Here's an update - received the basic gauge this morning after ordering yesterday afternoon, great service, went for this one at under £4:
40mm Pressure Gauge Base Entry 0-15 PSI AIR AND OIL on eBay (end time 20-Jul-11 16:25:01 BST)
And put the gauge together this morning, works an absolute treat, dead chuffed!
Here is the completed gauge next to the Zodiac floor/keel valve it fits:
I turned down a piece of PVC pipe to a taper so it fits the valve precisely, a couple of pieces of different diameter hose join the parts. A rounded 'prodder' (for want of a better word) is 'chemically metalled' into the tube at just the right position and depth to open the valve:
And in position:
Here checking the floor pressure:
And for reference this is the original Zodiac guage:
Obviously what I've made is to fit the Zodiac valves but with the right adaptor for your own valves it will work on any make SIB for a fraction of the cost of a 'marine' one. Makes a real difference having a proper gauge reading in 0.5psi increments rather than a 'guesstimate' pictorial gauge and of course being able to check the exact pressure at any time without getting the pump out especially in those 'leaving the boat in the sun' moments. All I need to do now is make a second tapered adaptor with central 'prodder' to fit the main tube valves.
A couple of observations - the Zodiac gauge reads under at the (LP) tube end and over at the (HP) floor end. Also, *importantly* when inflating the (small volume) keel to the recommended 3.4 psi - the difference between 2.4 psi and 3.4 psi is *less* than *one* complete up or down stroke of a typical stirrup pump!!
Similarily when you approach the correct 12 or so psi for the floor you have to take it easy. Again, all this shows how important it is to have a proper gauge and why perhaps so many people complain of flexy SIB air floors.