Do Hydrometers lose accuracy?

[Opdog]

I have two proof and tralle alcoholmeters that I bought from Brewhaus. I bought one years ago when I first got started and another one I bought recently when I broke my hygrometer and thought I should probably have an extra on hand just in case.

I’m about to proof down some high test to put in a barrel and was double checking the ABV that I wrote on the bottle to make sure that the dilution was correct. I used the old alcoholmeter and the numbers were consistently 5% lower on an uncorrected basis. I thought that maybe the temp was significantly different. I’m doing the measurement at my basement bar, which stays at a pretty consistent 68F. I grabbed the new meter and it measured dead on the number I wrote on the bottles. So one of them is off.

I took a gallon of distilled water that I’m using to dilute my spirit and both meters measure accurate at zero. How do I figure out which one is off? It is only about 5% but I’d still like to have confidence that my measurements are accurate.

I don’t have any store bought vodka on hand to do a reference check there at the moment.

Any thoughts or advice is welcome.