Getting the wall of a bowl the right thickness is very important. The wall doesn’t have to be the same thickness all the way down, but it must look and feel right. An experienced turner would likely rely entirely on their sense of touch and not use calipers at all. But simple, non-adjustable homemade calipers can help. They are quick to make and use. Calipers are normally adjustable, and made of steel. But adjustability is only necessary if you actually need to measure the thickness. I find that when using ordinary steel calipers to check the wall of a bowl, I set them to more than the thickness anyway. By watching the gap between the point and the wall, I can see that the thickness is right. I made a batch of fixed ones to help with the teaching I do.
Material for homemade calipers
I used scraps of 6 mm MDF. This is unlikely to scratch the finished surface of a bowl, which plywood might. But with care in use, other materials would be fine. Dimensions are not at all critical, as long as the central cut-out is big enough for the caliper arms to reach down to the bottom of the bowl. They could be made in any size.
Making the calipers
I cut the blank to a circle with a band saw and drilled a small hole in the middle for the revolving centre. I then pinned the disc to the jaws of a chuck with the tailstock. This is a very quick method of mounting discs in the lathe. Friction from the chuck jaws drives them. I trued up the edge of the disc. Turning MDF is dusty, but very easy with a scraper. Then I marked an off-centre circle and cut the middle out with the band saw. The entry cut is at the narrowest point. I sanded the inner curve with a drum sander, but this, like the turning, is purely for appearance. Hand sanding, or no sanding, would work just as well.
Then I enlarged the entry cut and sanded the ends of the arms. Although these calipers have no adjustment, it would be easy enough to cut them in half and drill for a bolt and wing nut. The gap should be a little more than the likely wall thickness, say 15 mm for walls of 12 mm. When one arm slides down the inside wall, the gap between the other arm and the outside of the bowl clearly shows changes in thickness. I drilled a small hole for hanging the caliper on a nail.