If youre planing to use one of those to test for voltage drop on a a mechanical mod then dont waste your money..They arent accurate and have components that inherently cause voltage drop in itself.
Use a basic volt meter you can buy for $10.00 to test for voltage drop at the atty itself after the coil or coils are built and working properly to get a more accurate voltage drop reading.
if indeed it is for a mech mod check , the rule is simple , when the vape gets soft or the clouds get smaller it is time to switch batteries , no devices needed , you get better with time ...