According to information provided by the University of Warwick (UK) ( http://www2.warwick.ac.uk/fac/sci/eng/research/dtu/pubs/tr/lift/rptr12/tr12.pdf , page 14), the pressure chamber should have a minimum volume of 20 times the expected delivery flow per "cycle" of the pump, with 50 times the expected delivery being a better selection. The chart below provides some recommended minimum pressure chamber sizes based on 50 times the expected delivery flow per "cycle." Note that larger pressure chambers will have not have any negative impact on the pump performance (other than perhaps requiring a little more time to initially start the pump). Some of the lengths indicated are quite excessive, so you may prefer to use two or three pipes connected together in parallel to provide the required pressure chamber volume. Well pump pressure tanks will also work well - just make sure they have at least the minimum volume required.