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Relationship between the Calibration Constant and the Calibration Multiplier
All LI-COR radiation sensors produce a current proportional to the radiation intensity. During factory calibration, sensor output (in microamps) is measured while the sensor is exposed to a standard lamp of known intensity. The sensor output at this intensity has general units of microamps per radiation unit and is called the Calibration Constant (Calconstant). Each sensor has a slightly different output at a given radiation intensity and will therefore have a unique Calconstant.
LI-COR Light Meters and dataloggers measure the current output of the sensor in units of microamps, and convert the measured current to units of radiation. To make this conversion, LI-COR instruments use the sensor Calibration Multiplier. The Calibration Multiplier is the negative reciprocal of the Calconstant.
The Calibration Multiplier is always a negative number (because the shield of the coaxial cable of the sensor is positive instead of negative), and is expressed in radiation units per microamp.
When a LI-COR Light Meter or data logger is not used, the sensor can be used with other millivolt recorders or data loggers by connecting a millivolt adapter, available from LI-COR. Table 1 lists the millivolt adapters required for each sensor and the resistance of each adapter.
The millivolt adapter connects to the BNC connector of the sensor, and the wire leads of the adapter are connected to the data logger. Sensor output (in millivolts) when using the millivolt adapter can be computed using "Ohm's Law" (Voltage = Current X Resistance).
Example: Calculate the millivolt output of an LI-190 Quantum Sensor which has a calibration constant of 8.0 µA/1000 µmol s-1 m-2. Assume the 2290 millivolt adapter is used with the sensor.
Type "SL" sensors are available for the LI-190, LI-191, LI-200, and LI-210 Sensors. The Type SL Sensor produces a standardized millivolt output, and may be used in place of the Type SA Sensor and Millivolt Adapter.
For complete information on configuring your instrument, please consult your instruction manual or contact LI-COR. If the calibration constant for your sensor has been lost or misplaced, it can be obtained from LI-COR by providing the serial number of the sensor.
