How much water to use when cooking pasta in an Instant Pot The de facto Instant Pot manual at hippressurecooking has a page on cooking pasta but doesn't explain how to determine a proper amount of water to use when cooking pasta.


  Figuring out the liquid is easy (no sputters) The hip method for
  pressure cooking pasta does not actually measure any of the cooking
  liquid.  As noted in the recipe, only the amount of water that is
  needed for the amount of pasta to be cooked is used.  This ensures
  that there is almost no liquid left in the cooker by the time the
  pasta is finished cooking – nothing left to foam or sputter when
  pressure is released.


There follows a recipe that instructs to "Pour in the pasta, the tomato puree, and just enough water to cover the pasta".  2 cups of tomato puree.

So not much help there.  Presumably the amount of water that is needed for the amount of pasta to be cooked is related to the weight of the pasta rather than its volume—covering farfalle or rotini is going to require a lot more water per cup of pasta (but perhaps the same amount per gram) than is covering lasagne or spaghetti, because their shape makes these flat pastas more dense.

Is there a simple ratio of water to weight of pasta that will give consistent results with a variety of pasta shapes?  If not, is there a chart somewhere that gives a water-to-pasta ratio for various shapes?
You're correct, the water ratios may be slightly different between different types of noodles. In general, you're covering the noodles so that they will actually have a chance to soak up the water though, as with a pressure cooker, you don't have a chance to stir the noodles and ensure equal saturation.

When making pasta in the instant pot, you'll also need to keep in mind the layering effect of your ingredients. Tomato based sauces are prone to burns, so you want to keep them on the top of the pot. Put your noodles on the bottom, and if you feel that it's taking an excessive amount of water to cover them, you can leave the last 1/2 inch or so exposed, so long as you cover it with your tomato sauce, or some other liquid.