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AN11QSP-L: Calculating Calibration Coefficients for Biospherical PAR Sensor with Built-In Log Amplifier

Document Number: 
Publication Date: 
Wednesday, July 26, 2017
PDF icon appnote11QSPLJul17.pdf231.84 KB


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

  • QSP-200L and QCP-200L — no longer in production
  • QSP-2300L, QSP-2350L, QCP-2300L, QCP-2300L-HP, QCP-2350L-HP, and MCP-2300 — current production

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

  • SBE 9plus
  • SBE 16 or 19 — These PAR sensors may not be compatible with 6-cell housing version of these CTDs; consult Sea-Bird.
  • SBE 16plus, 16plus-IM, or 19plus — CTD’s optional PAR connector not required when using one of these PAR sensors. The PAR sensor interfaces with an A/D voltage channel on the CTD.
  • SBE 16plus V2, 16plus-IM V2, 19plus V2, or 25plus — The PAR sensor interfaces with an A/D voltage channel on the CTD.
  • SBE 25 – CTD’s PAR connector (optional on older versions) not used with these PAR sensors. The PAR sensor interfaces with an A/D voltage channel on the CTD.

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

Make the following entries/selections 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)

conversion units = appears in data file header; does not modify calculated values, which are controlled by multiplier entry

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

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


  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).
  1. 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.
  2. Cw is the wet μEinsteins/cm2•sec coefficient from the Biospherical calibration sheet. A typical value is on the order of 9.00 x 10 -6; values have varied over the years.
  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 μEinsteins/m2•sec.
  4. Offset (μEinsteins/m2•sec) = - (104 * Cw * 10V),  where V is the dark voltage.
    The dark voltage may be obtained from:
  • Biospherical calibration certificate for your sensor (Average Dark volts on calibration sheet), or
  • CTD PAR voltage channel with the sensor covered (dark) — in Seasave V7, display the voltage output of the PAR sensor channel.

Example: If Cw = 9.00 x 10 -6 and Dark Voltage = 0.0021), offset = - (104 * 9.00 x 10 -6 * 100.0021) = -0.0904. 

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 = 9.0 * 10-6 μEinsteins/cm2•sec, then C = 1.1111 * 10 10 (for entry into configuration file).


  • See Application Note 11S for integrating a Biospherical Surface PAR sensor with the SBE 11plus Deck Unit (used with the SBE 9plus CTD).
  • See Application Note 47 for integrating a Biospherical 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).


Application Note Revision History

Date Description
  Initial release.
July 2001 Add information about offset term, and clarify units.
October 2004 - Update with new (2003) Biospherical PAR sensor part numbers.
- Expand / rewrite / reorganize.
June 2005 Provide output in microEinsteins/m2 sec, and refer to Application Note 11 General for conversion to other units.
May 2007 Incorporate Seasave V7, and eliminate discussion of Seasoft-DOS.
March 2008 - Update to include V2 SeaCATs (16plus V2, 16plus-IM V2, 19plus  V2).
- Remove 'amps' in description of calibration coefficients and units on Biospherical's calibration sheet, because Biospherical changed their calibration sheet.
February 2010 - Add information on QSP-2350L (same functioning as QSP-2300L, just different connector) and QCP-2300L-HP (same functioning as QCP-2300L, just higher depth range), which are on current price list.
- Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon files.
- Update address.
October 2012 Update to include SBE 25plus.
August 2016 Update example / typical values of Cw and dark voltage to better reflect current Biospherical calibration sheets.
May 2017 Update example to correct typo in value for C.
June 2017 Add information on QCP-2350L (same functioning as QCP-2300L, just different connector).