Salinity differences between refractometer and American marine pinpoint prob.

[Jmas4]

My two instruments are showing me different salinity measurements in the same water. Both are calibrated. My refractometer is showing me 1.035 while my probe is showing me 1.022. What is right?

 

[TUSI]

Some thing is not calibrated properly or broken.

 

[chipmunkofdoom2]

It's possible that neither is right.

First, be sure you calibrated the refractometer with a salinity standard, either a seawater mimic or a solution specifically for refractive index measurement. For various reasons, calibrating a refractometer with freshwater can produce incorrect results. If you did not calibrate the refractometer with a salinity standard, I would do so. This could be the source of the discrepancy. Also be sure that you calibrate the Pinpoint meter with a solution designed for conductivity measurement. A solution designed specifically for refractive index may not produce the same results when measuring using conductivity. Additionally, when measuring with the Pinpoint meter, try taking a cup of water from the aquarium and measure from that. Be sure to swirl the probe around in the cup to remove any bubbles, and give the unit 3 - 5 minutes to compensate for temperature. This will rule out any possibility of stray voltage in your tank affecting your results.

If the discrepancy still exists after calibrating with the proper salinity standards, then there is likely an issue with one or both of the standards. I would purchase a new salinity standard for the conductivity meter and the refractometer, then repeat the tests. A simple and cheap alternative to buying new calibration fluids would be making Randy's homemade salinity standards. Be sure to note that you need a different recipe for your refractometer and your conductivity meter.

For what it's worth, I also have the Pinpoint Salinity Meter. I also have a refractometer as a backup. I used the Aquacraft calibration fluid from BRS to calibrate my refractometer. To calibrate my Pinpoint meter, I used the standard that came with my unit. When I measure my tank's water, my Pinpoint meter reads dead on at 53.0 mS and my refractometer reads dead on at 35ppt salinity.

 

[Jmas4]

It's possible that neither is right.

First, be sure you calibrated the refractometer with a salinity standard, either a seawater mimic or a solution specifically for refractive index measurement. For various reasons, calibrating a refractometer with freshwater can produce incorrect results. If you did not calibrate the refractometer with a salinity standard, I would do so. This could be the source of the discrepancy. Also be sure that you calibrate the Pinpoint meter with a solution designed for conductivity measurement. A solution designed specifically for refractive index may not produce the same results when measuring using conductivity. Additionally, when measuring with the Pinpoint meter, try taking a cup of water from the aquarium and measure from that. Be sure to swirl the probe around in the cup to remove any bubbles, and give the unit 3 - 5 minutes to compensate for temperature. This will rule out any possibility of stray voltage in your tank affecting your results.

If the discrepancy still exists after calibrating with the proper salinity standards, then there is likely an issue with one or both of the standards. I would purchase a new salinity standard for the conductivity meter and the refractometer, then repeat the tests. A simple and cheap alternative to buying new calibration fluids would be making Randy's homemade salinity standards. Be sure to note that you need a different recipe for your refractometer and your conductivity meter.

For what it's worth, I also have the Pinpoint Salinity Meter. I also have a refractometer as a backup. I used the Aquacraft calibration fluid from BRS to calibrate my refractometer. To calibrate my Pinpoint meter, I used the standard that came with my unit. When I measure my tank's water, my Pinpoint meter reads dead on at 53.0 mS and my refractometer reads dead on at 35ppt salinity.

How does rodi water affect the refractometer for calibration?

 

[Patis55]

With RODI water your calibrating to 0 . There is a large difference between 0 and 1.026. If you calibrate both instruments using a calibration fluid i would bet they both would read the same as long as both are functioning properly.

 

[n2585722]

Don't use RODI to calibrate. Use a standard that is 35ppt to calibrate.

 

[Jmas4]

My refractometer instructions say to use to water for calibration. http://www.theaquariumsolution.com/seawater-refractometer
A difference of salinity of 0 and 1.026 doesnt matter because if you calibrated it with 1.026 then ro water should be 0 and if you calibrated it with 0 saltwater should be 1.026

 

[Patis55]

If only it really worked that way in real life

 

[n2585722]

Single point calibration are not as accurate as dual point. Since a refractometer is a simgle point calibration device it is only accurate at the calibration point. There is a greater error in the reading the futher you get from the calibration point. So it is best to make that point at where your expected reading will be. In this case that is 35ppt. Just make sure the standard you use is compatible with refractometers.

 

[chipmunkofdoom2]

How does rodi water affect the refractometer for calibration?

Randy Holmes-Farley's article on the subject details the many ways in which hobby-grade refractometers can produce inaccurate results (see figures 15 and 17, which describe freshwater calibration specifically). Calibrating with freshwater will only produce accurate results at 35ppt salinity if the refractometer is perfectly accurate across its entire scale of measurement. It's possible you have a perfect refractometer, and that calibrating with freshwater will work just fine. I personally would not make that assumption. Calibration fluid is just too cheap, and DIY standards are exceedingly simple to make if a commercial solution is unavailable.

The only way to be certain that your refractometer reads 35ppt salinity accurately is to calibrate with a solution known to be 35ppt salinity.