how do i measure my mains water pressure

[John D v2.0]

excellent idea, really thinking cleverly here.

Did you wear ear defenders? air at 50psi has a lot of energy compared with water at 50psi, so if something burst it wouldn't be pleasant at all!

 

[gco]

Hmm, you are certainly right about the energy (I'm an engineer and I have plenty of experience with compressed air through sub-aqua diving). I didn't wear ear defenders but before I modified the bottle-top I had already done a dry (well, very wet if the truth be told!) run just holding the hose pipe in the bottle, which meant I could feel what was going on and I was reasonably confident that I wouldn't blow the bottle. Based on anecdotes on the web, 1-2 litre PET bottles that are made for carbonated drinks appear to be able to hold well over 100psi, and the smaller ones over 200psi: not surprisingly, the folks who play with water rockets seem to be particularly interested in this sort of information! It's worth noting that the star shape design of the bottom of these bottles is designed to pop out if the bottle becomes over-pressurised, so there is a warning mechanism. If the pressure is completely unknown I'd recommend keeping a good distance (10m) from the bottle, which means finding some way of securing it vertically whilst it is filling; and keeping a hand on the tap so it can be turned off rapidly if the bottom of the bottle pops. Both ear and eye protection would certainly not be out of place.