From ReefPedia
Hand held Salinity (Salt) Refactometer
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Hand held Salinity (Salt) Refactometer is a useful tool to check the salinity of the salt water.
The Refactometer is designed for testing the concentration of salt water and brine. Its provides a direct reading of the specific gravity and concentration (Parts Per Thousand) of salt in water. It provides reliable refractive index and salinity reading of toal dissolved solids of aqueous solution. The model having ATC (Automatic Temperature Compensation) is ideal for field use.
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 'Hand held Refractometer' |
Theory
The speed of light in a vacuum is alway the same, but when light moves through any other medium it travels more slowly.
Whenever light changes speed as it crosses a boundary from one medium into another its direction of travel also changes. By measuring the angle of refraction and knowing the index of refraction of the layer that is in contact with the sample, it is possible to determine the refractive index of the sample accurately.
In the refractometer the salt water sample is sandwiched into a thin layer between an illuminating prism and a refracting prism. The refracting prism is made of glass with a high refractive index. The light source is projected through the illuminating prism, the bottom surface of which is roughened to diffuse the light. Sample with different refractve indexes will produce different angles of refraction and thus the salinity of the water can be found.
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Operation Step (Calibration)
Step 1
Open daylight plate, and place 2 - 3 drops of distilled water (DI water can be used too) on the main prism. Close the daylight plate so the water spreads across the entire surface of the prism without air bubbles or dry spots. Allow the sample to temperature adjust on the prism for approximately 30 seconds before going to step #2. (This allows the sample to adjust to the ambient temperature of the refractometer)
Step 2
Hold daylight plate in the direction of a light source and look into the eyepiece. You will see a circular field with graduations down the center ( you may have to focus the eyepiece to clearly see the graduations). The upper portion of the field should be blue, while the lower portion shoud be white.
Step 3
Look into the eyepice and turn the Calibration Screw until the boundary between the upper blue field and the lower white field meet exacly on the zero scale. This is the end of the calibration process. Make sure the ambient room temperature is correct for the solution you are using (20 C/ 68 F). If the instrument is equippped with Automatic Temperature Compensation system, the ambient working temperature of the room must be 20 C (68 F) whenever the instrument is recalibrated. Once calibrated, shifts in ambient temperature within the acceptable range ( 10 C - 30 C), should not effect accuracy.
Using the refractometer
Now place a few drops of the sample to be tested onto the main prism, close the daylight plate and check reading. Take the reading where the boundary line of blue and white cross the graduated scales. The scale will provide a direct reading of the concentration.
Additional Information:
Refractometers and Salinity Measurement by Randy Holmes-Farley
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