In refrigerated warehouses, sensors used for refrigeration system control include numerous field installed sensors such as evaporator zone temperatures, suction and discharge pressure transducers and outdoor temperature and humidity sensors. Sensors may also be factory installed on equipment such as a screw compressor package. To ensure efficient system operation, as well as meet the Construction Inspection requirements for the Acceptance Tests, all sensors used for operational control of the system must be calibrated to provide accurate values.
Sensors used for information or other purposes which do not relate to maintaining pressures, temperatures or routine equipment sequencing and operational control are not subject to these calibration requirements.
For field installed sensors, on-site calibration must be completed, even if the sensor was provided with a calibration certificate. For field installed sensor there are multiple potential sources of error in the readings between the sensor and the operator interface. Errors may include, but are not limited to: sensor error, transmitter error, conversion error, thermal drift, or electrical noise. In order to provide accurate values to the control system, the entire end-to-end (i.e. sensor to the operator interface) must be calibrated.
The calibrating instruments used to calibrate the field installed sensors must have a high accuracy so that the end-to-end accuracy is within an acceptable threshold. This calibrating instrument, also called the calibration “standard”, must be calibrated at least every two years using a NIST traceable reference. The Refrigerated Warehouse Refrigeration System Acceptance Tests requires calibrating instrument accuracies as follows:
•Temperature: ±0.7°F between -30°F to 200°F
•Pressure: ±2.5 psi between 0 and 500 psig
•Relative Humidity (RH): ±1% between 5% and 90% RH
The calibration process includes checking the sensor reading which is used for control (as read from the operators interface) vs. the calibration instrument reading. The values in the control system must be adjusted according to the control system procedures, which may include zero and span values or single offset values for calibration, so that the reading from the operator readout is within an acceptable deviation from the calibrating instrument reading. Calibration must be performed at more than one value (e.g. temperature or pressure) to insure proper sensor function and that the signal conversion (e.g. proper ranges and engineering units) is properly implemented, consistent with control system documentation. Calibration values should be tested for persistence in the event of a controller or computer power reset.
For factory installed sensors on an equipment package, which are used for system control, the package manufacturer may certify the sensor has been calibrated using a NIST traceable reference, or the preceding field calibration process must be used.
For refrigerated warehouses, the calibration process requires documentation to be provided to Field Technician completing the Acceptance Test, and the owner for documentation and use in ongoing system maintenance. Calibration documentation include records showing when the calibration was performed, what instruments were used in the calibration, and what offsets or other calibration values were used to adjust sensor readings in the control system. This is required for both field installed sensors and factory installed sensors on equipment packages.