Device calibration consists of verifying the measurement accuracy of a device and adjusting for any measurement error. Verification consists of measuring the performance of the device and comparing these measurements to the published specifications. During calibration, you supply and read voltage levels or other signals using external standards, then you adjust the device calibration constants. The new calibration constants are stored in the EEPROM. These calibration constants are loaded from memory as needed to adjust for the error in the measurements taken by the device. There are two kinds of calibration, external and self. For more information on calibrating your device with NI-DAQmx, refer to Device Calibration Considerations. For detailed external calibration procedures, refer to ni.com/calibration.
External calibration, which is typically performed by a metrology lab, requires using a high-precision voltage source to verify and adjust calibration constants. This procedure replaces all calibration constants in the EEPROM and is equivalent to a factory calibration. Because the external calibration procedure changes all EEPROM constants, it invalidates the original calibration certificate. If an external calibration is done with a NIST-certified voltage source, a new NIST traceability certificate can be issued.
Self-calibration adjusts the calibration constants with respect to an onboard reference stored on the device. The new calibration constants are defined with respect to the calibration constants created during an external calibration to ensure that the measurements are traceable to these external standards. The new calibration constants do not affect the constants created during an external calibration because they are stored in a different area of the device memory. You can perform a self-calibration at any time to adjust the device for use in environments other than those in which the device was externally calibrated. You use the DAQmx Self Calibrate (or DAQmxSelfCal) function/VI to perform a self-calibration.