To pressure cook black beans, a safe ratio to use is 3:1, 3 cups water for every 1 cup/8 ounces dried beans. You don't have to measure the water exactly. Just add enough water to cover the beans by at least 1-inch. Error on the side of adding too much water, because you can always just strain it off after the beans are cooked. If the Instant Pot doesn't have enough water in it, the beans will burn to the bottom.