Calibrating a refractometer

[Randy Holmes-Farley]

Why are so many people calibrating theirs so often? Can't you calibrate once and just put loc-tite on the screw so it will stay calibrated?

Maybe, but if it drifts for reasons that do not relate to the screw moving you are...screwed.

 

[scoopsthedog]

For seawater at 25 deg C, 35 ppt = sg of 1.0264 at 25 deg C and 1.0266 at 20 deg C.

I discuss it and show data here (tables 3 and 4):

http://reefkeeping.com/issues/2004-06/rhf/index.htm

So if I calibrate with solution but it is at a dif Temp then tank water should I try and get them to the same Temp first?

 

[Randy Holmes-Farley]

So if I calibrate with solution but it is at a dif Temp then tank water should I try and get them to the same Temp first?

If it is an ATC refractometer, and the ATC function is perfect, it won't matter as long as you are within the range of correction.

If it is not ATC, it MUST be at the same temp.

Getting them close is good in any case, but what is more important is the refractometer and room temp, as the little drop of water quickly adjusts to the temp of the refractometer.

 

[Frop]

Why are so many people calibrating theirs so often? Can't you calibrate once and just put loc-tite on the screw so it will stay calibrated?

Mine literally shifts .001 to .002 and I have no idea why. That's why I also check mine before every use. I don't mind it being off .001 just not any more lol. I am for the middle of the salinity range anyways

 

[cope413]

https://www.amazon.com/Standard-Sea...34&sr=8-2&keywords=salinity+calibration+fluid

I purchased my last bottle in April 2015. I calibrate every time I use it. Probably still have half the bottle remaining.

There's really no good reason not to get some proper calibration fluid given the possible issues without it.

$13 and be done with it

 

[Scott.h]

I've tried calibrating with the proper solution then checking it with RODI. It's way off and I'd never trust that. In fact I'd use my old glass bobber before I went that route.

 

[FishLover01]

A lot of refractometers require for a 45 second waiting period after you put test sample on the slide. This is to allow for temperature compensation. Secondly, you should most definitely be only calibrating with the calibration fluid recommended by your particular refractmeter manufacturer. Some refractometers need to be calibrated often and others don't need to be calibrated as much. If the calibration fluid is at 1.035 and it's at that often, then you don't need calibration every single time. Almost all calibration fluids can be purchased via Amazon.

 

[1000_smiles]

My refractometer just won't hold calibration. It is one with a set screw. If I calibrate it with 35 ppt solution it reads 37 ppt. I adjust the set screw to read 35 ppt, but within a half an hour if I check it again with the 35 ppt solution it reads 37 ppt. Kind of a pita calibrating the thing before every reading.
Would a Vee Gee STX-3 be any better?
I've looked at the digital ones (Milwaukee and Hanna seem to be the same), but the accuracy is + or - 2 ppt or 0.002 s.g. doesn't seem that great.
Any suggestions?
Randy, what do you use?

 

[RONBATTLE]

OK...MY 2 CENTS WORTH...I USED TO CALIBRATE MY RS REFRACTOMETER WITH RODI...THOUGHT I WAS ON TARGET BUT CONTINUALLY HAD PROBLEMS...I TOLD MY LFS GUY ABOUT IT AND HE TESTED MY WATER AND MY SALINITY WAS AT 1.043...I SCREAMED...HE GAVE ME SOME CALIBRATION FLUID...I SET IT AT 1.025...MOST ISSUES CLEARED UP IN ABOUT A WEEK, ALONG WITH WATER CHANGES...SO, NOW Y'ALL HAVE MY OL HEAD SPINNING...WHO'S ON FIRST...I DONT KNOW...

 

[BCSreef]

My refractometer just won't hold calibration. It is one with a set screw. If I calibrate it with 35 ppt solution it reads 37 ppt. I adjust the set screw to read 35 ppt, but within a half an hour if I check it again with the 35 ppt solution it reads 37 ppt. Kind of a pita calibrating the thing before every reading.
Would a Vee Gee STX-3 be any better?
I've looked at the digital ones (Milwaukee and Hanna seem to be the same), but the accuracy is + or - 2 ppt or 0.002 s.g. doesn't seem that great.
Any suggestions?
Randy, what do you use?

I am using the Milwaukee digital unit now. I've had to worthless optical units and have given up on them. My Milwaukee does read 0.001 high.

My procedure is the first zero on distilled water. Then test my tank water. And finally I test the calibration fluid. That's One reason why we need to have a calibration fluid.

From a reproducibility standpoint, the unit gives me the same results each and every time.

 

[Randy Holmes-Farley]

I've tried calibrating with the proper solution then checking it with RODI. It's way off and I'd never trust that. In fact I'd use my old glass bobber before I went that route.

Not sure what way off means, but a brine refractometer should be off with RO/DI. That doesn't mean it isn't working right.

 

[Randy Holmes-Farley]

My refractometer just won't hold calibration. It is one with a set screw. If I calibrate it with 35 ppt solution it reads 37 ppt. I adjust the set screw to read 35 ppt, but within a half an hour if I check it again with the 35 ppt solution it reads 37 ppt. Kind of a pita calibrating the thing before every reading.
Would a Vee Gee STX-3 be any better?
I've looked at the digital ones (Milwaukee and Hanna seem to be the same), but the accuracy is + or - 2 ppt or 0.002 s.g. doesn't seem that great.
Any suggestions?
Randy, what do you use?

I always used conductivity, not refractometers. lol

 

[jason2459]

...SO, NOW Y'ALL HAVE MY OL HEAD SPINNING...WHO'S ON FIRST...I DONT KNOW...

Don't worry your e-yelling had my head spinning.

But for how accurate one has to be I don't believe one has to be precisely at 35ppt. It's a good target.

Consistency is key. If you have established a resonable calibration protocol and fairly accurate method to determin salinity and stick with it you will be fine. If you believe you are reading 34-35ppt and maintain that level you should be just fine.

Corals can be found at various salinity levels. Salinity levels can change seasonally to daily. 35ppt is the canonical surface level average and a good number to set as the baseline to measure for. If you are not precisely there you should still be just fine.

 

[jason2459]

My refractometer just won't hold calibration. It is one with a set screw. If I calibrate it with 35 ppt solution it reads 37 ppt. I adjust the set screw to read 35 ppt, but within a half an hour if I check it again with the 35 ppt solution it reads 37 ppt. Kind of a pita calibrating the thing before every reading.
Would a Vee Gee STX-3 be any better?
I've looked at the digital ones (Milwaukee and Hanna seem to be the same), but the accuracy is + or - 2 ppt or 0.002 s.g. doesn't seem that great.
Any suggestions?
Randy, what do you use?

I've had a great experience with my veegee/vitalsine. Has been very reliable and easier for me to read then others I've used which have been quite a bit. Cheap brine refractometers worked well just needed calibration a little more often and has to be done near the end point you want to achieve for your salinity.

Redsea seawater refractometer was the worse refractometer I've ever used. Tiniest bump or just looking at it seemed to cause it to loose its calibration.

Others that I like are the Milwaukee digital seawater but I wish it had higher resolution and an option for two point calibration. H2Ocean also makes a true seawater refractometer but it was a little harder for me to read compared to the veegee/vitalsine.

I also like my pinpoint conductivity probe. My Apex conductivity probe is ok but highly susceptible to interference. Care must be used when calibrating and placement of the probe to get decent readings from it

 

[Nemo's Janitor]

My mistake, so 36.5 grams salt in a liter of DI water...

It is 36.5 grams of salt and 963.5 grams of DI water.

 

[Falcon53]

I use the Milwaukee digital refractometer and I have one of the Chinese no-name look-through ones as a back-up. Tested my display tank this weekend, since I have been skimming really wet and was concerned it would be low. Instead, it read 1.029! I checked it against the backup and got the same result. Thankfully, before panicking and taking drastic action, I let the Milwaukee and the Chinese unit warm up a bit, as they had been in my basement. After about ten minutes in a warmer room, the reading on both units dropped to 1.027. Still high but a lot lower than 1.029. And these units supposedly have ATC. So the answer to the question as to whether temperature can affect the readings even with ATC, the answer is yes!

 

[Scott.h]

Not sure what way off means, but a brine refractometer should be off with RO/DI. That doesn't mean it isn't working right.

Hey you're the science guy. I take your word for it. All I know is it doesn't read zero after I check it with water after calibrating it with calibration solution. How could I calibrate it with RODI and expected to read accurate at salinity?

I always used conductivity, not refractometers. lol

So how do we know what's right? My glass floating bobber reads 1022 from under the water at the surface. My calibrated ATC refractometer reads 1025, and my apex calibrated salinity probe reads 39.6. I would think if the probe was correct I'd have a problem with snails dying off, no? That's a big variation.

 

[cope413]

If you have a known solution of 35ppt, and your refractometer reads 35ppt for that solution, and your tank water doesn't, you can be confident your tank is not at 35ppt.

It doesn't have to be more complicated than that unless you want it to be. $13 for calibration fluid. Why not just buy a bottle and be done with it?

 

[jason2459]

If you have a known solution of 35ppt, and your refractometer reads 35ppt for that solution, and your tank water doesn't, you can be confident your tank is not at 35ppt.

It doesn't have to be more complicated than that unless you want it to be. $13 for calibration fluid. Why not just buy a bottle and be done with it?

Maybe

http://reef2reef.com/threads/does-your-calibration-solution-need-calibrated.250133/

 

[Randy Holmes-Farley]

Hey you're the science guy. I take your word for it. All I know is it doesn't read zero after I check it with water after calibrating it with calibration solution. How could I calibrate it with RODI and expected to read accurate at salinity?.

If it is a brine refractometer, and it is calibrated perfectly with RO/DI, IT MUST be inaccurate for seawater by a known amount.