calibration

[Matt_Wandell]

Originally posted by Thales
It seems that this is a very important part to point out:



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.

I'm just assuming that a second calibration point can only make it more accurate. I am also assuming that the same type of slope error he talks about with the refractometer (that requires a calibration at 53mS/cm or 35 ppt) could be occurring with the conductivity probe or meter.

Maybe it's just redundant busy work, but hey, what else do I have to do?

[Matt_Wandell]

Originally posted by 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

Just curious, what are you using for your second calibration standard for conductivity? We have a 12.85 mS/cm KCl solution.

[sfsuphysics]

Originally posted by Matt_Wandell
I'm just assuming that a second calibration point can only make it more accurate.

Now maybe I've just been out of the lab for too long, or I've been teaching a more rudimentary lab that doesn't deal with too much accuracy but... how can you get a 2nd calibration point to "work" with something that only has one mechanism to calibrate, i.e. a screw in the case of refractometer (I'm unfamiliar with conductivity probes maybe they work differently?)

Assume you calibrate to your 35ppt solution, then you go to the 0 tds solution, if you adjust to that second solution your first adjustment is now off. Or is there some rational as to why you'd measure at one level other than the other? I would think if anything the adjustment to a known solution that's closer to what we want to measure is more important.

[Matt_Wandell]

Originally posted by sfsuphysics
Now maybe I've just been out of the lab for too long, or I've been teaching a more rudimentary lab that doesn't deal with too much accuracy but... how can you get a 2nd calibration point to "work" with something that only has one mechanism to calibrate, i.e. a screw in the case of refractometer (I'm unfamiliar with conductivity probes maybe they work differently?)

The same way you do with a pH probe. There are standard solutions the probe is programmed to calibrate to. This portion of the discussion is somewhat moot if you are using a refractometer.

When you calibrate a pH probe you are supposed to calibrate at 7, then at 10 (or 4), then go back to 7, back to 10 (or 4), etc. until the meter reads the calibration standards correctly each time. The meter needs 2 points to be properly calibrated and refine its interpretation of the information it receives from the probe.

Assume you calibrate to your 35ppt solution, then you go to the 0 tds solution, if you adjust to that second solution your first adjustment is now off. Or is there some rational as to why you'd measure at one level other than the other? I would think if anything the adjustment to a known solution that's closer to what we want to measure is more important.

FWIW, the second point would not be zero, because no solution will have a conductivity value of 0.0 mS/cm. Even freshly made DI water will trap CO2 and give you a nonzero value as soon as it contacts air. This value rises the longer the sample sits out.

See the article by Randy where he discusses the "slope miscalibration" for refractometers. I assume (again) that this type of error is possible in conductivity meters as well. I want to be confident that my conductivity meter is accurate for ALL values of seawater I might be measuring (from 12ppt-35ppt), not just at 35 ppt. I would thus measure the conductivity at 53mS/cm, then at 12.85mS/cm, then back to 53 mS/cm, then back to 12.85 mS/cm, etc. until I was confident it was measuring both standards accurately. I hope this is clear.

[sfsuphysics]

Ahhh gotcha Matt, yeah I don't have any of them fancy dancy electronical do-dads with my tank

[Matt_Wandell]

Originally posted by sfsuphysics
Ahhh gotcha Matt, yeah I don't have any of them fancy dancy electronical do-dads with my tank

Even with a refractometer I would want to make a second calibration standard for 13ppt, just for the sake of being able to do hyposalinity correctly.

[Matt_Wandell]

A little more info about conductivity and its measurement for those interested, in another article by RHF:

http://www.aquariumfish.com/aquarium....aspx?aid=1804

[GreshamH]

Originally posted by Matt_Wandell
Just curious, what are you using for your second calibration standard for conductivity? We have a 12.85 mS/cm KCl solution.

I'll have my PHD's talk to your PHD's, then we'll discuss it over lunch

I'm not sure Matt. I'll have to talk to Eric next week. We have many different conductivity meters we use, from hand held to large wall mount ones that look very industrial.

[Mr. Ugly]

I made up about 2 liters of 35ppt refractometer calibration solution this afternoon per the Randy article.

36.50 grams Morton's Iodized salt
963.50 grams of distilled water

Two batches of the above with water and salt weighed to the nearest .01 grams.

I've been paranoid about my refractometer lately. I got it about 3 years ago from Aquatic Ecosystems. It's their "low cost" model. And it's supposed to be zeroed with distilled water, which I did.

http://www.aquaticeco.com/index.cfm/...detail/iid/931

Anyway, I checked it today with the Randy 35ppt solution that I made, and it reads spot on.

Calibration solution will be available to BAR members at the upcoming meetings.

I might bottle up small quantities if I run out of stuff to do

[Thales]

Great Norm! I checked mine, same model as yours, with pinpoint solution and it was off (low) by at least 3ppt from RODI which I have been testing it with. , so I am way interested in testing it with your solution.

[Matt_Wandell]

Originally posted by Thales
Great Norm! I checked mine, same model as yours, with pinpoint solution and it was off (low) by at least 3ppt from RODI which I have been testing it with. , so I am way interested in testing it with your solution.

If it's easier, just bring it in next time you see me.

[Mr. Ugly]

Hmmm... now that's interesting.

I"ll leave some at your doorstep on my way to work tomorrow.

PLMK how it reads.

If mine reads different than the Pinpoint, I'll want to check the balance and make another batch.

[Thales]

Whoo hoo! I love this club!

Thanks Norm! I'll let you know ASAP.

Matt, I was still puking today, so I'll see you next week. I'll bring the refracto then (and the underwater rig, so be ready to pose!)


Also, see this thread:

http://archive.reefcentral.com/forum...readid=1283818

Should the refractometers read below zero with RODI after calibration with a standard solution?

A perfectly made refractometer would do so. Many of the hobby refractometers appear to not be made correctly, so the calibration at 0 ppt with fresh water does not ensure correct calibration at 35 ppt, and likewise, if correctly calibrated at 35 ppt, it may not read fresh water to be 0 ppt. Since you typically use it at 35 ppt, that's's where you want it to read correctly.

[Thales]

DI - below 0, 3 or 4 ppt
PP - 35
Norm's - 34ish
± is supposed to be acceptable.

I'll try again in the afternoon when the big lights come on.

[GreshamH]

The big lights, sounds cool

[Thales]

Oh, it's cool alright!

[Mr. Ugly]

Originally posted by Thales
DI - below 0, 3 or 4 ppt
PP - 35
Norm's - 34ish
± is supposed to be acceptable.

Sounds good.

I'll check the calibration on the Mettler when I get a chance anyway.