Accuracy of salt test calibration

[shilala]

Originally Posted by 76GTFan

I believe the scientific validity, but I fear there are too many variables in the common setting. Like others have stated, specific amounts of water to salt would be very helpful. And do different forms of salt make a difference?

On specific amounts of salt to water...
It's real simple. The salt just needs to be saturated. There's a huge margin for error. Even if you have half salt and half water, you're still okay. The solution will still yeild 75% over itself.
That's why we use this test for calibration, it's so simple, and so hard to screw up. What happens is we start thinking too hard, and get scared of the unknown.

Your question about different forms of salt is kind of a loaded one. If you mean "does magnesium chloride yield a different RH than sodium chloride?", the answer is yes.
If your question is "Do different brands of table salt yield a different RH?", the answer is "yes, maybe a fraction of a percentage point."
If your question is "Can I use salt substitute like people use for low salt diets for the test?" the answer is no. It'll yield a different RH, as it isn't table salt, or sodium chloride.

The only thing I can add that may help is that by increasing the surface area of your salt/water mixture, you can move things along a lot faster.
That means that if you use a peanut butter jar lid rather than a soda bottle lid, the air inside your ziplock bag (or whatever) will come to 75%RH a lot quicker.

One last thing...
The depth of the salt/water solution in the peanut butter lid or pop bottle lid or shot glass or whatever doesn't matter at all. A half inch is as good as a mile.