A variety of sensors are used to control many facets of heating, ventilating, and air conditioning systems. Confirming that a sensor is measuring the respective parameter accurately is crucial to proper system operation and energy performance. For example, if a supply fan variable frequency drive is controlled based on duct static pressure, then it is imperative that the pressure sensor is measuring accurately. A precise definition of calibration is to perform a set of test procedures under specific conditions in order to establish a relationship between the value indicated by a measuring device and the corresponding values that would be realized by the standard being applied. The most common testing standards have been developed by the National Institute of Standards and Technology (NIST). However, the term “calibration” used in the acceptance tests simply refers to verification that the measured value from a sensor will correspond reasonably well (within 10 percent for pressure or light and within 2°F for temperature) to the actual state of the medium being measured.
The requirement found in a few test procedures for sensor calibration can be met by either having a calibration certificate provided with the sensor from the manufacturer or through field verification. A calibration certificate from the manufacturer verifies that the particular sensor was tested per a traceable standard (typically NIST) and confirmed to be measuring accurately. A factory-calibrated sensor is assumed to be accurate and requires no further testing. Field verification generally requires checking the measured value from the sensor against a calibrated instrument while the sensor is installed in the system. Typically most sensors can be checked at a single operating point if the expected measurement range does not vary significantly. Any adjustments that are necessary to make the field-installed sensor correspond to the value measured by the calibrated instrument can be made at either the transmitter itself or within the control system database.
The following sensors are required to be checked for calibration.
•Pressure sensors used in variable flow applications (i.e. supply fan or pump variable frequency drive is controlled to maintain a specific pressure setpoint). This is applicable to test procedure(s): NA7.5.6 Supply Fan Variable Flow Controls and NA7.5.9 Hydronic System Variable Flow Controls. Accuracy to 10 percent.
•Temperature sensors used to control field-installed economizers and supply water temperature reset. This is applicable to test procedure(s): NA7.5.4 Air Economizer Controls Acceptance and NA7.5.8 Supply Water Temperature Reset Controls. Accuracy to 2°F.
•Carbon dioxide sensors used to implement a demand-controlled ventilation control strategy. This is applicable to test procedure(s): NA7.5.5 Demand-controlled Ventilation Systems Acceptance. Accuracy to 75 PPM (parts per million) of CO2 concentration.
“System”
A “system” is used to control outdoor air dampers in variable air volume (VAV) systems. There are many different ways to control minimum ventilation in a VAV system, including (but not limited to):
•Supply/return flow tracking
•Direct outdoor air flow measurement
•Constant differential pressure across dedicated ventilation air damper
•Constant mixed air plenum pressure
The term “system” refers to whatever type of control strategy employed to actively control minimum ventilation air flow. This is applicable to test procedure(s): NA7.5.1.1 Variable Air Volume Systems Outdoor Air Acceptance. Overall, the “system” must be able to control flow to within the 10 percent of the design outdoor air ventilation value.