If you want absolute accuracy when mixing by weight, you’re going to need to figure out how much everything weighs.
Specific gravity (SG) is the weight of one milliliter of the liquid at a given temperature.
Some vendors (like TFA and Capella’s) thoughtfully provide MSDS links for most of their flavors that list specific gravity.
Generally PG has a specific gravity of 1.04 and VG has a specific gravity of 1.26, or there abouts.
Note:
Specific gravity of liquids does change as temperatures increase or decrease (temperature changes cause density changes). This is a really small variance and all density changes will be equal at the same temperature, so you can either ignore it or do all of your measuring and mixing at the same temperature.
If a vendor does not list SG, we have to measure it. If you are going to do this, I would recommend measuring as much of the liquid as you can (10ml should be enough to give an accurate measure), and then divide the resulting weight by the number of ml measured to give you a fairly accurate SG for that liquid.
This can be quite a pain, and does waste some flavoring (some is going to stick to the measuring vessel and dispensing utensil).
Alternative Method
Alternatively you can just assign a given weight to all of your flavorings. Many people mix by just assigning either the weight of PG to all of their flavors (because the vast majority of flavors are suspended in PG), or just assigning all flavorings a value of 1.
Both methods have their advantages, and I would not necessarily recommend one over the other. It’s all going to come down to what you want to do.
I will note that the primary reason that mixing by weight is the most accurate is that it is fully repeatable. Even assigning a random value to the weight of all of your flavors (I like 9, so all of my flavors will weigh 0.999g!), so long as you ALWAYS use that value, will give this advantage.
Which ever you choose, I would very much suggest that you do one or the other (if for nothing other than your own sanity).
source:
https://godofsteam.wordpress.com/2014/09/11/getting-started-diy-102-mixing-by-weight/
