refractometer calibrating
- Thread starter dmjordan
- Start date
scsinet
Active Member
If you want to get fancy, you can easily make the 2-3 drops of distilled water...
Put a dinner plate in the freezer for a few minutes...
Just get a pot of water on the stove to a rolling boil.
Take out the plate and hold it at an angle over the pot so the steam hits it. Water will condense on the plate and run to the bottom of the angle, where you can hold a measuring cup, etc to catch the drops.
Bingo Bango... distilled water.
Of course, one could argue that your plate and measuring cup aren't ever perfectly "clean" so once the water hits it, it's not distilled... but again.. semantecs.
Put a dinner plate in the freezer for a few minutes...
Just get a pot of water on the stove to a rolling boil.
Take out the plate and hold it at an angle over the pot so the steam hits it. Water will condense on the plate and run to the bottom of the angle, where you can hold a measuring cup, etc to catch the drops.
Bingo Bango... distilled water.
Of course, one could argue that your plate and measuring cup aren't ever perfectly "clean" so once the water hits it, it's not distilled... but again.. semantecs.
javatech1
Member
It is widely believed that only pure water is required to calibrate refractometers. That fact is true of many refractometers, and is certainly appropriate for routine calibration, but it assumes that they were manufactured correctly and have not been damaged since manufacturing. As refractometers used by aquarists become less and less expensive (with some now selling for less than $30), there is every reason to believe that at some point they will no longer be accurate enough.
The only way to be sure that a given refractometer gives useful information is to check its accuracy in a solution similar to aquarium water. I believe that all refractometers should be checked in this fashion when first purchased, and again any time there is a reason to be concerned. For example, an aquarist might be concerned if an aquarium that had been running for years at a salinity of 35 ppt suddenly reads 39 ppt.
In order to provide a standard for refractometers, a solution whose refractive index is similar to normal seawater is required. Seawater with S= 35 has a refractive index of 1.3394.1 Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252)4 has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as S=35 seawater, and can be used as an appropriate standard (Table 2).
http://reefkeeping.com/issues/2004-06/rhf/index.php
The only way to be sure that a given refractometer gives useful information is to check its accuracy in a solution similar to aquarium water. I believe that all refractometers should be checked in this fashion when first purchased, and again any time there is a reason to be concerned. For example, an aquarist might be concerned if an aquarium that had been running for years at a salinity of 35 ppt suddenly reads 39 ppt.
In order to provide a standard for refractometers, a solution whose refractive index is similar to normal seawater is required. Seawater with S= 35 has a refractive index of 1.3394.1 Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252)4 has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as S=35 seawater, and can be used as an appropriate standard (Table 2).
http://reefkeeping.com/issues/2004-06/rhf/index.php
trainfever
Active Member
What did he say? :notsure:
scsinet
Active Member
Layman's terms it aint...
But I think what he's trying to say is that the because you are trying to read a reading of about 1.025, the calibration point should be closer to that because it's so far away from where the manufacturer tells you to do it.
To elaborate... when you calibrate a refractometer that reads... say 1.000 - 1.070, the readings you are most interested in are in a narrow range of that... usually a few points give or take from 1.025, since that's what most aquariums run at.
When you calibrate something, the further you get away from the reading which was used to calibrate it, the more the refractometer's margin of error causes a deviation from that mark.
Ergo, if a refractometer has a deviation or margin of error of 1% per .001, that means that if you calibrate it to 1.000 with distilled water, that reading 1.025 might be as much as 25% off. Granted, this is a highly exaggerated situation for the purposes of explanation.
Therefore, what you want to do is calibrate the device using a reading that is as close to what you will usually be reading with the device, so if you are usually reading 1.025, then you should calibrate with a 1.025 solution, so if you read water that is 1.024, even with the ridiculous margin of error described above, you will only be 1% off at most....
Of course, the problem is that in order to do that, you need to prepare a calibration solution that has been tested using a "reference" refractometer. it's similar to how you sometimes set a clock according to a reference clock, such as an atomic clock. You have to have a trusted source.. .clock or refractometer, that you know to be exactly correct. The problem is that most people don't have access to such a refractometer.
So, in the end, just calibrate it with RODI water. None of them are as inaccurate as I described above, and the bottom line is that if your refractometer reads 1.025 and your seawater is 1.024 or 1.026, it's not going to hurt anything. The important thing is that your refractometer reads the same thing every time, because it's more important that you maintain seawater at a precise certain salinity than what exactly that certain salinity is.
But I think what he's trying to say is that the because you are trying to read a reading of about 1.025, the calibration point should be closer to that because it's so far away from where the manufacturer tells you to do it.
To elaborate... when you calibrate a refractometer that reads... say 1.000 - 1.070, the readings you are most interested in are in a narrow range of that... usually a few points give or take from 1.025, since that's what most aquariums run at.
When you calibrate something, the further you get away from the reading which was used to calibrate it, the more the refractometer's margin of error causes a deviation from that mark.
Ergo, if a refractometer has a deviation or margin of error of 1% per .001, that means that if you calibrate it to 1.000 with distilled water, that reading 1.025 might be as much as 25% off. Granted, this is a highly exaggerated situation for the purposes of explanation.
Therefore, what you want to do is calibrate the device using a reading that is as close to what you will usually be reading with the device, so if you are usually reading 1.025, then you should calibrate with a 1.025 solution, so if you read water that is 1.024, even with the ridiculous margin of error described above, you will only be 1% off at most....
Of course, the problem is that in order to do that, you need to prepare a calibration solution that has been tested using a "reference" refractometer. it's similar to how you sometimes set a clock according to a reference clock, such as an atomic clock. You have to have a trusted source.. .clock or refractometer, that you know to be exactly correct. The problem is that most people don't have access to such a refractometer.
So, in the end, just calibrate it with RODI water. None of them are as inaccurate as I described above, and the bottom line is that if your refractometer reads 1.025 and your seawater is 1.024 or 1.026, it's not going to hurt anything. The important thing is that your refractometer reads the same thing every time, because it's more important that you maintain seawater at a precise certain salinity than what exactly that certain salinity is.
mrdroctopus
New Member
^^^^^What He Said^^^^^ Is the most comprehensive thing I think I've ever read. I joined just to say that.
jbailey52
Member
This is an old thread but thought I'd share. My refractometer has given me my water results at 1.027 consistently. Just found me out swing are hydrometer so I decided just to give it a try... It was reading 1.022 and I thought there is no way they are that off. So I heard about the calibration solution at 1.026 so I bought it for a couple bucks..... Yep my refractometer was way off, and I constantly calibrated with ro di water.... Just FYI