Last time I looked a pound was exactly 444822.16152605 dynes (i.e. 453.59237 times 980.665). And a square inch is exactly 6.4516 square centimeters, so one psi is (the former divided by the latter) about 68947. 57293 16836 13367 dynes/sq cm. And a millibar is exactly 1000 dynes per sq cm, isn't it?