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.
  

[an error occurred while processing this directive]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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