Introduction

Salinity onboard is measured using a Fisher Model S66366 Refractometer. Higher concentration of salt in water results in more refraction of light through water. The amount of refraction of light through a sample on the refractometer can then be used to measure the salinity of the sample. The refractometer has scales for both specific gravity and ppt. The procedure used to measure salinity calibrates the refractometer with a seawater standard (IAPSO) to 35 ppt on the refractometer, although values are typically reported without units.

Calibration

1. Rinse and clean the blue prism surface with ultrapure water onto it and then wiping dry.
2. Add a drop of IAPSO standard seawater onto the blue prism surface. 50 uL from a pipette should work well.
3. Close the clear prism lid and the water should spread over the prism surface.
4. Look through the eyepiece of the refractometer towards a source of light. Sunlight from a window, or a ceiling light.
5. You should see a clear boundary between blue on top and white on bottom.
6. If this boundary is not at 35 (using the ppt scale on the right), then use a small screwdriver to adjust the calibration screw on top of the refractometer until the boundary between blue and white is at 35.
7. Rinse and clean the prism surface and lid with ultrapure water and then wipe dry.

Sample Analysis

1. Add a drop of sample onto the blue prism surface and close the prism lid.
2. Look through the eyepiece towards a source of light.
3. Read the number at the boundary between blue and white as the value for salinity.