It is super humid in Houston. Even with the AC on it is still more than 60% in my office. So I have my spools in a dry bag or in a dryer. The problem with dryers is that they tend to be opaque, so that I can never really tell how much filament is left without opening it up. Not a big deal but I just wanted a simple visible indicator to give me an idea of how much filament is left. So, I made a spring scale that can be easily calibrated. I just put the dryer box on the scale calibrate the scale to “E” when the box is empty and then load the filament. The scale gauge and pointer are designed to read full with an additional 1kg of mass. The gauge and needle can easily be re-mixed for different springs (different spring constant). I designed the pointer and gauge so that a change of ~5mm between the base and platform produces a change of ~35mm at the end of the pointer. Easy enough to read at a distance. I experimented with using 3d printed springs but I could never get them to print reliably and even the ones that did come out had a super low spring constant. So I had to use metal springs. In fact this scale could be used to weigh anything up to 1kg.

Hardware:

1. 9 M3 4mm heat inserts
2. 5 M3 4mm screws
3. 5 M3 6mm screws
4. 1 M3 12mm screw
5. 2 M3 washers
6. 1 M3 lock washer
7. 1 M3 lock nut
8. 1 M3 nut
9. 5, 11mm OD, 0.8mm thick, 25mm uncompressed height sprints

There are 9 printed objects:

1. 1 scale base
2. 1 scale platform
3. 4 scale slides
4. 1 scale gauge (v3 for E and F, v4 for a scale)
5. 1 pointer
6. 1 scale armature

The springs are on Amazon : https://www.amazon.com/dp/B0CGZPMMHR

This is super easy to print. I used PLA and PETG for everything and both work fine. 0.2mm layer height, 20% infill. Orientation as in stl files. I almost always use supports (sorry).

When I printed the scale gauge I did a filament change at layer 11 to get the letters in a contrasting color.

This is how I assembled it: ( There is a picture for each step)

1. Add two 4mm M3 heat inserts to the scale base where you want the gauge to be.
2. Add one 4mm M3 heat insert into the hole on each of the scale slides.
3. Secure each scale slide to the scale base using 4, 6mm M3 screws.
4. Place 4 springs into the recessed hole around the 4 scale slides and one in the center. 
5. Add two 4mm M3 heat inserts into the scale platform in the location corresponding to where you put them in the base.
6. Add one 4mm M3 heat insert into the bottom of the armature.
7. Secure the armature to the armature base using one 4mm M3 screw.
8. Secure the armature base to the scale base with 2, 4mm M3 screws.
9. Secure the scale pointer to the scale gauge using 1, 6mm M3 screw, 2 M3 washers and a lock nut. I put a washer on either side of the pointer. It has to be loose enough to allow the pointer to easily rotate.
10. Secure the scale gauge to the scale platform using 2, 4mm M3 screws.
11. Put a 12mm M3 screw ( it doesn't have to be 12mm but it needs to be long enough) with a washer and a lock washer through the rectangular hole in the armature and loosely secure it.
12. Place the scale platform on top of the scale base and put the end of the 12mm M3 screw through the rear hole :) of the pointer. I just bend the armature back a little.

Calibration:

I place the empty filament box on the scale and then move the 12mm M3 screw up or down until the pointer indicates E. The closer you put the center of mass toward the gauge the more the pointer will deflect. So find the optimal spot for the center of mass to get the right deflection. If the scale doesn't fully deflect with 1kg you could try removing the center spring.

I tested this with both of my dryers: Sunlu and Eibos.

I added Scale Gauge v4 with a kg scale with marks from 0kg to 1kg at approximately every 166g.