I use a refractometer and always calibrate it. The water is testing at 1.025. According to this test my water is at 1.029 ?????!!!! How is this possible? Can someone help?
Also, any issues in the results you see?
ICP Results - 38.9 ppt????
- Thread starter Eggpaul
- Start date
- Tagged users None
I use a refractometer and always calibrate it. The water is testing at 1.025. According to this test my water is at 1.029 ?????!!!! How is this possible? Can someone help?
Also, any issues in the results you see?
2Sunny
Active Member
I recently found my refractometer was off by 2 ppt just because I was using an indoor light source and not sunlight. I also know from past experience that if you do not calibrate a refractometer using BOTH RO/DI AND a 35 ppt AND not at the correct temp it's easy to get another 2 ppt off.
If it were me, I would calibrate my refractometer with both RO/DI and 35 ppt solution before accepting that result.
If it were me, I would calibrate my refractometer with both RO/DI and 35 ppt solution before accepting that result.
I would get another instrument to double check your refractometer.
Your refractometer or calibration solution could be off.
I really like my hanna HN8319 but don't like my hanna HN1411. The HN8319 calibrates with a 35ppt solution and the hanna HN 1411 calibrates with RODI water. The HN1411 is always reading my salinity as +2ppt off.
My apex and hanna HN8319 read the same measurements.
Your refractometer or calibration solution could be off.
I really like my hanna HN8319 but don't like my hanna HN1411. The HN8319 calibrates with a 35ppt solution and the hanna HN 1411 calibrates with RODI water. The HN1411 is always reading my salinity as +2ppt off.
My apex and hanna HN8319 read the same measurements.
2Sunny
Active Member
35 ppt solution, but it's many years old...
Now there's another potential variable. Time for some fresh solution
2Sunny
Active Member
Exactly.
92Miata
Valuable Member
Most refractometers have no way to adjust the slope - they're just really setting an offset - so for most you can't really calibrate two points. If you get it to 0 on RODI, it'll be off at 35ppt, and if you calibrate to 35, it'll be off at 0
Being on at 35ppt is way more important than being on at 0.
You're almost always making things worse touching the calibration know on a refractometer with RODI in it.
Philly Reefer
Valuable Member
Even with new solution it can be wrong. I used brightwell solution and it's off by 2 ppt. Bought a different solution from Aqua Craft and I am hopeful that it's correctly showing 35ppt
chipmunkofdoom2
Always Making Something
35 ppt solution, but it's many years old...
It would make sense that you're reading high then. As time goes along the water will evaporate, making the solution more concentrated.
You can make your own calibration fluid using table salt.
excell007
Active Member
Have a secondary intrument to compare it with. Twice a month I compare the result of Hanna to a floating hydrometer. Use the one specific for aquarium cause it is calibrated at 77 degrees, the ones used for brewing are calibrated at a much lower temperature. After about a month my Hanna gets off by 0.6ppt, signalling the need to recalibrate.
2Sunny
Active Member
Well you better tell that to Randy Holmes Farley. You can find his recommended procedure uner the section titled: Tips on Calibrating a Refractometer
Refractometers and Salinity Measurement
Philly Reefer
Valuable Member
I am curious about making DIY solution. Unfortunately it has to use 2L bottle. I would have tried it if its using smaller bottle
92Miata
Valuable Member
I'd suggest you reread that article and skip the section where he talks about "perfect refractometer" and go straight to the one where he talks about fixing real refractometers.
Ie, slope issues and offset issues.
2Sunny
Active Member
Tips to Calibrate a Refractometer from Randy:
Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. So my recommendation for calibration is as follows:
1. First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonable TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261.
This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I've never understood how that could matter and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article.
Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified to have passed the second calibration step below at least once. This is an offset calibration, as described above.
2. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. This step involves testing it in a solution matching the refractive index of 35 ppt seawater (or some similar solution near the range of measurement). Remember to let it sit for at least 30 seconds so it comes to the same temperature as the refractometer. Suitable commercial and do-it-yourself standards were described earlier in this article. Using one of them, place a drop onto the refractometer and read the value. If it reads approximately 35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940, then the refractometer is properly calibrated and is set to go.
. . .
Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. So my recommendation for calibration is as follows:
1. First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonable TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261.
This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I've never understood how that could matter and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article.
Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified to have passed the second calibration step below at least once. This is an offset calibration, as described above.
2. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. This step involves testing it in a solution matching the refractive index of 35 ppt seawater (or some similar solution near the range of measurement). Remember to let it sit for at least 30 seconds so it comes to the same temperature as the refractometer. Suitable commercial and do-it-yourself standards were described earlier in this article. Using one of them, place a drop onto the refractometer and read the value. If it reads approximately 35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940, then the refractometer is properly calibrated and is set to go.
. . .
2Sunny
Active Member
No actually I suggest YOU read the article because I have read it many times over. Since we can't know the errors that the OP's refractometer may or may not have Randy came up with a general process to fix any and or all different types of errors.
92Miata
Valuable Member
Again, you'll notice that the salt water step comes second - because it'll throw the calibration off at 0
Because the calibration at 0 does not matter. If you touch the calibration knob at 35ppt, it's no longer accurate at 0
Hobby level refractometers don't allow you to change both the slope and the offset - so it can only be accurate at one point.
You want that point to be as close to the desired salinity as possible.
