When I became more serious about dying fibers and fabric, I wanted to purchase a dedicated device to heat my water. I researched several options of hotplates and sous vide. The sous vide intrigued me, I couldn't see any disadvantages to it, and it was priced about the same as a quality hotplate.
I don't remember the brand, but it cost around $60 on Amazon. My fiber art friend now has it and she also really loves it. I want to describe the advantages and how to use it for dying fibers.
It heats your water evenly by circulating it so you don't have to worry so much about constant stirring. A hotplate will heat the water at the bottom of the pot much hotter than the water at the top. Therefore, the fabric at the bottom will pick up more dye than what's nearer the surface. To avoid this, you must stir the pot frequently. With the sous vide, you still have to stir, but the heating element is in the middle of the pot and it has a mechanism which is always moving the water around. This leads to very even temperatures throughout the pot.
Another thing is that you can set the temperature you want, unlike with a hot plate. When dying different materials, you may want a higher or lower temperature. You never want boiling water, which a hot plate might lead to if you're not careful. On the display you can set and monitor the temperature, and even through reviews say that it isn't always 100% accurate, for dying purposes, 1 or 2 degrees difference is not a big deal. Also, because of how the sous vide heats water, described above, it's better for fibers like wool, which like to gradually change temperatures. If it's shocked by becoming too hot too fast, it may felt on you.
There is also a timer, which is quite convenient.
There are some aspects of the sous vide which are not ideal for loose fibers especially. There is an opening on the tube which is where water is circulated. Inside there's a little propeller type device. This circulation of water around this propeller will take your fibers with it and they can get tangled inside. I didn't have dramatic problems with this, but I still looked for different solutions. One was to keep fibers separated in little bags, and regularly dip them so that they would receive fresh water and dye. This had the added advantage of keeping fibers in an orderly arrangement. When they are immersed in water they like to spread out really wide and get tangled a little with each other. I didn't have any problem with this method, but you do have to make sure to renew the water in the bags frequently. It makes it convenient to share the dye pot with other fibers and fabric as well. You don't want to be stirring fabric and tangle fibers up in the fabric.
The other thing which worked for me, if I wanted the fibers to be loose for stronger dye saturation, was to cut off the foot part of some old panty hose and using a string, tie it on around the sous vide tube. This allows water through but blocks the fibers from getting inside. This worked totally fine. I would recommend this method at all times just to be on the safe side and keep your sous vide clean and in good condition.
These are some results of one dye pot. Different fibers took up the dye differently.
Warning: If using a sous vide for dying, DO NOT use it for food preparation. As with all dying equipment, it should be dedicated only to dying because it can be toxic to use for food preparation.