Specifying Sensors
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 RTDs are more linear and accurate, thermistors are generally less expensive and work just as well. Here are some 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?F is adequate. But there are applications where 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.
Temperature Sensors
Other sensors commonly specified include humidity, pressure, CO2, flow, and occupancy. Let’s look at some key things to keep in mind about using these sensors.
- Humidity. You need to carefully consider the application when specifying a humidity sensor. More accurate sensors are more expensive, but they 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 changeover), we generally like to get one good-quality outdoor air humidity sensor for the building that will be regularly recalibrated.
- 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. ES
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