Calibrating your refractometer?

[RyanH]

I just checked my refractometer and it was about 5ppm off. How often do you guys check your refractometers? And why do they need to be recalibrated? Does the tuning screw turn by itself? I calibrated mine with RO/DI water, but could you use tap water? Thanks.

Ryan

[Mackrill]

You cant use tap water unless you know the exact salinity of it.

You need to turn the screw. Some never need to be recalibrated, some need to be tuned every once in a while. It never hurts to check the calibration to be sure it is right.

[XxDutchxX]

I dont think that you can use tap water. I check mine about once a month if im lucky...usually i forget. I think they get out of tune by puting/taking them out of the cases.

[RyanH]

I was really surprised that it would get that far out of wack in just a couple months. Gotta keep an eye on it, or the extra money spent on a refractometer is wasted.

[guitarfish]

I take RO water and make it the same temp as the tank water, then calibrate it. My unit is supposed to automatically compensate for temp, but I like to calibrate using the same temp water, since its accuracy is critica.

[laddy00]

Even using RO/DI water will not give you better than 5ppt accuracy because you are calibrating so far from of your sample salinity (~35) Most of the ones I have calibrated for friends where 5-8 off after using ro/di water for calibration. I'm not sure where you can order it but we use "standard" seawater of 35ppt, sold in glass viles.

[guitarfish]

Laddy00, I rely on the refractometer primarily for accuracy during hyposalinity, which at 1.009, is pretty close to the calibration. 5ppt is a very large differential isn't it?

[gcvt]

Originally posted by XxDutchxX
I think they get out of tune by puting/taking them out of the cases.

Yup, the only times I've found mine off was after bumping the adjustment knob while I was cleaning it or putting it back in its case.

[Habib]

Originally posted by laddy00
Even using RO/DI water will not give you better than 5ppt accuracy because you are calibrating so far from of your sample salinity (~35) Most of the ones I have calibrated for friends where 5-8 off after using ro/di water for calibration. I'm not sure where you can order it but we use "standard" seawater of 35ppt, sold in glass viles.

:eek1:

That is about 20% off.

Can you give some more details? :)

[jamal-188]

Originally posted by laddy00
Even using RO/DI water will not give you better than 5ppt accuracy because you are calibrating so far from of your sample salinity (~35) Most of the ones I have calibrated for friends where 5-8 off after using ro/di water for calibration. I'm not sure where you can order it but we use "standard" seawater of 35ppt, sold in glass viles.

Not to say your wrong but that what the directions from the manufactor says to do. I don't see why it would matter how far away the calibrations is from your sample salinty. If 0 SG(ro/di) is properly reading 0 on the refractometer than the correct sg of your aquarium water would show up. The way a refractometer works is by bending the light depending on the amount of salt in the water. I just don't see how it would mater how far away you are from the calibration setting.

[kbreese]

Originally posted by laddy00
Even using RO/DI water will not give you better than 5ppt accuracy because you are calibrating so far from of your sample salinity (~35) Most of the ones I have calibrated for friends where 5-8 off after using ro/di water for calibration. I'm not sure where you can order it but we use "standard" seawater of 35ppt, sold in glass viles.

I am no expert, but I don't understand what you are saying. ro/di water (provided good filter's are used) will have zero salinity, so as long as the refractometer is set at 0 with that, than it should be accurate. I don't understand why you would want to use seawater instead. I would think that would be a more innacurate method, b/c how do you know for sure the exact salinity of the sample of seawater? I think the chances are greater that the ro/di water will be at 0 than the chances of the seawater sample being exactly at 35ppt.

[laddy00]

kbreeze,
Its not just regular old sw we use it comes in an encased glass vile that must be broke open and is guaranteed to be 34.999 (pss).
As far as calibrating away from your sample point the problem is that inaccuracy in your instrument will be magnified the farther away from your calibration point you are. The refractometers I calibrated for my friends where all of the $50 dollar variety which are not that accurate without calibrating to 35. If you have a more expensive model it may not magnify the error of the instrument as much.

[mhurley]

I calibrated mine when I first got it, then it sat in a hot fish room for 2 months and it was completely out of whack, 9ppm. Recalibrated it with RO and it was fine. Now I keep it in a cooler place. I think the heat messed with the optics and threw it off.

Mike

[Daniel411]

Originally posted by laddy00
kbreeze,
Its not just regular old sw we use it comes in an encased glass vile that must be broke open and is guaranteed to be 34.999 (pss).
As far as calibrating away from your sample point the problem is that inaccuracy in your instrument will be magnified the farther away from your calibration point you are. The refractometers I calibrated for my friends where all of the $50 dollar variety which are not that accurate without calibrating to 35. If you have a more expensive model it may not magnify the error of the instrument as much.

Thats what you're paying for with higher priced precision instruments. Accuracy accross broader ranges or with less degree of error.

[Randy Holmes-Farley]

You can also calibrate it (or confirm it)with a salt/water solution as described in this thread:

http://archive.reefcentral.com/forums/showthread.php?threadid=65509

[laddy00]

I just read through all the replies and realized the 5-8 should be 5-8%. Makes a big difference. Sorry. But this still is pretty significant if you are trying to be accurate.

[kbreese]

Originally posted by laddy00
kbreeze,
Its not just regular old sw we use it comes in an encased glass vile that must be broke open and is guaranteed to be 34.999 (pss).
As far as calibrating away from your sample point the problem is that inaccuracy in your instrument will be magnified the farther away from your calibration point you are. The refractometers I calibrated for my friends where all of the $50 dollar variety which are not that accurate without calibrating to 35. If you have a more expensive model it may not magnify the error of the instrument as much.

Oh ok, I see what you are saying now. Yes the refractometer I have is the same $50 unit. I forget the model number but I know it has ATC in it.

Well are we talking a serious difference here? I mean I think 90% of the people here on RC use that same $50 unit and calibrate it with rodi water, so I can't imagine that it's significantly off. Well, I added basically 1/2 cup of Instant Ocean salt per gallon for my tank and basically got a reading right at 1.025 which as far as I know is the number I should be at. So how far off do you think it could possibly be??

PS- I don't know how "ppt" or "pss" translates. I have know idea what they mean. does 35ppt or pss= 1.025 specific gravity??

[laddy00]

1.025 is equal to a salinity of 33.2 using saltyzoo's converter. PSS stands for practical salinty scale which is for the most part the same as parts per thousand (ppt). PSS or PSU is just used due to the way it is calculated without units as a function of a ratio. Just think of it as ppt.

[Randy Holmes-Farley]

PS- I don't know how "ppt" or "pss" translates. I have know idea what they mean. does 35ppt or pss= 1.025 specific gravity??

The ocean has, on average, a salinity of 35 ppt. That corresponds to a specific gravity of about 1.0264.

[JB NY]

I check mine every 6-9 months. I had to calibrate it about 3 months after I first got it. But it has not needed to be re-calibrated in over 2 1/2 years after that calibration. I calibrate mine to RO/DI water as stated in the directions that came with my unit.