refractometer versus hydrometer

I think too many people put too much faith in

a) Cheaply made refractometers from China that only claim to be accurate yet have no certification documents and are not properly calibrated when you get them.

b) Their ability to properly calibrate and use any refractometer or hydrometer, skewing results in unpredictable ways.

c) Numbers over observation.


A cheap refractometer is really no better than a good certified Hydrometer. In the case of the latter they're calibrated and intended for use in a saltwater tank and generally have certificates of calibration. That's a legal document, not just a marketing tool.

No matter what instrument you're using to check salinity the most important factor is temperature. Check to make sure the test sample is the proper temperature before you test. Most hydrometers and refractometers are calibrated at 75º F and most reef tanks are in the 78-80 range. There are conversion charts, or you can just cool your water off a bit before testing.

In the case of a "Deep 6" or similar dip / swing arm hydrometers the test water is usually direct from the tank and since it's warmer it will appear to be lower than it really is.

And as with most things in this hobby stability within a given range is much more important than an exact value. Based on observation if everything seems to be doing fine and you're happy and the tank is happy it really doesn't matter if your SG is 1.022 or 1.028 as both levels exist in nature and are capable of sustaining life. If things are good and your instrument tests at 1.02x and never deviates more than 0.001 you should be ok.

If you really need to get an accurate base-line for your testing then send some water to a certified analytical lab and get an exact value, often to the 4th or even 5th decimal point. Or spend the $200-300 on a quality CERTIFIED conductivity meter, hydrometer, or refractometer from a quality supplier.

Even then the only way to get accurate readings is to follow good standard labratory practices. One unaccounted for variable and everything is a wash.

...that said I use a $50 Chinese refractometer, calibrate with glass distilled water, and set the calibration every time I use it. I make sure the room temperature is 75ºF and the refractometer has been in the room and open for at least an hour for it to warm up. If the tool is cold the water that hits it will chill and show a higher SG than it really is. The lens is washed and dried with distilled water. A gallon of glass distilled water is about $8 from a lab supply store and lasts a really long time. A gallon from a grocery store is almost as good and costs 99¢ either will work, I just have access to the better stuff for free.

-Doug

 

I use a hydrometer and the only reason I dont use a refractometer is I cant afford it right now. But I have used them before and have used water tests on multiple tanks in the LFS to determine they are accurate. much more accurate than my instant Ocean Hydrometer.

I would easily put my faith in a chinese made refractometer over that hydrometer. hands down.

 

Ok, don't take it personally... just my point here....I imagine a lot of people are thinking the same thing, and I'm just out to explain that science isn't variable. If you want accurate test results you have to follow the proscribed procedure EXACTLY. What someone overlooks or considers an unimportant detail can invalidate results.

If you think your hydrometer is faulty why not pursue a refund?

How did you verify it's the hydrometer and not in your method?

If all you have are two instruments and a sample to be tested and the results vary, how do you know which one is wrong? How do you know they're not both wrong? (There's a real answer, just seeing if anyone knows what it is)

And just because a LFS does a test doesn't mean they're doing it right either.

 

How available are these "good certified hydrometers" and how much do they cost?

You can get a good refractometer from marine depot right now for $40, and it will be accurate, consistent, and easy to use. Something I can't say for either of the hydrometers I have owned.

The refractometer I use: Marine Depot Aquarium Refractometer

 

I must have one of the only hydrometers that actually works. I bought a refractometer because I heard they way more accurate.......... upon first test the hydrometer (instant ocean) read 1.024, and the refractometer read 1.024. Used a friends refractometer and it also read 1.024. Maybe I just got lucky....idk. But the hydrometer is quicker to use......oh well. I still like the refractometer and I bust it out every now and then to make sure my hydrometer is still acurate

 

How much does a good quality certified calibrated hydrometer cost? Where can I buy one?

 

$20-30

Any lab supply should offer hydrometers.

General use hydrometers are calibrated at 60º and aquarium use ones are often calibrated at 75º. It's important to know your sample temperature and adjust the reading or temp accordingly.


It's important to define our needs as well. Good lab equipment is often calibrated and certified to a much higher level than we actually need, which will cost more.

Ultimately this is devolving to an issue of semantics. As I've already said stability is more important than accurate readings....and anything you have that's made for the purpose and isn't broken should yield usable results.

people seeing 0.005 variations or more probably are doing something wrong.

I will admit that a decent refractometer is probably going to be more accurate than a bad hydrometer, of which there are a few on the market, but I stand my by position that a quality hydrometer will give sufficient readings for a lower cost than a quality refractometer.

In the "cheapest" world, yeah, a $40 refractometer might be more accurate than a $5 swing arm hydrometer, but I don't think a $40 refractometer will beat any $30 hydrometers.

If money is no object you can get stuff that costs thousands and is accurate to 0.00001 or better. At the point it's usually a matter of someone is getting sued over something, or military / hospital applications.

Also, if an exact value is something that you're interested in consider an analytical testing lab, it's actually quite cheap... if there's no lawsuit or technical spec requirement and you just need good data a slow day and the right attitude can go a long way to getting free tests done.

You'd be amazed what a good lab can turn out if you pay for a full wet lab panel. You'll not only get a salinity, but exact composition of what salts make up that salinity. You'll see traces of copper that none of our test kits can detect. Ever want to know how much chrome is in your tank? lead? praseodymium? ... I mean really a GC/MS test can tell you all you'd ever want to know and more.

 

Check with a 3rd instrument?(i.e. a certified calibrated hydrometer) lol :

 

Would you consider this to be a good quality refractometer?

Vital Sine Refractometer

This part threw me off..

"Easy, no-tool calibration, with automatic compensation for shifts in ambient temperature (from 10°- 30°C)..." Then it goes on to say, "....Features built-in calibration ring." Is it jusy saying I don't need any extra tools in order to calibrate it?

It says it has ATC(auto temp calibration).

 

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