Your refractometer may be inaccurate if calibrated in RO/DI water

[Randy Holmes-Farley]

Many of those being sold right now are improperly made, and cannot be calibrated accurately in pure water (when the intended use is in seawater):

See this thread:
http://archive.reefcentral.com/forum...5&pagenumber=1

[mission man]

Now I'm rattled. I just used the homemade mix to check my refractometer and I came up with 39ppt. With RO/DI checks at 0. Now I'm going to hed to LFS to check it there.

[PaintGuru]

I just checked mine with the homemade mix and came up with ~39ppt as well. Readjusted it and of course my tank is coming out at 1.022 instead of .025.

[Randy Holmes-Farley]

Those results are, unfortunately, not unusual.

[Tang Salad]

Thanks for your very helpful posts, Randy.

After years of reading threads about how we should all ditch our hydrometers to buy refractometers, that's what I (and many others) did. And now this.

Very, very disappointing.

[HotHotHot]

If my refractometer came with a "pure water" calibration fluid, then I should be OK with RODI right?

Ed

[Tang Salad]

Originally posted by HotHotHot
If my refractometer came with a "pure water" calibration fluid, then I should be OK with RODI right?

Ed

Read the link.

The problem is that a refractometer calibrated at zero, may be unaccurate at the higher, normal-use range.

I believe the thread is only referring to one kind of refractometer, though.

[Snakebyt]

thanks for the heads up

[kelley_mc]

Is this what we are talking about?

[PaintGuru]

Originally posted by Randy Holmes-Farley
Those results are, unfortunately, not unusual.

Hopefully I didn't have too much error from my mixing. I have a balance, but it is only accurate to 2g. So I weighed the water and salt to the masses you specified as best I could. I can't imagine there is more error to my way then there is to volumetric measuring.

[mbbuna]

Originally posted by Tang Salad
Read the link.



I believe the thread is only referring to one kind of refractometer, though.

not just one kind. most of them

its an easy standard to make and worth the piece of mind to be sure

[Randy Holmes-Farley]

Hopefully I didn't have too much error from my mixing. I have a balance, but it is only accurate to 2g. So I weighed the water and salt to the masses you specified as best I could. I can't imagine there is more error to my way then there is to volumetric measuring.

What volume and mass did you use?

+/- 2 g out of 73.1 grams of salt will potentially cause an error range of 1.0257 - 1.0271.

[vessxpress1]

Oh boy. Here we go again! There's been so many new people join though, I guess it was about time to remind everyone again. Randy, maybe it would be helpful if you posted the smaller gram weights that you can have a pharmacist mix up for you in a small bottle. That seems like a lot more accurate way to come up with a solution and what I calibrated mine to. My premium blue now drops the blue below the zero when reading pure RO/DI and I'm confident with it.

[PaintGuru]

Originally posted by Randy Holmes-Farley
Hopefully I didn't have too much error from my mixing. I have a balance, but it is only accurate to 2g. So I weighed the water and salt to the masses you specified as best I could. I can't imagine there is more error to my way then there is to volumetric measuring.

What volume and mass did you use?

+/- 2 g out of 73.1 grams of salt will potentially cause an error range of 1.0257 - 1.0271.

Bloody heck. Yes I used 73.1 grams. Well it really wouldn't be +/- 2g, more like +/- 1g since I know it was somewhere between 72 and 74. I'll mix up a better batch tomorrow at work.

[cwegescheide]

I did the 2 litre bottle and tablesalt trick that Randy suggested and I discovered I had a problem with my refractometer. I didn't use and special instruments for supper accurate salt calculations or anything either. I think the point is more if you get something WAY off when doing this, then you know you have a refractometer that is incorrect. FWIW when I followed Randys recipie I came out with 1.026

I realized my refractometer was .09 off!! OUCH!

[techreef]

Just call Randy's salt formula the new Heart Attack Special.

[Amphiprion]

Originally posted by Randy Holmes-Farley
Hopefully I didn't have too much error from my mixing. I have a balance, but it is only accurate to 2g. So I weighed the water and salt to the masses you specified as best I could. I can't imagine there is more error to my way then there is to volumetric measuring.

What volume and mass did you use?

+/- 2 g out of 73.1 grams of salt will potentially cause an error range of 1.0257 - 1.0271.

This isn't really that bad, at all.

[Flobajob]

Is there a standard amount the rhs10atc refractometers are out by? If there is then can you just make an adjustment for that amount it is out, without having to prepare any solution?

[2fishy]

Originally posted by kelley_mc
Is this what we are talking about?

Yes that is the one that they are talking about. So has anyone taken a measurement using a standard hydrometer in comparison to their refractometer to make sure that they are getting different readings?

[2fishy]

I'm a little concerned about this only because I am testing the same with both a hydrometer and refractometer, both reading at 1.025, and distilled water at zero. I even tested my tap water with the refractometer knowing that it would be giving a salt reading of some sort, but not sure of what the content of my tap would normally read at. Tap water is on a water conditioner. I do have that same refractometer as stated. Does anyone know why the hydrometer would be giving the same reading????

[r00onmac]

so... after freaking out that my new refractometer read 1.038 and spending 4 days lowering the sg to 1.025-1.026 i may have lowered it too much? man... ok i need to find that recipe for calibration solution... Randy, care to repost it here?

[dcombs44]

what is the 2 liter bottle method

[PaintGuru]

2 liter calibration method

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

[dcombs44]

thanks..i actually just found that article.

[LobsterOfJustice]

Randy saves the day again.

Thanks