How many PSI of helium does it take to lift 150 pounds?

Answer 1

It's not the pressure that provides buoyance, it's the volume. Each 22.4 liters of helium in a balloon (which means at low pressure) has a mass of 4 g and displaces 29 g of air, for a net lift of 25 g. So you can figure out what volume is needed for 150 lb of lift:
V = 22.4 L / 25 g * 150 lb * 453.6 g/lb = 60964 L = 2153 ft^3.
P.S. I know it's a sign of ignorance and bad taste to express a force in grams, but doing so avoids an unnecessary conversion to Newtons. Sorry to anyone I have offended.