refractometer calibration- distilled or calibration fluid?

[velocityoflove]

I felt like my refractometer was giving me funny readings. It would go out of calibration, and my salt mix "recipe" would read high. I've never dropped it.
I went to my lfs, and we got the same reading. I've checked it at two stores. Both of these stores calibrate with RO or distilled water.

I went to a store that calibrated with calibration fluid and they got a completely different reading, that my salt was very low.

My fish and coral seem fine. My refractometer reads fine (I calibrated with distilled). Should I not trust my reading? I'm worried about my salt, but I'm not sure what to do.

 

[rdnelson]

When calibrating any precision device, it is always best to calibrate at levels close to those you will be testing. In otherwords, calebrating to a solution that has a salinity close to whay you want your salt mix to be will give you much more accurate readings.

 

[kilralpine]

I would probably do as he said, find a store with some 1.025 calibration fluid or like 1.020 and then reverse check it against RO and make sure its reading dead 0. I think at that point I would trust your meter. You said it was different between the two stores?? When they checked their own salt levels? or with calibration fluids??

 

[ichthyoid]

rdnelson99;790309 wrote: When calibrating any precision device, it is always best to calibrate at levels close to those you will be testing. In otherwords, calebrating to a solution that has a salinity close to whay you want your salt mix to be will give you much more accurate readings.

+1
Few instruments are accurate over their entire range. By using DI water you are calibrating at the extreme bottom of the refractometer's range.

The inaccuracy is usually caused by the device being nonlinear. That is, there is often an 'S' shaped response over the measuring range. Knowing this, you will want to calibrate near the middle of the devices range.

You also want to use a device where the quantity to be measured is near the center of the devices measuring range, as Rich mentioned above.

 

[velocityoflove]

kilralpine;790328 wrote: I would probably do as he said, find a store with some 1.025 calibration fluid or like 1.020 and then reverse check it against RO and make sure its reading dead 0. I think at that point I would trust your meter. You said it was different between the two stores?? When they checked their own salt levels? or with calibration fluids??

I went to a store to have the salt mix I made for my water change tested. They got 1.027, the same as I was reading. My salt mix usually gets me a 1.025 every time. They checked their calibration there with RO water. I went to another place that used calibration fluid. They read my tank at 1.018. They also checked their calibration. I went home and calibrated mine with distilled water, then tested my tank which read 1.025.

 

[kilralpine]

I would safely assume that your probably in the range of that 1.027 reading and your 1.025 reading. The calibration fluid stores 1.018 seems so out of line; I would place my bet on they just messed up the calibrating somehow. Some refractometers are designed to be used with RO or Distilled. Just remember to keep it simple, test a single batch multiple times and make sure your not getting different readings with your refract. Then base your WC water and your tank water off of your own meter and keep it as stable as possible.

Edit: Also with the testing of the salt batch, how do you determine that your "salt mix" is always 1.025?

 

[velocityoflove]

kilralpine;790353 wrote:

Edit: Also with the testing of the salt batch, how do you determine that your "salt mix" is always 1.025?

I test it every time to make sure, of course. I take 5 Gallons of RO minus 10 cups, and add 2 cups of salt.