Calibration of a Refractometer
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Dr.Pat
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never rely on fresh water to calibrate.while it will read at 0.0 it will not tell you if the refractometer is off.I lost a reef tank because of that . while the meter read 0.0 with ro /di it was 10 points off when checked with a salinity calibration solution.I now use a salinity meter with a probe and it requires much less calibration. the cheaper the refractometer the less reliable they are!
Seriously, it's $14 for a bottle on amazon. You should calibrate at 2 points, 0 and 35ppt. Use the solution, check it at 35ppt, then use Rodi and check that 0 is correct.
You'll likely find your refractometer is a bit off if you've never done it before.
That $14 dollar bottle will last you a long time. It's such a small cost for peace of mind with your large tank investment
You'll likely find your refractometer is a bit off if you've never done it before.
That $14 dollar bottle will last you a long time. It's such a small cost for peace of mind with your large tank investment
Dr.Pat
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Downbeach
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I don't usually have to make an adjustment, but I check it with each WC, in my case about every two weeks. Most refractometers now have an ATC(auto temp correct) feature, so let it sit for about 30 seconds after adding the sample before reading. Almost all have a single screw adjustment, and should be adjusted @ 35ppt with the solution.
Randy Holmes-Farley
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With the exception of certain electronic refractometers that physically won't allow calibration with a 35 ppt seawater standard, it is never wrong to use such a standard.
If you have a true seawater refractometer (which will say so), then it is also OK to calibrate with Ro/DI water. Both should give the same calibration, although using the 35 ppt standard is calibrating closer to what you want to measure so may avoid or reduce certain manufacturing imperfections that might be present.
If it doesn't say it is a true seawater refractometer, it almost certainly is a brine refractometer resold to hobbyists for seawater. It is not accurately calibrated with RO/DI for purposes of measuring seawater. It is accurate that way for measuring pure sodium chloride solutions (assuming it was made correctly).
If you have a true seawater refractometer (which will say so), then it is also OK to calibrate with Ro/DI water. Both should give the same calibration, although using the 35 ppt standard is calibrating closer to what you want to measure so may avoid or reduce certain manufacturing imperfections that might be present.
If it doesn't say it is a true seawater refractometer, it almost certainly is a brine refractometer resold to hobbyists for seawater. It is not accurately calibrated with RO/DI for purposes of measuring seawater. It is accurate that way for measuring pure sodium chloride solutions (assuming it was made correctly).
saltyphish
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I calibrate mine every time I use it which is about once a week. Sometimes it is off. I think that is a result of my children playing with it or it getting banged around.
X2
ANY calibration should be done with 2 known points of reference, otherwise you can not verify the correctness of a scale...
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A refractometer can only be "calibrated" with a single fluid. It can be checked (before or after calibration) with other fluids, but...
If it is a brine refractometer, and you calibrate in any fashion so that it reads 35 ppt sewater correctly, it MUST read fresh water (RO/DI water) incorrectly by a fixed known amount. It will read incorrectly by 1.7 ppt (if you use that scale) or by 0.0003 if you read in specific gravity.
Downbeach
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Calibration should be done with a "known" or "set" amount to determine the correctness of a sample. The problem with using actual seawater is that it will vary from place to place depending on a number of factors, i.e. rain water run off, proximity to FW sources, i.e. creeks, rivers, etc. incoming vs outgoing tides, etc.
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I have a commercial true 35 ppt Atlantic seawater sample, but it would be very expensive to buy such a thing for routine use. It is much simpler to use a standard with the same known refractive index as 35 ppt seawater.
You cannot collect your own and have any confidence in the exact salinity as it varies considerably, especially near land where fresh water runs in from rain and such.
Nemo's Janitor
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Calibration of refractometers is a pet to me. Whilst two popular seawater refractometers (the D-D and the Red Sea) claim to be seawater refractometers. Both are ATC refractometers. One has a calibration temperature of 20c the other at 25c. This difference should be indicated by a different SG indication corresponding to the Salinity scale.
What i am asking is that the pinpoint salinity calibration fluid 53ms is calibrated at 25c. Would that not give an incorrect reading if used on a instrument that has a calibration temperature of 20c?
What i am asking is that the pinpoint salinity calibration fluid 53ms is calibrated at 25c. Would that not give an incorrect reading if used on a instrument that has a calibration temperature of 20c?
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The Pinpoint fluid is a 35 ppt seawater mimic solution. It has a conductivity of 53 mS/cm at 25 deg C, although one need not use it at that temp for a conductivity probe since they all measure and correct for temperature. It is a 35 ppt solution, regardless of temperature.
In using it for an ATC refractometer, the temp won't matter within the range of the ATC capability. It doesn't make any sense (regardless of what the manufactuer claims) to require an ATC refractometer to be calibrated at a particular temp, as long as you are within its range. Using the Pimpoint fluid, it will be fine at temps other than 25 deg C since it essentially is seawater (the major ions, anway) and is a 35 ppt standard regardless of the temperature.
If it is not an ATC refractometer, then yes, one must be concerned with calibration and use temperatures.
In using it for an ATC refractometer, the temp won't matter within the range of the ATC capability. It doesn't make any sense (regardless of what the manufactuer claims) to require an ATC refractometer to be calibrated at a particular temp, as long as you are within its range. Using the Pimpoint fluid, it will be fine at temps other than 25 deg C since it essentially is seawater (the major ions, anway) and is a 35 ppt standard regardless of the temperature.
If it is not an ATC refractometer, then yes, one must be concerned with calibration and use temperatures.
Nemo's Janitor
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Thank you Randy. The Pinpoint solution is not readily available in where I live. I had read your article on making a standard calibration solution using coke bottles. I tweaked it a little and used a balance to measure the salt mix I use, and the RO water to get a S=35. Some few years ago i made up 2 liters and use that as a standard. It might not be 100% accurate but it enables me to keep salinity consistent.
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You're welcome.
Happy Reefing.
Happy Reefing.
