I vape and I mix my own e-liquids. When I started doing this I used a small digital scale that I previously bought to weigh coins with. I discovered that the scale I had would auto power off after a few minutes. Not only that, if the increase in weight was very small it wouldn't reset the auto shut off timer and it could turn off halfway through adding a flavour as they are sometimes in very small quantities. This scale also only has a resolution of 0.1 gram and a maximum load of 200 gram. I needed a scale that had a resolution of 0.01 gram so I could be more precise. I also needed a scale that could handle about 500 gram of weight and, very importantly, that did not auto shut off.
I searched high and low for a scale that met these requirements. It was easy to find something that had a 0.01 gram resolution and that could do 500 gram of weight at a time. The difficult part was finding one that would not auto shut off. It seemed that every cheap scale I looked at would have an auto shut off feature and no way to turn it off. I soon realised that this was because all of the scales I was looking at were battery powered. With further searching I found scales that were mains powered and where you could turn the auto shut off feature off if you wanted to. These were all very expensive so I decided to keep looking.
After some extensive searching I finally found the perfect scale for my task. It was cheap, could handle 600 gram total weight, had a resolution of 0.01 gram, could be calibrated (a bonus feature) and could alternatively be powered via a micro USB connection. The scale does have auto shut off when powered on battery, but when powered via USB it does not shut off, it only dims the screen until you change the weight on it, then it promptly lights up again. On top of this it has a nice large digital display that is very easy to read and supports some other functions like TARE, COUNT and UNIT changing.
It is worth noting that the first time I tested it I plugged it into an Anker portable USB battery pack. The Anker battery pack is clever and will turn off if there is almost no current being drawn from it. Because the scale is very low powered the battery pack turns off in a short time. If I power the scale with an old battery pack that I have that has none of the fancy features of the Anker battery pack, it stays on for as long as the battery pack can supply power. It also stays on when powered from a computer USB port and works just as well connected to a mains USB phone charger.
The Auto Power Off setting adjustment is not documented in the manual. The Auto Power Off setting should only have an effect when you run the scale on battery, on USB it should not auto power off at all. To adjust the Auto Power Off timer, turn the scale on and wait for it to zero. Now press and hold the TARE and COUNT buttons simultaneously and the display should show AU on the left, let go when AU is displayed. If you get Err showing instead of AU, make sure you press the COUNT button a fraction ahead of the TARE button. On the right it will either show a hyphen or a number. The number is the time in seconds before the scale will auto power off. When set to the hyphen the scale will not auto power off. You can change the setting by pressing the UNIT button to cycle through. Once you have the screen showing the setting you want, press the TARE button once. Then turn the scale off and back on with the ON/OFF button. The setting is now active and you can check it by pressing and holding the TARE and COUNT buttons together again until AU is displayed.
The scale is branded "J&J" and the box showed "EP Series" and "DS-29" on it. There was also a barcode sticker on the box that showed "E1604-1", this is the same model number that was shown on the eBay listing I bought it from. That particular model number is for the 600 gram max weight scale with a resolution of 0.01 gram. There are other models of this scale, for example: a 1000 gram max weight scale that has a 0.1 gram resolution, but that looks the same otherwise (that is model "E1640-2".)
There is little point in linking the eBay listing as they often go missing or change. With the above information and the below picture you should be able to search eBay/Amazon/Google and find the exact scale. For reference, I paid £7.06 for the scale, delivered from China.
For those that are interested in calibrating the scale, all you need is to buy a calibration weight. I bought a 500 gram weight for £4.99, also from an eBay listing. Why 500 gram? Because I figured it'd be more accurate to calibrate near the upper weight of the scale in comparison to using, for example, a 100 gram weight which is near the bottom range of the scale. Hopefully my thinking isn't flawed.
Have fun mixing your own e-liquids!