calibrating refractometer issues

[fishgate]

I just got the salinity probe for my RKE. I was calibrating it and when done, figured I would use the 35ppt fluid to calibrate my refractometer. It was WAY off. I had to significantly move it. It says to calibrate with RO water which is what I have always done. So I was done, and checked it, and no where near where it should be. My tank water was reading like 41 ppt. The newly calibrated RKE probe read 32.5. So I recalibrated the refractometer with RO water and now it reads almost dead on 32.5! Does this make any sense at all? My refractometer truly cannot be calibrated by anything but RO water?

 

[GimpyFin]

What kind of refractometer do you have? The digital ones use RO/distilled only.

 

[fishgate]

What kind of refractometer do you have? The digital ones use RO/distilled only.

It is BRS branded.

 

[GimpyFin]

Hmm, Not sure then. They're supposed to use the calibration fluid. The one BRS sells on their website says not to use RO water, if that's the same one.

 

[Scott07]

Doesn't make sense to me... I had similar issues a few years ago when i read Randy Holmes Farley's article and learned about the importance of calibrating a standard brine refractometer with fluid instead of RO. Specifically that you could calibrate your refractometer with 35ppt fluid and then read your tank water to be that much off says to me a measurement error somewhere. The scale on your refractometer would have to be wayyyyyyyy off for there to be that much of a discrepancy between 32.5 and 35 ppt. Likley culprits I would think would be sample contamination, maybe leftover dried saltwater or calibration fluid on the glass. I would repeat calibrating the refractometer and double check everything. Calibrating with 35ppt and reading 41ppt just doesn't add up to me.

 

[Ou8me2]

It would be pointless to buy their refractometer kit which I believe comes with 35pt calibration fluid if all you needed was RO water. :hammer:

Also after each test make sure you wash off the saltwater off the top plastic thing and lens with fresh RO/DI water. Yes, this can throw off results if you don't. All you need is a couple drops and not large globs of water. You'd be surprised how well just two drops covers the whole lens.. or whatever you want to call it.

I thought I had a calibration issue and the whole problem was I wasn't consistently putting the same amount of water. Also, I believe it says if it is an ATC refractometer then you need to wait 45 seconds for the reading. I was growing frustrated cause everytime mine would read differently. Been rock solid since.

 

[fishgate]

Doesn't make sense to me... I had similar issues a few years ago when i read Randy Holmes Farley's article and learned about the importance of calibrating a standard brine refractometer with fluid instead of RO. Specifically that you could calibrate your refractometer with 35ppt fluid and then read your tank water to be that much off says to me a measurement error somewhere. The scale on your refractometer would have to be wayyyyyyyy off for there to be that much of a discrepancy between 32.5 and 35 ppt. Likley culprits I would think would be sample contamination, maybe leftover dried saltwater or calibration fluid on the glass. I would repeat calibrating the refractometer and double check everything. Calibrating with 35ppt and reading 41ppt just doesn't add up to me.

I used the brand-new fluid that came with my ReefKeeper probe. I calibrated the probe, then used the fluid with a new pipette for the refractometer. There was no contamination or even a chance for contamination.

 

[Scott07]

I used the brand-new fluid that came with my ReefKeeper probe. I calibrated the probe, then used the fluid with a new pipette for the refractometer. There was no contamination or even a chance for contamination.

I hear ya.... not saying you did anything wrong, just if the scale on your refractometer is far off enough to calibrate at 35ppt, then read 32.5ppt water as 41ppt.... I would think the scale when you look in your refractometer would be noticably out of proportion. Thats a BIG discrepancy. Just trying to help.

All that said ive never used the solution that comes with a reefkeeper probe. Nor do i know 100% if its maybe different somehow from regular lab standard calibration fluid... could be for all i know. My advise was based on the assumption that it's a 35ppt standard.

 

[fishgate]

I hear ya.... not saying you did anything wrong, just if the scale on your refractometer is far off enough to calibrate at 35ppt, then read 32.5ppt water as 41ppt.... I would think the scale when you look in your refractometer would be noticably out of proportion. Thats a BIG discrepancy. Just trying to help.

All that said ive never used the solution that comes with a reefkeeper probe. Nor do i know 100% if its maybe different somehow from regular lab standard calibration fluid... could be for all i know. My advise was based on the assumption that it's a 35ppt standard.

This is it...

 

[Scott07]

This is it...

Because its calling itself a conductivity standard im guessing thats the missing variable here. Makes sense thats how a probe would work vs. a visual instrument. Ill bet if you calibrated with pinpoint or another refractometer solution it would be a lot closer than 32.5 to 41 ppt lol.... again, don't have a salinity probe, and have never used that fluid... but id put money thats the culprit. Really interesting though that it would different from other calibration fluids.

 

[thegrun]

You are measuring two different things and the conductivity probe reads the amount of electrical conductivity of the solution. Your conductivity standard is made to conduct electricity at a given rate, it cannot be used for either salinity or refraction calibrations.

 

[fishgate]

You are measuring two different things and the conductivity probe reads the amount of electrical conductivity of the solution. Your conductivity standard is made to conduct electricity at a given rate, it cannot be used for either salinity or refraction calibrations.

OK yeah this makes sense to me now. This is 35ppt using the conductivity probe but not for a refractometer type measurements. I'll just keep using my RO water.

 

[nereefpat]

I agree with thegrun.

As far as calibrating, you can use RO water if that is what the refractometer instructions call for. However, you need to be using a 35ppt refractometer calibration fluid as a "check standard."