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APPLICATION NOTE NO. 11QSP-L
Calculating Calibration Coefficients for Biospherical PAR Light Sensor with Built-In Log Amplifier
 Revised February 2010

Print version

This application note applies to the following Biospherical Instruments PAR light sensors, which all have a built-in log amplifier:

These PAR sensors are compatible with the following Sea-Bird CTDs:

Note: The CTD voltage channel for use with the PAR sensor can be single-ended or differential.

SEASOFT computes PAR using the following equation:

PAR = [multiplier * (109 * 10(V-B) / M) / calibration constant] + offset

Enter the following coefficients in the CTD configuration file:

M = 1.0   (Notes 2 and 3)

B = 0.0   (Notes 2 and 3)

calibration constant = 105 / Cw   (Notes 2 and 4)

multiplier = 1.0 for output units of μEinsteins/m2.sec   (Note 5)

offset = - (104 * Cw * 10V) (Note 6)

Notes:

  1. In our SEASOFT V2 suite of programs, edit the CTD configuration (.con or .xmlcon) file using the Configure Inputs menu in Seasave V7 (real-time data acquisition software) or the Configure menu in SBE Data Processing (data processing software).
  2. Sea-Bird provides two calibration sheets for the PAR sensor in the CTD manual:
  1. For all SBE 911plus, 16, 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19, 19plus, 19plus V2, and 25 CTDs, M = 1.0. For SBE 9/11 systems built before 1993 that have differential input amplifiers, M = 2; consult your SBE 9 manual or contact factory for further information. B should always be set to 0.0.
  2. Cw is the wet μEinsteins/cm2.sec coefficient from the Biospherical calibration sheet. A typical value is 4.00 x 10 -5.
  3. The multiplier can be used to calculate irradiance in units other than μEinsteins/m2.sec. See Application Note 11General for multiplier values for other units.
    The multiplier can also be used to scale the data, to compare the shape of data sets taken at disparate light levels. For example, a multiplier of 10 would make a 10     μEinsteins/m2.sec light level plot as 100 μ    Einsteins/m2.sec.
  4. Offset (μEinsteins/m2.sec) = - (104 * Cw * 10V),  where V is the dark voltage. 
    For typical values (Cw = 4.00 x 10 -5 and Dark Voltage = 0.150), offset = -0.5650. 
    The dark voltage may be obtained from:

Instead of using the dark voltage to calculate the offset, you can also directly obtain the offset using the following method: Enter M, B, and Calibration constant, and set offset = 0.0 in the configuration (.con or .xmlcon) file. In Seasave V7, display the calculated PAR output with the sensor dark; then enter the negative of this reading as the offset in the configuration file.

 

Mathematical Derivation
  1. Using the sensor output in volts (V), Biospherical calculates:
    light (    μEinsteins/cm2.sec) = Cw * (10Light Signal Voltage - 10Dark Voltage).
  2. SEASOFT calculates μEinsteins/m2.sec = [multiplier * 109 * 10(V - B) / M) / Calibration constant] + offset
    where M, B, Calibration constant, and offset are the SEASOFT coefficients entered in the CTD configuration file.
  3. To determine Calibration constant, let B = 0.0, M = 1.0, multiplier = 1.0. Equating the Biospherical and SEASOFT relationships:

104 (cm2/m2)* Cw * (10Light Signal Voltage - 10Dark Voltage) = (109 * 10V) / Calibration constant + offset

Since offset = - (104 * Cw * 10Dark Voltage), and V = Light Signal Voltage:

Calibration constant = 109 / (104 * Cw) = 105 / Cw

Example: If Wet calibration factor = 4.00 * 10-5 μEinsteins/cm2.sec, then C = 2,500,000,000 (for entry into configuration file).

Notes:

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Last modified: 05-May-2010

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