APPLICATION NOTE NO. 18-1
SBE 18, 27, and 30, & AMT pH Sensor Calibration (PHFIT Version 2.1)
Revised April 2013

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This application note applies to the SBE 18 pH sensor, SBE 27 pH/ORP (Redox) sensor, SBE 30 DO/pH/ORP sensor, and AMT Analysenmesstechnik GmBh pH sensor.

Sea-Bird software calculates pH as:

pH = 7 + (Vout - offset) / (1.98416 x 10 ^-4 x T * slope) (see Appendix for derivation of equation)

T = temperature (ºK)

Vout = output voltage from pH sensor (0 - 5 volts)

Offset and slope = calibration coefficients, determined by a least-squares fit of voltage and pH in a series of buffer solutions, using the measured temperature of the buffer solutions; coefficients are calculated using PHFIT software

Sea-Bird includes a calibration sheet with the shipment. The calibration sheet provides values for offset and slope, which have been input by Sea-Bird in the configuration (.con or .xmlcon) file.

Note: If you purchase an AMT sensor as a separate item, not integrated with a Sea-Bird CTD, Sea-Bird provides only the calibration sheet from AMT. This calibration sheet calculates pH as:
pH = a + bV
where a and b are calculated by AMT using a least-squares fit of voltage and pH in a series of buffer solutions. If you will be integrating the AMT sensor with a Sea-Bird CTD, use the voltage and pH values from AMT’s calibration sheet (and 25 ºC for the temperature) to generate the offset and slope as described below in User Recalibration.

Sea-Bird provides PHFIT software to use when calibrating pH sensors. PHFIT is installed when you install SBE Data Processing (part of the Seasoft software package). PHFIT can be used on computers running Windows XP service pack 2 or later, Windows Vista, Windows 7, or Windows 8.

• PHFIT installation was added in SBE Data Processing version 7.22.5a, and Seasoft version dated April 26, 2013 (it was previously available as part of our DOS software package, and was incompatible with Windows 7 and 8).
• The latest version of SBE Data Processing and Seasoft are available for download from our website.
• phfit.exe in installed in the same directory as SBE Data Processing (default location C:\Program Files\Sea-Bird\SBEDataProcessing-Win32).
• phfit.exe must be run from a DOS window.

When needed, recalibrate the pH sensor as follows:

1. Prepare a series of buffer solutions (up to 25 solutions); maintain each at approximately the same temperature. Record the temperature.
2. Measure and record the output voltage from the pH sensor for each buffer solution.
   Note: For the SBE 18, 27, and 30, an electrical connection between the buffer solution and the anode on the top of the sensor must be made while measuring pH.
3. Run PHFIT from a DOS window:

1. At the prompt, enter the sensor serial number and the temperature (in ºC) of the buffer solutions.
2. At the prompt, enter the pH and output voltage (Vout) for up to 25 buffer solutions. When you have finished, the program outputs the offset and slope, along with the residuals.

4. Enter the new offset and slope in the CTD’s configuration (.con or .xmlcon) file. Instructions are provided below for modifying the configuration file using SBE Data Processing (in Seasoft V2):

1. Double click on SBEDataProc.exe.
2. In the Configure menu, select the applicable CTD.
3. In the dialog box, click Open and select the applicable .con or .xmlcon file for the CTD.
4. In the sensor list, double click on the pH sensor.
5. Enter the new offset and slope in the dialog box and click OK.
6. Click Save or Save As to save the changed configuration (.con or .xmlcon) file.

 

Vout = offset + [ slope * (R * T / F) * ln (10) * (pH - 7) ]

Where

R = gas constant = 8.31434

F = Faraday constant = 9.64867 x 10 ⁺⁴

T = temperature (ºK)

Vout = output voltage from pH sensor (0 - 5 volts)

Substituting for R, F, and ln (10):

Vout = offset + [ slope * 1.98416 x 10 ^-4 * T * (pH - 7) ]

Therefore,