My refractometer is way off when testing with Pinpoint 53mS calibration fluid

[ostrow]

Thanks Randy. Yeah, I realized that in my 4300th post

[Randy Holmes-Farley]

The pinpoint stuff will calibrate the cheaper "blue handled" refractometer correctly to 1.026 and make my cheap unit pretty accurate for testing my tank?

I presently believe that to be true, yes.

[Boomer]

Cham

The PinPiont will work with any refract that we use in this hobby, even a $200 one or a $400 one

[cham]

Originally posted by Boomer
Cham

The PinPiont will work with any refract that we use in this hobby, even a $200 one or a $400 one

Right, but I've heard how the "blue handle" refract can be an innacurate test because its inexpensive design they were hard to calibrate correctly. If what we see is true, the pinpoint fluid may help to make the blue handle as accurate (or close enough) to some of the more expensive models.

[Randy Holmes-Farley]

That concern applies when calibrating at a point far from where you'll use it. If you calibrate at the exact point where you use it (35 ppt seawater), it will be fine.

[seattlerob]

OK, I received my bottle of Pinpoint Salinity Monitor 53.0mS Calibration Fluid from PA today and did some tests on my refractometer (blue RHS-10ATC model from MD):

Prior to any testing, I checked the refractometer using RO/DI water and read 0, which is exactly what it read when I received it originally.

1st test: I made up some of Randy's Homemade Calibration solution using Mortons Salt (non-iodized). The recipe called for iodized salt, but I didn't have any. Not sure what difference, if any, this makes...so take this result with a grain of salt (sorry, could not resist). I used a scale to weigh out the water and the salt, so I think the solution was pretty accurate (scale was 1/10 g resolution). Reading on the refractometer was 38ppt, instead of the 35ppt I should have seen.

2nd test: Using the Pinpoint 53.0mS fluid, the refractometer read just slightly higher than 38ppt (maybe 0.5 higher).

Using the pinpoint solution, I have recalibrated my refractometer to 35ppt. Now, with RO/DI water, the refractometer reads below 0. I believe my results match those previously posted, but wanted to share the results for those with inquiring minds

rob

Originally posted by seattlerob
Thanks guys for this thread. I just ordered a bottle of the calibration solution from PA. I have never calibrated my Marine Depot refractometer (blue one) except using RO/DI (which read zero when I received it and still reads zero), so I am anxious to find out how out of whack my salinity really is...

[Randy Holmes-Farley]

The iodine should not matter, and I'm glad my DIY matched the Pinpoint (close enough), but its disappointing that the refractometer cannot really be used as claimed (calibrating in RO/DI water).

[boxfishpooalot]

This is a stupid question but does calibration solutions expire? I suppose it depends on what the solution is made of.

[techreef]

jeez, is nothing in this hobby as advertised? sigh. not only am I measuring cup challenged, but I don't have a scale that measures 1/10g amounts. (I just wanted a saltwater tank, darnit! not a laboratory's worth of supplies and kit!!! )

My refractometer has a black rubberized handle. Does anyone trust the colors of the handles to mean anything reliable when trying to determine whether or not the refractometer model is a "reliable" one? Guess I'll buy the Pinpoint solution and use that to be certain my refractometer is correctly calibrated.

Thank you everybody for thoroughly hashing this out. It makes me feel dumb, but it really helps me to read the same point said 2 or 3 different ways when it comes to chemistry stuff like this!

[Randy Holmes-Farley]

This is a stupid question but does calibration solutions expire? I suppose it depends on what the solution is made of.

A salt (NaCl) solution will not go bad. A seawater solution might change a little, but not enough to be important, IMO.

[barjam]

If I calibrate using the fluid won't I still be off a tad for things like hyposalinity? Or is 1.09 close enough to 1.26 not to matter?

[Randy Holmes-Farley]

Yes, you may still be off if you use it far from seawater salinity. You might want to take tank water and dilute it to get to the sg you intend to use, and see where it falls.

sg of 1.009 would come (closely enough) from 1 part sg 1.0264 seawater plus 2 parts RO/DI water.

[Boomer]

tech

Does anyone trust the colors of the handles

This was an issue many months ago. These blue-handled ones come from one and only one Chinese company. A number of these where tested against a std and were right on with no calibration, when they should not have. They should have read below the zero mark and should not have read 35 ppt right out of the box as the std off-set for a NaCl meter is -1.5 ppt to low. A str lab grade would read such, -1.5 ppt to low if tested against a seawater std. It appeared at the time then that these blue-handled one where good as is out oif the box. As time has pasted we have seen those that got them where just lucky and many of them now are off out of the box. The black cheap one are also made by one and only one Chinese company. Sellers of these refracts use the same exact model # as the manufacture. Thus, the reason for "is it a black handle or blue handle"

There are no available real seawater refracts, which can be calibrated with RO/DI about $100.

http://thefilterguys.biz/refractometers.htm

[cham]

Ok, I just got my pinpoint 53mS solution via UPS a few minutes ago. I have a blue handle meter that is a few weeks old.

I first put RO/DI water on it to clean the prisim fully and to check the FW calibration. I read dead on at zero (have yet to adjust this thing, came at zero out of the box)

Then I put on the pinpoint solution and it reads 1.028 exactly.

The solution should read 1.026 right? So I need to adjust down .002 for a correct reading?

[techreef]

scared to see what salinity my tank water is sitting at right now...

[Boomer]

Chan

Yes that is correct. Turn till get reads 1.0264 = 35 ppt NSW. You do not have to turn the screw to get 1.0264 if you do not want, just remmber that you want it to read 1.028 =35 ppt if you do not turn the screw

[barjam]

Yes, you may still be off if you use it far from seawater salinity. You might want to take tank water and dilute it to get to the sg you intend to use, and see where it falls.

sg of 1.009 would come (closely enough) from 1 part sg 1.0264 seawater plus 2 parts RO/DI water.

Thanks for the work around. It will be interesting to see how far off 1.009 is on one calibrated to 1.026. If you don't mind could you share the formula you used to get that figure? Thanks

[Randy Holmes-Farley]

I took a short cut and assumed the specific gravity was linear with salinity (it is pretty close) and did:

9/26.4 = 0.34

So you want 34% of the salinity of 35 ppt seawater, or 1/3. So that is very close to 1 part seawater (33%) and two parts fresh water (67%).

I'd guess that refractometer is a lot closer there, if calibrated in RO/DI, then at 35 ppt. In next months reefkeeping article I'll show graphs explaining why, but I might be able to upload one now.

[Randy Holmes-Farley]

Here's a graph of one of two ways that a refractometer may be mismade and when calibrated in RO/DI, will not give the correct answer when using seawater.

RO/DI is the bottom left point at the axis, while the green drop line is at seawater refractive index (other graphs in the article make this clearer, but I can't upload them all right now):

Note that the error here is smaller as the lines come together near the RO/DI point, so errors will be smaller for hyposalinity than for seawater.

[Randy Holmes-Farley]

This graph shows how calibration at 35 ppt (refractive index = 1.339 or so) reduces the error when using it at 35 ppt, but might actually increase the error when measuring very low salinity (like hyposalinity near the bottom left corner).

[cwegescheide]

Originally posted by barjam
Thanks for the work around. It will be interesting to see how far off 1.009 is on one calibrated to 1.026. If you don't mind could you share the formula you used to get that figure? Thanks

Actually when I saw a big discrepancy between what my refracto I took my refracto to the Florida Aquarium to have the pro's verify my reading. I know that sounds a little extreme but I can walk there from my place. I was surprised that they were VERY helpful and cool people. Once they dialed it in and I came back I then tried the Randy's Homemade calibration fluid again. It measured 1.026

Chris

[barjam]

Believe it or not I was stuck at the dilution math. My common sense-o-meter was failing me, apparently.

My plan is to calibrate to 35ppt using the calibration fluid and make note of where 2 parts RO/DI to one part 35ppt hits on the refract for hypo purposes.

Thanks again.

[Randy Holmes-Farley]

Actually when I saw a big discrepancy between what my refracto I took my refracto to the Florida Aquarium to have the pro's verify my reading. I know that sounds a little extreme but I can walk there from my place. I was surprised that they were VERY helpful and cool people. Once they dialed it in and I came back I then tried the Randy's Homemade calibration fluid again. It measured 1.026

That's good to hear. Did you make it by cup sand coke bottles, or by weight?

Do you know what they did to the refractometer?

My plan is to calibrate to 35ppt using the calibration fluid and make note of where 2 parts RO/DI to one part 35ppt hits on the refract for hypo purposes.

Sounds good. Good luck.

[goreefer]

After reading this thread I must say that I'm a little suprised with what some of you have said.
I also tend to look at the whole picture of my tank health, of which salinity is ONLY ONE factor.
I do use a refractometer, and it is a mid-priced one. Off the top of my head I don't remember the color, it might be blue.
My point is that if you always mix your new SW to the same salinity, wether it is 1.024 or 1.025, and your tank is healthy, does it really make that much difference?
Does it ever rain in the ocean? and what effects does that have on the animals in the ocean?

Above all, we need to thank Randy for all that he does for our hobby. With out him many of you would be lost.

[inwall75]

I agree with you about SG stability.

However, since corals are osmoconformers, keeping them at the same SG that they typically live in on the reefs is a good idea. I want to keep them stable at or near that SG as opposed to a hyposaline condition.