IMPORTANT! YOU MUST MAKE SURE Open the link from Pressure Scale below USING IPHONE 6S OR IPHONE 6S PLUS.
Lay the iPhone down on a flat and steady surface, then place a metal spoon or something similar* onto the screen – the items rough weight should appear on screen but you’ll be using this to hold other items to weigh, thus hit the “Tare” button to zero out the scale to “0” with the spoon on the screen
Now place your item(s) to weigh in the spoons cup to see that items weight
LINK: Pressure Scale