refractometer versus hydrometer

An easier way to find out than using a third instrument (which may also be wrong) is to use a reference fluid. My salinity probe came with some, I imagine any retail-boxed refractometer of decent price will have some included, or you can get some on Foster and Smith Aquatics.

 

Ok, on the hydrometers all that tells you is that one or both are inaccurate. How do you know which right and which is wrong? Maybe both are wrong. Were they the same brand? New or old? Clean or dirty?

On the refractometer, ok they read the same, how do you know the readings are accurate? they could both be wrong.


... the answer is you have to calibrate it the right way.

The wrong way is to put a few drops of RO water on the lens and use the screw to set it to zero, rinse it off, and put your tank water on it.

Calibration is a science.

Before you start make sure everything is clean. Clean the lens/prism and the cover panel on the refractometer. Use a new pipette or dropper, or make sure whatever you use is really cleaned of all salts from previous tests. Use only distilled water for cleaning.

First step is to calibrate your test environment. Refer to the manual that came with the refractometer and find out what temperature it's made to be accurate at. Even ATC's work best at a certain temperature. Usually it will be 60º or 75º.

Next adjust the temperature in the room to the needed level, let's say 75º. If you have access to an incubator even better. Its pretty easy to make one as well from a 10g tank, a light bulb, a dimmer, and a good thermometer (again, an instrument you have to depend on being accurate, redundancy in this case is a good idea).

"Soak" (in air, not water) the refractometer at 75º for at least an hour to make sure it is 75º.

Prepare a distilled water sample at 75º as well. Soaking it in the same place as the refractometer will suffice, but evaporation will result in a slightly lower temp than "room" temp, so consider a water bath to hold it steady.

At the same time you also need to temp. correct a reference solution. You can buy these premade, the LFS may sell them, if not a lab supply will. You'll want 1.025 SG reference solution. They usually come in foil pouches or dropper bottles. Whatever you get you don't need whole lot for a refractometer.

Also prepare a sample to be tested from the tank.

Make sure all samples are sealed. Evaporation will change SG. the distilled shouldn't really matter, but it's good practice.

Once everything is 75º follow the procedure in the manual to calibrate to zero with distilled water.

Now verify accuracy with the reference solution. If the instrument is good it should read 1.025. If it doesn't the refractometer is pretty much worthless, but can still be used to get a rough idea within 0.0005 or so. Adjust the calibration screw to read the right level for the solution.

Now you're calibrated and verified.

Test your tank sample, and you should be pretty certain the reading is accurate.

...

the same process can be used for a hydrometer, but you'll need more volume on the reference/test solutions and generally speaking "swing arm" hydrometers cannot be calibrated to zero, so ideally you'll have at least two reference/calibration solutions within it's testing range, i.e. 1.022 and 1.028. There's no adjustment, so if the values don't read properly after all that take it back and get a refund or exchange.

Cleanliness counts big time.

Also, plastic swing arm hydrometers _MUST_ be cured by soaking in salt water at around 1.025 for 24 hours. After that it may be useful to keep them in a sealed zip-loc bag wetted with distilled water. Don't let them dry out or the curing process should be repeated.

No matter which tool you use, always clean it after use.


... see... it's a time consuming exact process.... and the only way to make sure you're getting a proper reading. Anything else is just tool-assisted guesswork.

 

Yeowch Greysoul....I think I heard the kathunk from all that procedural information hitting at once. The internet likes TL;DR versions ;D

 

Ok I think I understand what you are debating now. You believe a refractometer could be off as could a hydrometer but the fact of the metter is for someone like me who doesnt know how to calibrate either one, I would rather take my chances with the one thats know to be more reliable.

Its like a gun and a knife, neither one are garunteed to kill but in the line of defense, Id rather take my chances with the gun.

 

Refractometer - not a requirement but a real nice to have. If you can't get one I would recommend getting 2 different brand hydrometers, borrow a refractometer and use it to figure out where you want your SG. Then check your SG with the 2 hydrometers, mark the spot (use a knife or something to scratch it). Then going forward use both hydrometers to check eachother so you'll know if you're off.....of course it's better to just get a refractometer, but I've used 2 hydrometers since I setup my current reef. I also have a refractometer (ebay) that I use to tripple check things every once in a while, it's just a bit less work with the hydrometers.

 

+1 on refractometer. I bought mine for $35 at my LFS. Much more accurate and easier to maintain clean.

You won't regret it.

 

Also ATC (automatic temperature compensation) refractometers will adjust for room temp.