Refractometer

[athompson]

Learned a lesson today. When applying the solution to ur prism, make sure you do only use 2-3 drops. Just enough to make a thin layer across your prism. If u put too much sample on ur prism, it will throw ur results by as much as 2-3 ppt. Follow the directs exactly. I did an experiment with calibration solution because depending on the amount of water i put on my refractometer from my tank id get anywhere from 32-35. The more the sample, the lower the ppt. Tested it with my 35ppt sample and it did the same thing. Maybe im the only moron that thought more is more but i thought id still put it out there

[dkeller_nc]

Something is a bit "off" here. The refractive index of a solution is defined as the ratio of the speed of light in the solution to the speed of light in a vacuum.

Obviously, one can't easily measure these two velocities, so a property that is directly related to the refractive index is measured, which is the change in angle of incidence of light when it crosses the interface of the solution and a reference material. In the case of a refractometer, the reference material is the glass prism itself.

The reason the above is relevant is that the refractive index is an "intrinsic property", meaning that the total volume or path length of the light going through the sample doesn't affect the refractive index. It's a bit like temperature - it is also an intrinsic property, so the temperature of a gallon of water is the same as 5 gallons of the same water.

There are a lot of possibilities as to why your observation would be so, most of them related to the refractometer itself. One that isn't related to the refractometer is temperature - the refractive index is quite sensitive to this parameter, and 2 drops of a solution on the prisim would change temperature faster than 5 to 10 drops. If you want to test this, put your refractometer, calibration solution and seawater samples in the oven (with the oven off and "cold", obviously!) and leave them for 20 to 30 minutes. The calibration solution and seawater samples must be in sealed containers to prevent concentration by evaporation, and also to ensure that you don't get evaporative cooling.

Because the interior of the oven isn't subject to drafts, the refractometer, calibration solution and seawater samples should be within a tenth of a degree farenheit.

If it turns out that your observation about your refractometer isn't due to temperature, you might consider returning it for an exchange if you've just purchased it. If not, I wouldn't throw it out - even 2 ppt salinity inaccuracy won't matter for your tank so long as the salinity is stable.

[athompson]

Hmmmm, that seems like a lot if work. If i follow instructions and only put a few drops on the prizm, making sure its completely covered with no air bubbles, its dead accurate. I guess my whole point was to say, follow the instructions on ur equipment, whether its ur refractometer, test kits, or whatever else. It really does matter.

[dkeller_nc]

Indeed it does (following instructions matters). The reason I wrote the reply, though, is so that someone else doesn't draw the conclusion that precise volume measurements are necessary to correctly measure refractive index in general and with every instrument. For example, I use a digital refractometer, and the number of drops placed on the prism doesn't affect the readings at all so long as the prism is completely covered with seawater.

But your note was nonetheless useful to others. I don't know this for certain, but I suspect that almost all handheld manual refractometers typically sold to aquarists are made in the same factory out of the same components, regardless of the brand. So it's a reasonable conclusion that others would experience what you observed.

[athompson]

Indeed you are right about that. I didnt think about types other than the handheld kind i am using ( which i assume many beginners also use).

Its also noteworthy that if you notice that ur line (where blue and white meet) is kind of fuzzy, that is another indication that u have too much saltwater on the prizm. atleast for my type of refractometer

[sleepydoc]

I've found that the type of light makes a difference in the sharpness of the line. Incandescent lights are the worst. Sunlight and fluorescents are much better.

[hogfanreefer]

On my refractometer it's not too much water that makes the line fuzzy, it's when the cover is not all the way down.