Hydrometer Vs. Refractometer

L

LJLKRL

New member
Had an interesting eye opener this weekend at the LFS.
I brought a water sample in for testing because my tank is finished cycling and I want a fish.
He tested the water with a hagen kit, I have an API kit.
Both ammonia and nitrite were zero, so at least I know my API kit is ok for now.
I told him my salinity is 1.024 according to my hydrometer and I was curious what his refractometer said.
He tested it and said your salinity is 1.029, and my jaw almost hit the counter.
Well, I bought a refractometer from him right then. When I got home I put some RODI water on it and it read zero, so I put some water from my tank on it and it read 1.027. Still high but not as high as his reading.
Next time I go in I am going to bring my refractometer and some tank water and RODI water and see what his says compared to mine.
I would think if they are both calibrated they should read the same.

I did not buy a fish yet.

I did change about 8 gallons of water out with RODI over 2 days and bring my salinity down to 1.025 according to my refractometer.
 
You should be calibrating it with a solution that is close to the refractive index of the sample you are testing. Here is an example of the fluid.
 
A refractometer is only as good as its calibration. I would get the recommended calibration solution, then you will know what your salinity is. some will say to just use DI to zero and you are good. I do not believe that. IMO you must calibrate to the range you are looking for, example 1.026.

On a side note, once you know for sure what you Water is put some in your hydrometer and you can tell how off it is. Then you can use it for a quick reference. Just my 2 cents.
 
wr80 said:
So is a Refractometer > Hydrometer? I'm guessing it's more reliable.
Click to expand...

No. This is a common misconception. Both density and refractometry measurements are equally valid ways to determine salinity. But neither instrument is accurate without calibration.

Here are some pros/cons:

Refractometer

Pros: Requires very, very little sample. This can be extremely useful in determining the salinity in a bag of fish/coral for acclimation purposes. They're easy to clean, and they're (usually) easier to calibrate.

Cons: Refractive index readings are more prone to temperature fluctuations than specific gravity readings. The instrument is more fragile than a plastic hydrometer. If you're not careful to rinse the previous sample/di water off of the measurement cell, very inaccurate determinations can result. They're usually more expensive.

Hydrometer:

Pros: Inexpensive. Rugged - you can drop them, and if there's no visible physical damage, they're good to go. The pointer-style hydrometers are very easy to read.

Cons: Subject to inaccuracies from salt encrustation. Most plastic hydrometers sold for saltwater aquarium use must be carefully calibrated, otherwise, the readings may be way off. Difficult to clean some styles. Takes a larger sample size than refractometers.
 
dkeller_nc said:
No. This is a common misconception. Both density and refractometry measurements are equally valid ways to determine salinity. But neither instrument is accurate without calibration.

Here are some pros/cons:

Refractometer

Pros: Requires very, very little sample. This can be extremely useful in determining the salinity in a bag of fish/coral for acclimation purposes. They're easy to clean, and they're (usually) easier to calibrate.

Cons: Refractive index readings are more prone to temperature fluctuations than specific gravity readings. The instrument is more fragile than a plastic hydrometer. If you're not careful to rinse the previous sample/di water off of the measurement cell, very inaccurate determinations can result. They're usually more expensive.

Hydrometer:

Pros: Inexpensive. Rugged - you can drop them, and if there's no visible physical damage, they're good to go. The pointer-style hydrometers are very easy to read.

Cons: Subject to inaccuracies from salt encrustation. Most plastic hydrometers sold for saltwater aquarium use must be carefully calibrated, otherwise, the readings may be way off. Difficult to clean some styles. Takes a larger sample size than refractometers.
Click to expand...

For a beginner which one should I buy? I like how a hydrometer can be put in a tank to constantly watch.
 
Buy a refractometer (and calibration fluid) and be done with it. Most that start with hydrometers will eventually replace it with a refractometer. There are just too many things to screw you up when using a hydrometer.

As far as putting a hydrometer in a tank and leaving it in there, that's probably not a good idea. It will foul up with algae and calcium deposits and end up not being accurate pretty quick. Once you get your tank stabilized, and you fall into a routine as far as top-off water, your salinity shouldn't change much on a day-to-day basis.
 
kurt_n said:
Buy a refractometer (and calibration fluid) and be done with it. Most that start with hydrometers will eventually replace it with a refractometer. There are just too many things to screw you up when using a hydrometer.

As far as putting a hydrometer in a tank and leaving it in there, that's probably not a good idea. It will foul up with algae and calcium deposits and end up not being accurate pretty quick. Once you get your tank stabilized, and you fall into a routine as far as top-off water, your salinity shouldn't change much on a day-to-day basis.
Click to expand...

Perfect thank you, i'll be getting one this week!
 
I've used refractometer, swing arm hydrometer & floating glass hydrometer. Personally I found the floating one very reliable & accurate provided I compensated for fluid temperature. Floating hydrometers are factory calibrated at close to the temperature range we keep our tank water at, so there's really no discernible error with respect to temperature. I preferred using it over all others, including refracto. Perhaps it's because I also use a floating hydrometer in my home brewing & wine making endeavours. Unfortunately I broke the floater for my aquarium. One of these days I'm going to pick up another. I seem to recall using my alcohol floating hydrometer to check tank water once & the reading was the same as my refractometer. I don't like the swing arm type, they're clunky & finiky. Both mine were also reading incorrectly. I used my refractometer readings as a standard & put sharpie marks on the swingarm hydrometers at 1.025. They're handy to keep around as a backup I suppose.

With respect to calibrating refractometers, my own experience, I've never used calibration fluid. I understand the concern with using RODI for calibration is possible non linearity of the refractometer scale between the 0 & 1.026 we like to measure. I've used RODI to adjust the refractometer to 0. I then tested a drop of 210 TDS tap water & the reading was still bang on 0. I then took a reading of my tank water & it was 1.025. Next step, I adjusted the refractometer to read 0.001 with a RODI sample. Another sample of tank water & guess what, the reading was 1.026, proving in my case that my refractometer is linear. Not very scientific, but to me logical. Maybe I got lucky & as mentioned, it's my personal experience. Your mileage may vary.

Generally I don't worry too much about a difference in reading or even a swing of salinity between 1.022 & 1.027, provided it's not a sudden swing caused, say, by doing a very large water change using significantly different salinity water. I'm not using it in a lab to make medicine, it's a hobby & I don't see the need to get that concerned over a .001 or .002 point variation in readings. From what I've seen on forums over the years, much of our livestock is tolerant of a surprisingly broad range in salinity provided, as mentioned the change is not too sudden. There are of course exceptions with certain animals more sensitive than others, so those with valuable sps are generally more picky about keeping salinity as stable at the preferred level as possible. Once you have your system up & running for a few weeks/months at a stable salinity, if you keep it topped up, there should be no significant variation in salinity. The only thing that will change it quickly, is failure to top up and/or water change with different salinity. If you have a nano or very small system, it doesn't take much, but with a large volume system, once it's stable, you have to really be trying hard or neglecting things for a sudden change in salinity.

Photos from a number of years ago of tank water samples on an IO swing arm, Coralife swing arm & Made in Taiwan floating hydrometer. The Coralife read 2 points lower than the other two. If I remember correctly I also sampled with the refractometer & it read 1.025. The floating hydrometer read virtually identical to the refractometer.

IOSwingArmB.jpg


CoralifeSwingArmB.jpg


TaiwanGlassHydroB.jpg
 
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Funny, the instructions that came with it say to calibrate with distilled water. The LFS guy said to use RODI water since I had some already.
 
Distilled, RO/DI... for the purposes of what they're doing, it's the same. They just want something that is as close to pure (and a SG of 1.000) as possible. And yes, most instructions will tell you to use one or the other. In fact, when I bought my first refractometer, it actually came with a small bottle of distilled water which was labelled as "distilled water/calibration fluid." :)

It really comes down to the linearity of the specific refractometer. Some will be dead nuts on when calibrated with distilled water. Some won't. It probably even varies within manufacturing batches from the same manufacturer. That's why you hear differing stories from folks. The only way to know about your own specific one is to get the calibration fluid and see where you're at when you use distilled water to calibrate.
 
Makes sense. I asked the LFS what they use just now and they said RODI.
I will order some fluid and calibrate it with that.
 
Got my calibration fluid yesterday. Check the refractometer with it and wouldn't you know it was dead on 35 ppm.
I have to trust that it is accurate.
So, my salinity in my tank reads 1.025 on the refractometer
This should be fine right?
 
LJLKRL said:
Got my calibration fluid yesterday. Check the refractometer with it and wouldn't you know it was dead on 35 ppm.
I have to trust that it is accurate.
So, my salinity in my tank reads 1.025 on the refractometer
This should be fine right?
Click to expand...

The problem with calibration fluid is that some need them and some dont. For example, a refractometer purchased from Bulk Reef Supply has calibration instructions using the fluid. The one I purchased from Marine Depot only requires distilled water per the instructions. This is not to say you should purchase one over the other, I'm just saying that it depends on the model. I haven't had to calibrate mine in awhile, but I believe that the way I was supposed to do it was to put the distilled water on there at 72 degrees and then adjust it to where the bar showed exactly zero.

The difference with calibration fluid is, as others have noted, is that it's a bottle of expensive liquid with a known level of SG. You then obviously adjust your unit to that SG listed on the solution.
 
I like the Hydrometer once is "calibrated", how do you that? well find a friend or LFS with a well calibrated refractometer, measure the salinity with the refractometer and then measure with the Hydrometer. Mark with a pet on the hydrometer where the neddle point and right down what that really is according to the refractometer.

If your firend or lfs have a salinity that you dont want, you can mix water to an "ideal" salinity (or SG, whatever you like to measure) using a refractometer then measure that water with the hydrometer and mark the point.

The hydrometer will never change. Just don't be dropping or leaving full of salt to the point the needle wont move anymore, rinse after you use it.
 
Good idea marking the hydrometer.
I just want to make sure my readings are correct, and was really shocked when they told me it was 1.029!
That could have been a problem for my tank and I have spent too much on this thing to have something like high salinity kill things before I even get started good.
Definitely a learning experience, and I have lots more to learn.
 
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