calibration

[rhythmicfire]

I am getting my refractometer in the mail tomorrow!
When I first calibrate it I hear you should calibrate it with RO/DI water for the most accurate results in the future. I don't use RO/DI water (I know, I know, I know ) but can I use bottled water to calibrate my refractometer?
And I think the SG of regular water is 1.000.

[delsol650]

Can't you use Distilled bottled water?

[rhythmicfire]

Originally posted by delsol650
Can't you use Distilled bottled water?

All bottled water is distilled right?

[troutonmyline]

NO! A lot of bottled water is tap water, or at best filtered water. The distillation process is expensive, so if it doesn't say "distilled' bet it's not. Distilled water has a specific gravity of 1.00. My refractometer came precalibrated, so check to see if yours is, but it would be good to check it. I checked mine and it was right on.

[rhythmicfire]

Originally posted by troutonmyline
NO! A lot of bottled water is tap water, or at best filtered water. The distillation process is expensive, so if it doesn't say "distilled' bet it's not. Distilled water has a specific gravity of 1.00. My refractometer came precalibrated, so check to see if yours is, but it would be good to check it. I checked mine and it was right on.

What'dya know, I learned something new
Thanks for the information regarding distilled water. I'll check if it is pre-calibrated!

[Kinetic]

You could find someone with the calibration solution, it tells you exactly what specific gravity your refractometer should be reading.

[Matt_Wandell]

This exact thread is happening in the MARS forum too.

Best to calibrate it with a 35 ppt standard solution and DI water sample, if possible. I would not necessarily trust any manufacturer's claim that their refractometer comes pre-calibrated.

[Thales]

Hey Dr Matt,

How does the 53ms cal solution for probes measure out in ppt?

Also, there have been a lot of people posting that calibrating with RODI and a calibrated solution have pretty much negligible differences.

[troutonmyline]

Isn't that assuming your rodi unit in working perfectly and putting out 0 tds? I bought mine used and I think I need new resin cause my finished water reads 25 ppm. Does anyone have extra resin I could purchase?

[Thales]

That shouldn't be an assumption - you need to be checking the TDS.

[delsol650]

I calibrate using my RO/DI.

[jellygeee]

I thought all the refractometers sold online (Liveaquaria and marinedepot) are already calibrated or at least that's what mentioned in the website. Am I wrong?

[rhythmicfire]

Originally posted by jellygeee
I thought all the refractometers sold online (Liveaquaria and marinedepot) are already calibrated or at least that's what mentioned in the website. Am I wrong?

Thanks for the feedback everyone!
I am getting my refractometer from DF&S, but I still plan to calibrate it anyways, because I don't want to take anything for granted

[ianiwane]

The one I got from premiumaquatics was WAY off to begin with. I did calibrate it with some calibration solution (the type Matt is talking about).

[rhythmicfire]

do i need to pay any attention to the temperature of the distilled water I will be using to calibrate my refractometer? The water won't be ice cold or anything, but does it need to be at the exact same temperature as the prism in refractometer?

[colotl]

I got my refractometer over the weekend from DFS and it was off by about 1ppt using distilled water at 70C. I would be more comfortable with the solution.
Matt were did you get yours?

[africangrey]

Please read through the article written by Randy Holmes-Farley, Matt is right about the 53 mS/cm Conductivity solution by American Marine, it's a full strength sea water with salinity of 1.0264. Most hobbist refractometers are over estimating the salinity if calibrated with RO/DI with 0 TDS only, the correct way is to perform a 2 pt calibration with RO/DI and 53 mS/cm conductivity solution, but most likely you won't get both reading at o and 35 ppt with a refractometer that were less than $100. At this point, as Randy suggested use 53 mS/cm as your reference standard because this is the number you want to target if you are reading this thread and have a reef tank.

You will find the article here: http://reefkeeping.com/issues/2006-12/rhf/index.php

By the way, a bottle of calibration fluid is about $4.00, which isn't very expensive way to make a full strength sea water correctly.

Spencer

[Matt_Wandell]

Originally posted by Thales
Hey Dr Matt,

How does the 53ms cal solution for probes measure out in ppt?

Also, there have been a lot of people posting that calibrating with RODI and a calibrated solution have pretty much negligible differences.

What Spencer said. ^^^ That's a great link.

[Matt_Wandell]

Originally posted by Thales
Hey Dr Matt,

How does the 53ms cal solution for probes measure out in ppt?

Also, there have been a lot of people posting that calibrating with RODI and a calibrated solution have pretty much negligible differences.

Or in my own words...

53 mS/cm is a conductivity standard. It is not necessarily 35 ppt of salinity, unless the standard explicitly says it is.

You would want to calibrate using 2 points outside the target measuring range for the same reasons you do it with pH meters. Randy's article shows this really well. This thread has convinced me I need to talk to Laurie about how we calibrate our conductivity meter.

[Thales]

It seems that this is a very important part to point out:

One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. It is sold as a 53 mS/cm calibration fluid for the company's electronic salinity probe (a conductivity probe), but it also is suitable for use in a refractometer. NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The Pinpoint fluid happens to be made to match seawater in other respects, not just conductivity, but other brands, or do-it-yourself 53 mS/cm standards, may not be appropriate to use with a refractometer because, while they have the same conductivity as 35 ppt seawater, they may not have the same refractive index.

Even the Pinpoint solution doesn't say it is 35ppt.

Matt, maybe I am missing it, but I'm not seeing anything about needing to calibrate a conductivity meter using two points.
In this article, http://www.carlosreef.com/WordPress/...arley-ph-d/#14 , he mentioned calibrating a conductivity probe, but only mentions one solution of known value.

[GreshamH]

We calibrate our conductivity meters with two points Rich. We also make our own calibration solution for both refracts and conductivity meters maybe we should offer them to the public

[rhythmicfire]

Originally posted by delsol650
I calibrate using my RO/DI.

does the water need to be at a certain temp?

[Thales]

Originally posted by GreshamH
We calibrate our conductivity meters with two points Rich.

Cool. 0 and 35?

We also make our own calibration solution for both refracts and conductivity meters maybe we should offer them to the public

Again with that discussion!?

[GreshamH]

That discussion has never left your ear my friend. I've only had it with you, well, and Eric/Randy

Calibration wise, dunno, I'll have to ask Eric.

[africangrey]

In regards to the Pintpoint conductivity solution here is the assert from Randy's article, he even called American Marine to confirmed this many times in a separate RC thread.
"One suitable commercial standard is made by American Marine and sold under the brand name Pinpoint. It is sold as a 53 mS/cm calibration fluid for the company's electronic salinity probe (a conductivity probe), but it also is suitable for use in a refractometer. NOTE that this is not necessarily true of all 53 mS/cm conductivity standards. The Pinpoint fluid happens to be made to match seawater in other respects, not just conductivity, but other brands, or do-it-yourself 53 mS/cm standards, may not be appropriate to use with a refractometer because, while they have the same conductivity as 35 ppt seawater, they may not have the same refractive index."

And yes you can make your own calibration solution with a calibrated weight scale.

"In a previous article I have described how to make a do-it-yourself refractometer standard matching 35 ppt seawater, and I will just summarize that recipe here.

To provide a standard for refractometers requires a solution whose refractive index is similar to normal seawater. Seawater with a salinity of 35 ppt has a refractive index of 1.3394. Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252) has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as 35 ppt seawater, and therefore can be used as an appropriate standard (Table 5).

Table 5. Refractive Index as a function of the concentration of a sodium chloride solution. The darker blue row represents the standard.
Sodium Chloride Concentration (weight %) Refractive Index Equivalent Seawater Salinity (ppt)

3.3 1.3388 31.65
3.4 1.3390 32.8
3.5 1.3391 33.3
3.6 1.3393 34.4
3.65 1.3394 35.0
3.7 1.3395 35.6
3.8 1.3397 36.7


This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater. This recipe can be scaled to any appropriate size if suitable instruments are available (36.5 grams in 963.5 grams (mL) of water, 0.365 grams in 9.635 g (mL) of water, etc.).

This concentration roughly corresponds to ¼ cup (73.1 g) of Morton's Iodized Salt dissolved into two liters (2000 g) of water (giving very slightly more than 2 L of total volume).

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g).
2. Add one teaspoon of salt (making about 79.3 g total salt).
3. Measure the full volume of a plastic 2 L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g).
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

[Note: the standard described here using soft drink bottles is subject to variation in the volume of the bottle. It turns out that such bottles can vary in total volume, and this can lead to at least a one ppt error in the salinity of standards matched to seawater of 35 ppt salinity. Standards made with accurate measurements of salt and water, however, will accurately match 35 ppt."