refractometer calibration fluid discrepancy

[boxed_reef]

I just got new refractometer calibration fluid, and it is 2ppt lower than my old calibration fluid. I have been known to forget to put the cap back on the old fluid on a few occasions, so I guess evaporation could have occurred.
The calibration fluids are different brands, so before I start making changes should i get another new bottle to verify which is accurate? Or safe to assume the old was bad, and trust the new bottle?

 

[Flippers4pups]

Is the old bottle years old? If so, it very possible that it's evaporated some.

Come to think of it, mine is pretty old and I've done the same thing from time to time leaving the top off. It's years old.

 

[boxed_reef]

It’s about 1.5 years old. I guess I’ll try to remember to put the cap back on now!

 

[Dkeller_nc]

If you'd like to make your own, here's an article written by Randy Holmes Farley. That article uses things like kitchen measuring spoons and 2-liter coke bottles. While I understand why he wrote the article that way, since a lot of reefers would want to use whatever they have on hand, you can achieve very high accuracy for very little $$ - just get a 1 Liter class-A volumetric flask off of Amazon, and a kitchen scale that will measure at least to grams (tenths of a gram is better, and generally not any more expensive). Because salt is so soluble, you can get it to dissolve very quickly by filling the volumetric 1/2 full, adding the salt, swirling it to dissolve, and then adding enough water to get to the calibrated line to make 1 liter. It's a bit more cash, but a magnetic stir plate and a magnetic stir plate makes it much easier - about $30.