Best Salinity Measurement Tool?

[DivingTheWorld]

The one measurement in my tank that I can't seem to get an accurate read on is salinity. I've tested with 4 different devices and the readings are considerably different. All were calibrated just prior to testing.

Standard Refractometer - 34 ppt (as close as my eyeball can read)
Milwaukee Digital Refractometer - 34 ppt
Pinpoint Salinity Monitor - 32 ppt
Apex Salinity Module/Probe - 37.3 ppt

So what gives? Do I just chalk this up to an imperfect measurement and wing it? Do I just go with the Milwaukee since that reads in the middle?

What is everyone using to measure Salinity and are you getting matches between different devices?

 

[jamie1981]

Salinity Refractometer calibrated with DI water to 0 I think your too concerned with the exact reading, tank stability in the proper range is more important.

 

[E Rosewater]

Disagree with calibration to 0. Calibrate with a 35ppt standard.

 

[DivingTheWorld]

Disagree with calibration to 0. Calibrate with a 35ppt standard.

Yes and no. A standard refractometer can be (and I did) calibrated with 35 ppt like an Apex or Pinpoint. The Milwaukee digital refractometer can not. That has to be calibrated to 0.

The only potential variable is that I zero'd my Milwaukee with RODI. The instructions recommend either RODI or distilled water. I'm going to try distilled and see if that makes a difference.

 

[jamie1981]

Disagree with calibration to 0. Calibrate with a 35ppt standard.

Never understood why people say not to use di water to calibrate a refractomer. No way to know that the 35ppm solution is actually 35ppm without some kind of certification either. I know that DA released some solution with their salinity probes a while back that people were reporting all over the place. DI water is cheap and readily available to most hobbyist and highly accurate for our purposes. Saltwater aquariums are not an exact science. Some times I think people chase test kits to much and over complicate things and end up causing more issues than they solve but that's just my .02

 

[E Rosewater]

Many handheld refractometers do not have a linear response over the entire range of their use. Just because it's calibrated at zero doesn't mean that calibration still holds true at 35ppt.

 

[outssider]

Many handheld refractometers do not have a linear response over the entire range of their use. Just because it's calibrated at zero doesn't mean that calibration still holds true at 35ppt.

+1

mine is way off if I use ro/di water to calibrate...

it's also way off if I calibrate it one day at room temperature of 70 degrees then use it a few days later at 80 degrees. It has atc too !

I always calibrate with 35ppt right before every use.

 

[bertoni]

I preferred the PinPoint conductivity meter, personally.

 

[RG Rude]

I've often wondered myself. I cross my fingers and use a Pinpoint Salinity Monitor. I'm not sure if its accurate but as Jamie1981 pointed out I do have consistency. I just hope I'm not consistently way off.

 

[acabgd]

Best tool for measuring salinity
http://www.tropic-marin.com/meerwasseranalytik/de-araeometer-en-hydrometer-fr-areometre/?lang=en

Handmade, resolution down to 0.0001

I use it to calibrate my refractometer which is easier to use on a daily basis.

 

[DivingTheWorld]

Best tool for measuring salinity
http://www.tropic-marin.com/meerwasseranalytik/de-araeometer-en-hydrometer-fr-areometre/?lang=en

Handmade, resolution down to 0.0001

I use it to calibrate my refractometer which is easier to use on a daily basis.

Interesting, I've never heard of these. Might be a great way to calibrate my other devices. Thanks.

 

[Randy Holmes-Farley]

FWIW, I like to use conductivity for salinity measurement. Fast, accurate, easily checked in standards, and works for other things too, such as limewater potency.

Interesting, I've never heard of these. Might be a great way to calibrate my other devices. Thanks.

I had one and it was quite accurate. I reviewed it here:

http://www.advancedaquarist.com/2002/1/chemistry

 

[DiscusHeckel]

Several years ago, D-D carried out an experiment on their refractometer's accuracy after I initiated a debate on calibration solutions on a UK forum. They used ATC refractometer designed for brine applications. They also used a lab grade hydrometer (TMC brand) to measure the salinity (and specific gravity) of salt water solution at a specific temperature. They then repeated the experiment with their aforementioned refractometer calibrated using RODI. The result was that the refractometer's reading was approximately 1.5 ppt higher than the hydrometer's reading.

Based on this experiment and D-D's advice, I calibrate my D-D refractometer (designed for brine applications) at a room temperature of 20C with RODI and set it to zero. Then, I deduct 1.5 ppt from the reading. Hence, to get 35 ppt, I target 36.5 ppt. It has worked for me. For what it is worth, I use a certified deionised water to ensure that it has a zero TDS with 99.99% accuracy.

The following link provides more information on their advice.

http://www.theaquariumsolution.com/measuring-salinity-and-specific-gravity-your-aquarium

 

[nibor75]

Several years ago, D-D carried out an experiment on their refractometer's accuracy after I initiated a debate on calibration solutions on a UK forum. They used ATC refractometer designed for brine applications. They also used a lab grade hydrometer (TMC brand) to measure the salinity (and specific gravity) of salt water solution at a specific temperature. They then repeated the experiment with their aforementioned refractometer calibrated using RODI. The result was that the refractometer's reading was approximately 1.5 ppt higher than the hydrometer's reading.

Based on this experiment and D-D's advice, I calibrate my D-D refractometer (designed for brine applications) at a room temperature of 20C with RODI and set it to zero. Then, I deduct 1.5 ppt from the reading. Hence, to get 35 ppt, I target 36.5 ppt. It has worked for me. For what it is worth, I use a certified deionised water to ensure that it has a zero TDS with 99.99% accuracy.

The following link provides more information on their advice.

http://www.theaquariumsolution.com/measuring-salinity-and-specific-gravity-your-aquarium

How do you know that your refractometer has the same slope error as the one they used for their test?

 

[jimmcwibb]

I use the DIY NaCl (table salt) reference solution to calibrate my refractometer. 36.5 ppt gives the equivalent of 35 ppt seawater, and you can make it easily using baked salt (to drive off moisture) and a set of scales that measure to 0.1g.

See http://www.reefkeeping.com/issues/2004-06/rhf/

I have access to a lab at work and I've checked my refractometer against a lab grade seawater reference solution which is certified 35 ppt - it was spot on. So I'm happy with that.

In my opinion calibration with RO/DI would be OK for precision (and therefore consistency in your aquarium) but for accuracy? Who knows. It's a long way off 35ppt. It's best to calibrate as close to the measured value as possible.

 

[DivingTheWorld]

For Apex probe calibration, I used the Neptune Salinity solution (53,000). For my standard refractometer and Pinpoint, I used the Pinpoint Salinity solution.

I ordered a Tropic Marin hydrometer. When it arrives, I'll test it out and post my results.

I agree that the important thing is consistency and my salinity has been consistent. I guess I just want to also know that my consistent measurement is correct.

 

[outssider]

Best tool for measuring salinity
http://www.tropic-marin.com/meerwasseranalytik/de-araeometer-en-hydrometer-fr-areometre/?lang=en

Handmade, resolution down to 0.0001

I use it to calibrate my refractometer which is easier to use on a daily basis.

Maximum deviation: 0.001 at 77° F (25° C) = accuracy

Resolution: 0.0001 = that means that's the divisions that are visible on the instrument, not the accuracy.

 

[laga77]

Yes and no. A standard refractometer can be (and I did) calibrated with 35 ppt like an Apex or Pinpoint. The Milwaukee digital refractometer can not. That has to be calibrated to 0.

The only potential variable is that I zero'd my Milwaukee with RODI. The instructions recommend either RODI or distilled water. I'm going to try distilled and see if that makes a difference.

When I opened my Milwaukee there was a card added to the box stating to use only distilled. I e-mailed Milwaukee customer service with concerns on the accuracy with RODI and distilled. They sent me a sample bottle of distilled and a bottle of 1.026 SG control solution. I also purchased a bottle of 35ppt saltwater control. The only way I could get repeatable results was with the distilled water calibration. Even with new membrane and fresh DI in my RODI system I came up with variations in my tests. I even came up with variations using store bought distilled water. I now make my own distilled for calibration and come up with consistent results.

 

[Randy Holmes-Farley]

The Milwaukee MA887 is a true seawater refractometer and so the brine vs seawater issue is not a concern and RO/DI should be fine if the refractometer is perfectly made.

If it physically could allow a 35 ppt standard it would work perfectly, but the software doesn't allow it.

A 35 ppt standard will never be wrong (if the device allows it), so is the best choice for many refractometers.

 

[rayjay]

I've checked the accuracy listed for many refractometers over the years, and the results don't really impress me enough to buy one of ANY brand available.
For example, see the accuracy listed for the M A 8 8 7 D I G I T A L R E F R A C T O M E T E R
Others I have seen are worse and few are significantly better.
I'm with the faction that is more interested in a reasonably stable s.g. over being exact.
For over 20 yrs now I've been using Seatest "box" swing arm hydrometers that I check against my Fisher Scientific certified calibrated hydrometer (used in proper cylinder) about 2 to 3 times a year.
Of course the swing arms are thoroughly rinsed after each use, and soaking in white vinegar overnight once a week.
When I first started, there were no refractometers in any stores here, and when they did show up, I never saw the need to buy something that wasn't going to give me any significantly better results, or to make my tanks better.