There isn’t much that can be said about these. Simply screw this device onto your mod’s 510 connector and it will tell you the battery voltage. Since this one is “Inline” I’d assume you can also attach your atomizer to this device to see your voltage while vaping.
At $11.99 this is cheaper than other models I’ve seen on sale so far.
Get yours here http://101vape.com/home/207-inline-volt-meter.html