Refractometer question

[AlanM]

Try it!

 

Going to do that now.  The prism in mine gets stuck, from time to time, so I calibrate to solution every time I use it.  I have to loosen the screw, whack it like crazy, then put drops on it and tighten it down while watching the line.  I need a new one, but this one seems to still basically work and am too cheap to buy another one.

[AlanM]

It reads 0 with DI water, 35 with the solution, and 37 on my tank water, uh oh. 

[jaddc]

It reads 0 with DI water, 35 with the solution, and 37 on my tank water, uh oh. 

 

That's my point. Your tank would be 37 ppt (uh oh...) whether you used your 35 ppt solution or RODI water to single-point calibrate.

 

But RODI water is easy, costs nothing, and there is little doubt that zero is zero.

[AlanM]

I never test my tank water, heh.  Haven't in a very long time.  I test the tub of new salt water and that's it.

[Origami]

Calibrate a brine refractomer with a trusted 35ppt seawater standard if you're going to measure seawater at 35 ppt. This compensates for what they call integral linear (slope) and differential (offset) errors. Calibrating with RO or distilled is only accurate if these two errors are zero.

[jaddc]

<sigh>

 

waste of time.

[Origami]

Maybe this will help. It's an article by Randy Holmes-Farley from 2006 that talks about refractometer errors and why you want to calibrate using a trusted salinity standard. The key section pertains to slope errors. Here's the caveat: You need to have an accurate and trusted standard. If you don't then your refractometer is not calibrated properly. 

[jaddc]

Why yes. It does help. 

 

If you read down to the section "Tips on Calibrating a Refractometer" you will see my point.

 

Most refractometers will not have those errors. Once you prove to yourself that you purchased a decent device, RODI water is fine for a blank. 

Edited August 3, 2014 by jaddc

[jaddc]

Randy does an excellent job of talking about the linear nature of these devices.

 

The take home message is that you need to know how well your refractometer was calibrated. Once you convince yourself that your refractometer is correctly factory calibrated, then there is no reason to use 35 ppt standard.

 

It is not wrong to do so, just unnecessary.

 

If you wanted to save $$ and bought a bad refractometer, then yes, you MUST use a 35ppt solution.

[jaddc]

And I agree with Randy:

"As with many devices, however, you sometimes get what you pay for, and sometimes less. Very inexpensive refractometers can be prone to errors and may need to be checked in a solution matching seawater, not just pure freshwater."

 

So spend $35 and get a refractometer that is high quality.

[GraffitiSpotCorals]

I have seen a number of expert reefers have major problems with their tank because of ro water and junky refractometers. That is why I always tell people to use 35ppt, along with calibrating often just in case. A lot of people don't know or don't bother to buy a good device also.

I always use the 35ppt solution because I started with a refractometer that wouldn't show readings under 0 so I was not getting a good calibration, since I would go past the 0 but it wasn't reading lower. It would still read 0 and my water was a little lower than I thought.

[lnevo]

If you know that 0 is 0 and 35 ppt we are no longer talking about single point of calibration now are we. So yes go ahead and check both. Personally I calibrate every time I check. It's not extra effort by any stretch.

 

And finally the OP obviously has an issue with his refractometer being off with ro or he has bad calibration. Either way he needs to check them both out. If he goes only with RO he could be off by as much as 5ppt.

[gmerek2]

Mine has the same issue as his. I bet Neto has a BRS refractometer with BRS calibration fluid. The only difference is that I have a second calibration fluid from pinpoint. Both read nearly the same at 35ppt. However when I use RODI it is way off. Probably 4-5ppt. I'm willing to bet the calibration fluids are correct and the BRS refractometer has a bad downward slope

[jaddc]

Wow. 4-5 ppt??? 

 

I did a sanity check because I am a chemist and so lets do a Sunday evening experiment. I quickly made up a 30 ppt and 35 ppt saltwater solution. I checked it with my HI 96822 (which only does a single-point calibration with RODI water). Side-note, if you are looking for the next expensive toy to invest in, I do recommend this fancy refractometer. Talk about zero-guess work!

 

http://www.hannainst.com/USA/prods2.cfm?id=039001&ProdCode=HI%2096822

 

My portable refractometer reads 0 with RODI, 30 ppt and 35 ppt. I did it three times. Each time I spun my calibration screw a half turn and the performed a single-point calibration at either 0, 30 or 35 ppt.

 

My refractometer is model RHS-10ATC, if you want to search for it. 

 

Here is a protocol from South Carolina: 

http://score.dnr.sc.gov/ktmlpro10/files/uploads/elearning/refractometer.pdf

 

I get the same readings (+- 1 ppt error, which is the best we are going to get for under $50) no matter what I concentration I use for my single-point calibration.