AN74: Calculating Calibration Coefficients for Turner Cyclops-7 Fluorometer or Turbidity Sensor

Document Number: 
74
Publication Date: 
Sunday, August 8, 2010
AttachmentSize
appnote74Aug10.pdf176.51 KB

Introduction

The Cyclops-7 provides a 0 to 5 volt output voltage proportional to fluorescence or turbidity. The Cyclops-7 is factory-configured at Turner to detect one of the following:

Measuring Chlorophyll a Rhodamine WT tracer dye Phycocyanin Phycoerythrin Turbidity CDOM Crude Oil
Designation on sensor C R P E T U O

 

The user can customize the 0-5V Cyclops-7 range to correspond to the expected data range by changing the interface cable, thus improving data resolution. The ranges are:

Cable
Gain
Chlorophyll a
(µg/l)
Rhodamine WT
tracer dye (ppb)
Phycocyanin or
Phycoerythrin (cells/ml)
Turbidity
(NTU)
CDOM
(ppb QS*)
Crude Oil
(ppb QS*)
X100 0 - 5 0 - 10 0 - 1500 0 - 30 0 - 25 0 - 15
X10 0 - 50 0 - 100 0 - 15,000 0 - 300 0 - 250 0 - 150
X1 0 - 500 0 - 1000 0 - 150,000 0 - 3000 0 - 2500 0 - 1500
* QS=Quinine Sulfate

 

Example: You have a Cyclops-7, factory configured to detect Chlorophyll a (range 0 - 500, 0 - 50, or 0 - 5 µg/l, depending on interface cable). You expect a range of 0 – 20 µg/l for chlorophyll a fluorescence. If using an X1 cable, the maximum voltage will be approximately 0.2 V (= 20 µg/l / [500 µg/l / 5 V] ). This limits the resolution and multiplies the noise level of the instrument. Changing the range to 0 - 5 µg/l (using an X10 cable) provides the best results.

When interfacing a Cyclops-7 to a Sea-Bird CTD, Sea-Bird defines the concentration as:

concentration = (scale factor * voltage) + offset

The factory range can be used to calculate the factory default scale factor and offset (see below for field calibration):

scale factor = Range / 5 V
offset = - scale factor * measured voltage at 0 concentration (i.e., blank voltage)

To determine the blank voltage, place the Cyclops-7 in its field configuration (with or without a flow-through cap, as applicable) in de-ionized water and take a voltage reading.

 

Setting up Configuration (.con or .xmlcon) File in Seasave or SBE Data Processing

Change the range by changing the interface cable, as needed, before you set up the configuration file in Sea-Bird software.

  1. Use the Configure Inputs menu in Seasave V7 (real-time data acquisition software), or the Configure menu in SBE Data Processing (post-processing software), to create / modify the .con or .xmlcon file (see software Help files).
  2. Select Fluorometer - Turner Cyclops as a voltage sensor when editing the configuration file (the Turner Cyclops fluorometer was added to the list of voltage sensors in software version 7.20g; earlier software versions do not list it). The software prompts for scale factor, offset, and units, and calculates:

Chlorophyll a — Equation shown is for units of µg/l; other units available
chlorophyll a (µg/l) = (scale factor * voltage) + offset
where Scale Factor is in µg/l-volt and Offset is in µg/l

Rhodamine — Equation shown is for units of ppb; other units available
Rhodamine (ppb) = (scale factor * voltage) + offset
where Scale Factor is in ppb/volt and Offset is in ppb

Phycocyanin or Phycoerythrin — Equation shown is for units of cells/ml.
Phycocyanin or Phycoerythrin = (scale factor * voltage) + offset
where Scale Factor is in cells/ml-volt and Offset is in cells/ml

Turbidity — Equation show is for units of NTU.
Turbidity (NTU) = (scale factor * voltage) + offset
where Scale Factor is in NTU/volt and Offset is in NTU

CDOM or Crude Oil — Equation shown is for units of ppb QS.
CDOM = (scale factor * voltage) + offset
where Scale Factor is in ppb QS/volt and Offset is in ppb QS.

Note: The configuration file can only be saved as an .xmlcon file (not a .con file) if Turner Cyclops fluorometer is selected as one of the sensors.

Example of Chlorophyll a Concentration Calculation:
If
fluorometer scale factor = 10.5 µg/l-volts,
offset = - 0.03 µg/l, and
measured voltage from fluorometer = 2.65 volts,
Calculated concentration (µg/l) = (scale factor * voltage) + offset = (10.5 * 2.65) - 0.03 = 27.80 µg/l

 

Note on Field Calibration

While the nominal scale factor and offset based on factory range and your measured blank voltage can be used to obtain approximate values, field calibration for chlorophyll a, Phycocyanin, Phycoerythrin or turbidity is highly recommended.

  • The relationship between fluorescence and chlorophyll a, Phycocyanin, or Phycoerythrin is highly variable, and is not easy to determine in the laboratory. Species distribution, ambient light level, and health of the stock are just some of the factors that affect the relationship.
    To accurately measure concentrations, perform calibrations on seawater samples with concentrations of plankton populations that are similar to what is expected in situ. Determine concentrations independently, and use those concentrations, as well as readings from the Cyclops-7 (with or without a flow through cap, as applicable), to determine the correct Scale Factor. It is only through the use of these calibrations that a meaningful and accurate measure of these parameters can be obtained. The scale factor is correct as long as the condition of the plankton population does not change; the condition does change with season and geographic location.
  • The relationship between Cyclops-7 turbidity sensor output and turbidity is somewhat variable, and is not easy to determine in the laboratory. Particle size and shape are just some of the factors that affect the relationship. To accurately measure concentrations, perform calibrations on seawater samples with concentrations of particles that are similar to what is expected in situ. Determine concentrations independently, and use those concentrations, as well as readings from the Cyclops-7, to determine the correct Scale Factor. It is only through the use of these calibrations that a meaningful and accurate measure of turbidity can be obtained. The scale factor is correct as long as the distribution of particle sizes and shapes does not change; the condition does change with season and geographic location.

To accurately measure the offset, perform a calibration on a blank (de-ionized water) sample, with the Cyclops-7 in its field configuration (with or without a flow-through cap, as applicable).

See Turner’s Cyclops-7 manual for calibration details.

 

Application Note Revision History

This application note is for use with Seasave and SBE Data Processing version 7.20g and later. An earlier version of the application note is compatible with earlier software versions.

Date Description
July 2004 Initial release.
November 2004 Add information on Phycocyanin and Phycoerythrin fluorometers.
February 2005 Remove reference to units being calibrated at Turner factory. Turner is no longer providing a calibration sheet with their instruments, so user is responsible for determining blank voltage.
March 2009 Updated to reflect Turner's changes in fluorometer ranges
July 2009 Updated to add Turner Cyclops-7 Turbidity Sensor.
March 2010 - Remove old ranges.
- Add information on .xmlcon configuration file.
- Update Sea-bird address.
June 2010 - Modify ranges to match Turner's latest specification.
- Add CDOM and Crude Oil sensors.
August 2010 SBE Data Processing and Seasave 7.20g software revision: Turner Cyclops added to list of fluorometers (previously needed to select Turner Scufa for this sensor).