I wanted a simple hydrometer holder that would fit inside my spools and be able to use vacuum bags without ripping the bag. I tried a few others and didn't find exactly what I wanted and this was born. It fits spools with an inside diameter of at least 54.8mm. There are two models provided; one of which fits the larger hydrometers typically found on Amazon. The other model is specifically for Bambu hydrometers (or those that are slightly smaller).