I have a 3D printer that is going crazy with x-axis shift, and I need a tension gauge belt to measure the tension.
I've never used one before, and looking online, I can't tell which one would be the right fit.
Any ideas? What things should I look for?
It's extremely unlikely that belt tension is actually your problem. I've never heard of anyone using a gauge to measure their belt tension. Typically you just pull your belt tight by hand so that it produces a low note when plucked. It's far more likely that you're experiencing shifts due to too high or too low stepper current.
Unless your belt is so loose that it easily skips over the pulley (which should be obvious without using a gauge) or so tight that it completely binds up (it would be impossible to get it that tight without some kind of superhuman force) it's definitely not the problem.
You may not need to adjust the tension, but you may nevertheless want to measure the belt tension, at least to ensure it's not too tight: the shaft on stepper motor has limitations. For example, NEMA 17 may have 30 N maximum tension then the belt is located at 17 mm from the motor face.
In that case, no need to buy a tool, you can use an audio app showing the frequency spectrum and then you pluck the belt. The lowest frequency peak can be related to the tension as explained here:
https://benchtopmachineshop.blogspot.com/2019/04/printer-belt-tension.html
For GT2 belts and 27 N tension, assuming the distance between belt and motor face is 10 mm, the relation is
$$f [\text{Hz}] = \frac{28531 \frac{ \text mm}{\text s}}{length [\text{mm}]}$$
Be aware that by plucking the belt you will excite the higher harmonics more than the base harmonic, therefore identifying the lowest frequency may take some time and multiple tries. You should display the frequency spectrum as "waterfall". The app "Spectroid" for Android has it.
If the distance between middle of the belt width and motor face is twice the value assumed (for example, 20 mm instead of 10 mm), the belt tension should be halved (based on an inverse relationship).
For those interested in alternative approaches, this page calculates the belt tension based on the datasheet of the belts, but it completely ignores the maximum load tolerated by the motors. So you can follow their directions but do not exceed the values recommended by the motors. The calculations mentioned there are discussed also on the following video:
People are using this
to tighten the belt but I would totally agree with @Tom - there is no need to use such devices or gauges and your problem lies in stepsticks current.
It would be also problem with endstops so PCB doesn't know it's time to stop :)
Added for reference. To my understanding, especially if you run multiple identical printers, having the same belt tension on all of them is essential to getting accurate and consistent results. The effects of overly loose or overly tight belts have been demonstrated in a YouTube video by CHEP.
As mentioned in the comments above, a frequency analyser on a smartphone can be used, a web-based analyser is available at belt.connect.prusa3d.com. Additionally, the article Adjusting belt tension (MK3/MK3S/MK3S+/MK4) explains the concepts and steps that can be taken for belt tensioning.
Also, a simple belt tension measurement tool called Tension Meter for the GT2 belts of i3 MK3S+ or Prusa MINI+ that can easily be printed has been published by Prusament.
There is a similar printable model called True Open source tension meter for GT2 belts by ChipCE.
