trevorhiller
Reefing Nurse
Since getting into the hobby, I've been down the rabbit hole on salinity/specific gravity measuring devices. I've had the Hanna HI98319 Salinity Pen, a $20 Amazon White-Label refractometer, the Milwaukee MA887 digital refractometer, a VeeGee STX-3 & now the Misco Aquar-H2O digital refractometer. Years ago I even had the Instant Ocean Swing Arm Hydrometer. 
I'm going to leave the review of that out since I don't currently have it and it's been several years since I used it. I also would not currently recommend it.
I figured since I am probably one of the few to try so many devices I would post some pros and cons of the various options for those looking. These are listed in order of reliability & accuracy based on my personal experience with them.
1. Misco Aqua-H2O
-Pros: VERY precise and accurate. Build quality is on point. This device "feels" like a laboratory instrument. It can measure down to the ten-thousandths (ie. 1.0264 one more decimal place than other available options). This is nice because you can really catch drifting salinity quickly. It is rated to an accuracy of +/- 0.0005 and also comes with a PSU scale if your more interested in the 35 ppt-style readings. The device is easy to calibrate with RO water before a test, no special calibration solution required. You can also customize the measurement units available on the device, Misco offers several and custom programming is available via their website.
-Cons: $$$. The main con is the price. You're in for $470 for the device only, but to get the silicone case (which you'll surely want to protect your serious investment from accidental drops) you're at around $530. When using the PSU scale the screen only displays whole number values. No temperature readout.
2. VeeGee STX-3
-Pros: Typical precision, very consistent. In the time I've had mine it has held zero since the initial setup. When you feel this device in your hand you know it is quality made. The adjustment ring locks tightly via a set screw. It is very beefy which gives it a lot of thermal inertia, I believe this helps with the automatic temperature compensation (which is very important to accurate measurements if you didn’t know). It is also designed to be zeroed using RO/DI water unlike most of the other refractometers on the market. This makes it very easy to verify that you are getting consistent & accurate results with every measurement. Price when I bought it was around $75 on sale at Amazon. It can be had today for about $105 via the same source.
-Cons: It is analogue requiring a drop of water on the glass and a light source to allow you to view the results though the eyepiece. However, the whole measurement process including zeroing and cleaning can be done in about 30 seconds. It can be slightly annoying to zero as sometimes tightening the set screw shifts the scale unintentionally. I recommend looking through the eyepiece while tightening the set screw down to verify your zero actually lands on zero.
3. Hanna HI98319 "Salinity Pen"
-Pros: Price is around $80. I don't think this is bad for combination salinity/temperature measuring tool. This is the only device I tested that provides a useful temperature readout. It is by far the fastest of all the options IME. It is also water resistant. I run mine under the tap to clean it off-no issues. I still use mine for mixing saltwater as it's easy to dunk it in my mixing barrel and rinse under the tap when done. I often use it while adding salt as you can watch the salinity rise as you add it to give you a rough idea of where you are at. As for accuracy, mine seems to always read about 0.7 ppt lower than the above two devices. I highly recommend making a batch of RHF DIY 53 mS/cm calibration solution which allows for frequent recalibration at almost no cost.
-Cons: The calibration process is a bit annoying, but it is certainly doable. I always calibrate before use when taking actual tank measurements and this has the most elaborate calibration process (similar to a pH probe calibration with a packet of standard). For mixing water change water "close enough" is good enough for me and I don't calibrate it before every use when used in this fashion. If you buy the Hanna calibration solution this device has a recurring expense. This device also uses a small watch-style battery which you probably don’t have on hand. However, it does display battery life as a percentage on the screen.
4. Milwaukee MA887
-Pros: Quick & easy to use, provides consistent results. This device had the most innacurate results when compared to the others. It is easy to zero with RO/DI water, no special calibration solution required (although I would recommend making a standard just to check the accuracy of the device. It is water resistant and easy to clean. The build quality was satisfactory.
Cons: Accuracy: It is rated to +/- 2 ppt or +/- 0.002 specific gravity - not great IMO. Again, I would recommend making Randy's refractometer calibration solution and checking it's accuracy; mine was off by +2 ppt. While the device does provide a sample temperature readout, it is not particularly helpful as the refractometer is designed to cool or heat the water to the temperature of the metal sample ring (ambient temperature).
Note: I sold my MA887 as I did not like the level of accuracy or precision it affords.
5. Budget While-Label Refractometer
-Pros: Cheap, works. I purchsed one called "Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 PPT with Automatic Temperature Compensation" for around $20 on Amazon. I suspect they are all the same though.
-Cons: It measures salinity, but not very reliably. The only way I would recommend these is if you are using a float arm hydrometer. I found mine easily coming off of zero requiring constant calibration before use. I wouldn't trust my reef tank to one of these after using any of the above devices unless you are going to be strict about calibrating before EVERY measurement. If you can be disciplined to do this, it's a great budget option. However, they feel cheap in the hand and have low build quality. You can find these relabeled under various brands, but they all appear to be made in the same asian factory. Mine is relegated to my fish-only quarantine setup where accurate salinity is non-essential.
Hopefully this information helps you make your decision. Perhaps some day I will get the Tropic Marin floating hydrometer to test out just to make this comparison a little more complete. Remember, whichever device you choose to use, CALIBRATE BEFORE EVERY USE! It's the only way you can be sure your results are accurate.
Note: I purchased all of these with my own money and have no conflicts of interest.
I'm going to leave the review of that out since I don't currently have it and it's been several years since I used it. I also would not currently recommend it.I figured since I am probably one of the few to try so many devices I would post some pros and cons of the various options for those looking. These are listed in order of reliability & accuracy based on my personal experience with them.
1. Misco Aqua-H2O
-Pros: VERY precise and accurate. Build quality is on point. This device "feels" like a laboratory instrument. It can measure down to the ten-thousandths (ie. 1.0264 one more decimal place than other available options). This is nice because you can really catch drifting salinity quickly. It is rated to an accuracy of +/- 0.0005 and also comes with a PSU scale if your more interested in the 35 ppt-style readings. The device is easy to calibrate with RO water before a test, no special calibration solution required. You can also customize the measurement units available on the device, Misco offers several and custom programming is available via their website.
-Cons: $$$. The main con is the price. You're in for $470 for the device only, but to get the silicone case (which you'll surely want to protect your serious investment from accidental drops) you're at around $530. When using the PSU scale the screen only displays whole number values. No temperature readout.
2. VeeGee STX-3
-Pros: Typical precision, very consistent. In the time I've had mine it has held zero since the initial setup. When you feel this device in your hand you know it is quality made. The adjustment ring locks tightly via a set screw. It is very beefy which gives it a lot of thermal inertia, I believe this helps with the automatic temperature compensation (which is very important to accurate measurements if you didn’t know). It is also designed to be zeroed using RO/DI water unlike most of the other refractometers on the market. This makes it very easy to verify that you are getting consistent & accurate results with every measurement. Price when I bought it was around $75 on sale at Amazon. It can be had today for about $105 via the same source.
-Cons: It is analogue requiring a drop of water on the glass and a light source to allow you to view the results though the eyepiece. However, the whole measurement process including zeroing and cleaning can be done in about 30 seconds. It can be slightly annoying to zero as sometimes tightening the set screw shifts the scale unintentionally. I recommend looking through the eyepiece while tightening the set screw down to verify your zero actually lands on zero.
3. Hanna HI98319 "Salinity Pen"
-Pros: Price is around $80. I don't think this is bad for combination salinity/temperature measuring tool. This is the only device I tested that provides a useful temperature readout. It is by far the fastest of all the options IME. It is also water resistant. I run mine under the tap to clean it off-no issues. I still use mine for mixing saltwater as it's easy to dunk it in my mixing barrel and rinse under the tap when done. I often use it while adding salt as you can watch the salinity rise as you add it to give you a rough idea of where you are at. As for accuracy, mine seems to always read about 0.7 ppt lower than the above two devices. I highly recommend making a batch of RHF DIY 53 mS/cm calibration solution which allows for frequent recalibration at almost no cost.
-Cons: The calibration process is a bit annoying, but it is certainly doable. I always calibrate before use when taking actual tank measurements and this has the most elaborate calibration process (similar to a pH probe calibration with a packet of standard). For mixing water change water "close enough" is good enough for me and I don't calibrate it before every use when used in this fashion. If you buy the Hanna calibration solution this device has a recurring expense. This device also uses a small watch-style battery which you probably don’t have on hand. However, it does display battery life as a percentage on the screen.
4. Milwaukee MA887
-Pros: Quick & easy to use, provides consistent results. This device had the most innacurate results when compared to the others. It is easy to zero with RO/DI water, no special calibration solution required (although I would recommend making a standard just to check the accuracy of the device. It is water resistant and easy to clean. The build quality was satisfactory.
Cons: Accuracy: It is rated to +/- 2 ppt or +/- 0.002 specific gravity - not great IMO. Again, I would recommend making Randy's refractometer calibration solution and checking it's accuracy; mine was off by +2 ppt. While the device does provide a sample temperature readout, it is not particularly helpful as the refractometer is designed to cool or heat the water to the temperature of the metal sample ring (ambient temperature).
Note: I sold my MA887 as I did not like the level of accuracy or precision it affords.
5. Budget While-Label Refractometer
-Pros: Cheap, works. I purchsed one called "Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 PPT with Automatic Temperature Compensation" for around $20 on Amazon. I suspect they are all the same though.
-Cons: It measures salinity, but not very reliably. The only way I would recommend these is if you are using a float arm hydrometer. I found mine easily coming off of zero requiring constant calibration before use. I wouldn't trust my reef tank to one of these after using any of the above devices unless you are going to be strict about calibrating before EVERY measurement. If you can be disciplined to do this, it's a great budget option. However, they feel cheap in the hand and have low build quality. You can find these relabeled under various brands, but they all appear to be made in the same asian factory. Mine is relegated to my fish-only quarantine setup where accurate salinity is non-essential.
Hopefully this information helps you make your decision. Perhaps some day I will get the Tropic Marin floating hydrometer to test out just to make this comparison a little more complete. Remember, whichever device you choose to use, CALIBRATE BEFORE EVERY USE! It's the only way you can be sure your results are accurate.
Note: I purchased all of these with my own money and have no conflicts of interest.
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