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 * 10^V)   (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:

• Calibration sheet generated by Biospherical, which contains Biospherical’s calibration data.
• Calibration sheet generated by Sea-Bird, which incorporates the Biospherical data and generates M, B, and calibration constant needed for entry in Sea-Bird software (saving the user from doing the math).

3. For all SBE 911plus, 16, 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19, 19plus, 19plus V2, 25, and 25plus 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.
4. Cw is the wet μEinsteins/cm²•sec coefficient from the Biospherical calibration sheet. A typical value is 4.00 x 10 ^-5.
5. 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 μEinsteins/m²•sec.
6. Offset (μEinsteins/m²•sec) = - (10⁴ * Cw * 10^V),  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:

• Biospherical calibration certificate for your sensor, or
• CTD PAR voltage channel with the sensor covered (dark) — in Seasave V7, display the voltage output of the PAR sensor channel.

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 * (10^{Light Signal Voltage} - 10^{Dark 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 * (10^{Light Signal Voltage} - 10^{Dark Voltage}) = (10⁹ * 10^V) / Calibration constant + offset

Since offset = - (10⁴ * Cw * 10^{Dark 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:

• See Application Note 11S for integrating a Surface PAR sensor with the SBE 11plus Deck Unit (used with the SBE 9plus CTD).
• See Application Note 47 for integrating a Surface PAR sensor with the SBE 33 or 36 Deck Unit (used with the SBE 16, 16plus, 16plus V2, 19, 19plus, 19plus V2, 25, or 25plus CTD).