
August Issue -
BAS Column
|
Sensors
are a key part of any control system and have an impact on the cost,
stability and accuracy of any control system. But they are also
something that frankly gets little attention as we design systems,
develop specifications and write sequences. For many designers the
sensor section sits in the back of the product area of the
specification and may get little attention or changes from project to
project. In general that is not all bad, but you should go back and
periodically review your sensor specifications.
Temperature Sensors:
The vast majority of sensors are used for temperature including zones,
duct, pipe, and outdoor air. Sensing technology for BAS is
generally either an RTD (resistance to temperature device) or a
Thermistor. While RTD’s are more linear and accurate, thermistors are
generally less expensive and work just as well. Key items to note
when specifying:
- Zone Sensors:
Most suppliers offer a variety of options ranging from a simple blank
plate, to elaborate digital displays and adjustments. This is an
area that is worth discussing with the owner in advance of completing
your specification. Suppliers are also generally willing to
provide samples for the owner’s approval.
- Sensor Accuracy:
For most applications accuracy of +/- .5 degrees F is adequate.
But there are applications were you would want to specify more accurate
(and expensive) sensors. This includes any area where you need to
hold tight control. Care also needs to be taken where the intent is to
measure temperature differentials or deltas. Examples of this
include BTU measurement, or the inlet and outlet evaporator
temperatures on a chiller. When measuring deltas you should
specify that the two sensors are a matched pair.
Other
sensors commonly specified include humidity, pressure, CO2, flow and
occupancy. Some key things to keep in mind on these sensors
includes:
- Humidity:
You need to carefully consider the application when specifying a
humidity sensor. More accurate sensors are more expensive, but
should be used to control critical environments. Humidity sensors
for use in zones can be ordered with integrated temperature sensing
eliminating the need to have two sensors on the wall. When
sensing outdoor air conditions (i.e. for economizer change over) we
generally like to get one good quality outdoor air humidity sensor for
the building that will be regularly re-calibrated.
- Pressure:
For water differential pressure be sure to specify the needed
accessories such as isolation valves and manifolds so that the system
can be properly started and maintained.
- CO2, IAQ:
These are valuable for strategies such as demand controlled
ventilation. Be aware, however, that most CO2 sensors do need to
be regularly calibrated.
- Others:
Carefully check your design to see if other sensors such as flow,
occupancy, etc. may be required. Many of these are more
specialized devices and some additional research may be needed in order
to make sure you are specifying the right options.
Like any element of
a control design, selecting the right sensors, and making sure that
what is specified is being provided, helps assure a quality control
installation.
About the Authors
Paul and Ira first
worked together on a series of ASHRAE projects
including the BACnet committee and Guideline 13 – Specifying DDC
Controls. The formation of Building Intelligence Group provided them
the ability to work together professionally providing assistance to
owners with the planning, design and development of Intelligent
Building Systems. Building Intelligence Group provides services for
clients worldwide including leading Universities, Corporations, and
Developers. More information can be found at
www.buildingintelligencegroup.com
We also invite you to contact
us directly at Paul@buildingintelligencegroup.com or
ira@buildingintelligencegroup.com
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