Calibrating refractometer

[crumbletop]

<a href=showthread.php?s=&postid=9588470#post9588470 target=_blank>Originally posted</a> by jdieck
Hope this helps to clarify the issue.

Not really. I follow what you are saying, but based on the graph from fig 23, the slopes actually appear to be very similar. Given the margin for error for a refracto in obtaining accurate readings, it seems as though you would have a wider range salinities that could be measured than. In addition, the NaCl solutions I made up track appropriately -- i.e. both appear dead-on when calibrated with the 36.5 ppt solution to either 36.5 ppt or 35 ppt (seawater). That lends credence to the slopes being very much the same. So the refracto appears to track well for NaCl salinity readings as well as the adjustment for seawater.

Perhaps what you are saying is that at lower salinities, the refractive index of the 1.009 solution I made up with NaCl is quite off from where seawater would be at 1.009 and that the slope I am seeing is an NaCl slope. If so, that would be puzzling, because the offset at 35ppt appears very close to the offset I see with RO/DI once calibrated to 35ppt using Randy's solution. I.e. both appear to be linear to me, and to have substantially similar slopes. What I mean is, I run hyposalinity in my QT, and the dilution needed to create a mathematical 1.009 from a measured 35 ppt, matches the NaCl 1.009 standard.

 

[WaterKeeper]

<a href=showthread.php?s=&postid=9588574#post9588574 target=_blank>Originally posted</a> by sagwala
Yes you did write what you wrote! What the heck are you talking about! Well....... Nevermind, no not "nevermind" I'm confused.

JD does indeed know what he is talking about. We just need to work on his presentation skills.

Overall, he is saying that seawater is a mix of compounds and has a refractive index slightly different than a single compound solution of Morton's. Even when it rains Morton's to the same apparent weight of sea water the refractive indexes won't match. The light is bent slightly more in the mixed compound solution than the single compound solution. You can compensate for this by adding slightly more Morton's into a litre of water. This makes the refractometer read the same in 35 ppt seawater as it does with the Morton's standard.

Ok, so far? The little fix is fine but, as is usual in life, " into everything a little rain must pour". If the refractometer's manufacturer used 35 ppt Morton's as his calibrations standard when he built the refractometer it was linear for Morton's solutions from zero to maybe 50 ppt Morton's. If we change the refractometer to read 35 ppt with the more concentrated 37.9 ppt salt in the calibration standard our line now will not pass through zero. We cannot use plain RO/DI to calibrate the instrument but must always use the 37.9 standard, adjusting the instrument to read 35 ppt, to set it up before use. This works fine for checking our tank's salinity, which we want near 35 ppt sea water anyway. However, if we are going to measure a less concentrated solution, like when performing hypo, we will have an error and would need to make an adjusted standard of Morton's near that value for our readings to be correct.

Here's one little catch I will throw in. The "Hypo Gurus" say you must hit 1.009 exactly for hypo to work. They don't tell you if their refractometer was designed using a sea water calibration or pure salt when they tested their method. Ah, the mystery of it.

 

[jdieck]

<a href=showthread.php?s=&postid=9589216#post9589216 target=_blank>Originally posted</a> by WaterKeeper
JD does indeed know what he is talking about. We just need to work on his presentation skills.

Thanks! actually I am better at it in Spanish

 

[sjm817]

When I go to the beach this summer, I'm going to calibrate mine with real ocean water!

I'm sure you guys will tell me why that wont work either :lol:

 

[WaterKeeper]

All that suntan oil in the water causes....

 

[crumbletop]

<a href=showthread.php?s=&postid=9589216#post9589216 target=_blank>Originally posted</a> by WaterKeeper
Here's one little catch I will throw in. The "Hypo Gurus" say you must hit 1.009 exactly for hypo to work. They don't tell you if their refractometer was designed using a sea water calibration or pure salt when they tested their method. Ah, the mystery of it.

I was thinking of that as well. When it rains it pours

 

[jdieck]

<a href=showthread.php?s=&postid=9588909#post9588909 target=_blank>Originally posted</a> by sagwala
Thats better, more like the language i know, Especially for us noobs.

Cheers JD

Do you mean it because I used "litre" rather than "liter"?

 

[WaterKeeper]

:lol:

I never though of that. Usually on the Chemistry Forum I use litre but on the NTTH Forum I use liter. This seems more like a chemistry question so I did use litre above.

 

[sagwala]

Oh i quite believe that you know what your talking about JD, there is no way you could write all that down if you didn't and i don't think it was badly written, just a bit heavy thats all. And thats the subject doing that.

In regards to your spellings, i'm English so it has to be "Litre".

WaterKeeper thanks for the breakdown, so are you saying it would be best to have to two refractometre's and calibrate them individually for normal SW and Hypo

 

[WaterKeeper]

Not two refractometers just two standards a 1.026 and a 1.009. You can use either to adjust the calibration span on the same refractometer.

 

[sagwala]

Ok, I get it now, thank you all experienced ones!!! Time to rope my sister in to make up some solutions, she's a chemistry teacher and will probably do far better than Hammer and Spanner hand here!!

Cheers

 

[WaterKeeper]

You will need around 13.2 grams per litre of Morton's to duplicate a SW SpG of ~1.009.

 

[crumbletop]

<a href=showthread.php?s=&postid=9590109#post9590109 target=_blank>Originally posted</a> by WaterKeeper
Not two refractometers just two standards a 1.026 and a 1.009. You can use either to adjust the calibration span on the same refractometer.

My refractometer seems to be linear no matter which I use. When I calibrate with 1.0264, and check with 1.009, the 1.009 is dead on. Are the slopes that different between NaCl and seawater? I'm having a hard time finding index of refraction tables for NaCl.

 

[WaterKeeper]

Only slilghtly different but are you calibrating at 1.0264 with an NaCl solution and checking with a 1.009 NaCl solution? If that is the case and the meter was orginally calibrated using NaCl even the zero should be right on. If is was manufactured as a seawater meter then the zero will be off.

 

[crumbletop]

<a href=showthread.php?s=&postid=9592239#post9592239 target=_blank>Originally posted</a> by WaterKeeper
Only slilghtly different but are you calibrating at 1.0264 with an NaCl solution and checking with a 1.009 NaCl solution? If that is the case and the meter was orginally calibrated using NaCl even the zero should be right on. If is was manufactured as a seawater meter then the zero will be off.

I don't think it should work that way. Using the NaCl solutions designed to match saltwater should essentially only shift refractometer to the left if you look at the pic of "figure 23" (shifts things to the green line). If the slope is the same, then once calibrated for NSW using the NaCl solution, then all other salinities should measure out okay. If the slope is quite a bit different, then that is another issue.

 

[jdieck]

The zero will be off not only because you shift the chart but also because the slopes are different.
Having said that, depending on the scale, the accuracy, the accuracy change along the range of measurement and linearity of the refractometer the difference in slope may sometimes fall within the inaccurate range and the refractometer may not even detect it.
Say if for a certain level of salinity the difference in slope is 0.0002 sg and the accuracy at that level is +/- 0.0005 sg the instrument may not even see the difference in the slope.
As I mentioned before some of the cheaper salwater refractometers measured better in seawater than the most expensive seawater because they were innacurate in salwater.

 

[WaterKeeper]

The bottom line here is that in a normal reef tank environment the inaccuracy is not enough for the aveage hobbyist to worry over.

 

[crumbletop]

<a href=showthread.php?s=&postid=9592611#post9592611 target=_blank>Originally posted</a> by jdieck
Say if for a certain level of salinity the difference in slope is 0.0002 sg and the accuracy at that level is +/- 0.0005 sg the instrument may not even see the difference in the slope.

That is exactly why I wanted to know how different the refractive index vs salinity slopes are for NaCl vs seawater. If they are substantially similar, then it doesn't make a difference once the refracto is calibrated properly.

<a href=showthread.php?s=&postid=9592673#post9592673 target=_blank>Originally posted</a> by WaterKeeper
The bottom line here is that in a normal reef tank environment the inaccuracy is not enough for the aveage hobbyist to worry over.

Yes, that's what I was getting at

 

[WaterKeeper]

You wouldn't hear me say that if this was a discussion over on the Chemistry Forum.

 

[jdieck]

<a href=showthread.php?s=&postid=9592979#post9592979 target=_blank>Originally posted</a> by WaterKeeper
You wouldn't hear me say that if this was a discussion over on the Chemistry Forum.

Coward :lol: