Other than unrolling it and measuring it, is there a method for telling how much filament is left on a spool, for example calculating a length bases on weight or number of turns left on the spook?
Is it possible to extract the amount of filament used from the printer's firmware?
If you have an empty spool of the same brand, you could weigh the empty spool and the one you're trying to "measure" to get an approximate weight of the remaining filament. Divide by the (presumably available from manufacturer) weight per meter to get a rough length in meters, if that's more useful to you than weight.
If you use klipper you can use this script (by zellneralex) to calculate the filament length used since the last manual reset. Obviously it works with a single spool, if you switch spools it doesn't work.
If you want to know how much filament is left in the spool, the exact formula based on inner radius of the spool $r_{int}$ and outer radius of the spool when new $r_{ext}$ and based on the current outer radius of the remaining filament $x$ should be:
$$ 100 \left( \frac{x-r_{int}}{r_{ext}-r_{int}} \right)^2 $$
You can see that you get 100% when $x=r_{ext}$ (spool is new) and 0% when $x=r_{int}$.
It's a simple integration in $(x-x_0)\,dx$.
If you are down to one layer of filament, count the number of loops left on the spool, and multiply by the circumference of the spool to get the length. ($ \pi \times diameter \times loops $)
This can work if you have more than one layer and know the core diameter and the outer diameter of the filament left, but there would be some integration and a lot of estimation.
Some filament vendors put a window to see the remaining spooled filament with a decal showing graduations to match how much is left based on the diameter visible. You could do the same as a printed (in the paper sense) insert you slip along the inside of any spool between the filament and outer wall. You just need to compute the relationship between diameter and amount of filament based on the filament diameter and the number of turns per layer. Or you could just copy the design from a vendor who does this and figure it will be close enough.
I made an Excel spreadsheet that calculates this based on the spool dimensions. You can download it here.
There are products available that will keep track of the length of spools once you give it a starting length. The starting length could come from the spreadsheet.
Besides using the window on a spool that estimates the amount of filament left, I've used large calipers to measure the diameter of an empty spool and the diameter of the filament left on the spool.
Why not use a database online with a filament consumption counter? See this link.
You can create a database without buying counters. I have two printers an Ender and Mega Zero and 2 counters that automatically consume the selected filaments.
Very cool tool. I love it
