This depends on your method. If you are starting from pre-tempered chocolate (like a candy bar) then it is a very easy and basic skill… see below. If you are starting from a completely melted batch of chocolate, it is a very difficult skill, almost an art… if you are tabling as opposed to using a machine.
If you search tempering on YouTube seeding is the predominant method of tempering. Since I am making batches of chocolate from scratch, I don’t have any pre-tempered to seed with. Thus far I’ve tempered by tabling, but I plan to make a tempering bowl soon.
I think you are referring to Sous Vide. The thing that sous vide is good at is maintaining a specific temperature for a long period of time. Putting your chocolate into a bag and into the water can work, but to ensure that all of your chocolate is at the correct temperature to temper properly, you would have to leave it in for a very long time. Part of tempering is agitation, and unfortunately it is very difficult to agitate the chocolate while it is under water. I’ve attempted it twice now, and each time I have lost at least one bag to seizing… and I finally figured out why. I wasn’t using vacuum sealed bags, and since there was air (and therefore moisture) inside the bag, it condensed in the empty spaces above the chocolate, then as I agitated it started to mix in and the bag seized. This last time I figured this out on the first bag, so with the other bags I didn’t agitate at all, and I cut them open below the chocolate line. This saved the chocolate, but the temper was horrible.
I do intend to make a tempering bowl. I’m going to make an acrylic box with two holes in the top. One is for the sous vide and the other is for an aluminium bowl. Fill the box with water, drop in the bowl and the sous vide and I have a completely temperature controlled bowl. For now I will hand stir, but eventually I might add a motor system to spin the bowl against a fixed spatula.
Oh, and when I tempered by tabling, I used an infrared thermometer and it works very well.