Tadhana - which RIB dealer did you work for? I dealt with many of them on the East coats all the way down to Florida where I bought 3 of my RIBS.
"A fully inflated dinghy has a pressure of about 3.2 psi. For every 35 degree F temperature rise, the air pressure rises about 1/4 psi. So if you fully inflate your dinghy at 70 degrees and then pull it up onto a tropical beach where the temperature gets up to 140 degrees, the tube pressure is only 3.75 PSI. A well built hypalon boat won't rupture till 17 PSI."
Perhaps the math works out like that (I do not really know) but for those who have had pressure gages installed on the tubes we see pressure changes from right about 3 psi in the mornings to over 4 psi in the mid day sun. That is a 25% change and it is not advised to go to 4 psi with many "well made hypalon' boats". Some specs are even lower so check your owners manuals please. Now that static pressure needs to also account for people sitting jumping and otherwise straining the tubes and seams as well as the effects of hitting objects both sharp and not so sharp. Many high quality RIBS (Hypalon and PVC) are tested to 5-6 psi static at max.
"But you would need one in every chamber so you are looking at 6 hours of labor to fit the valves"
I agree with this 100% if you are filling by a compressed air tank - I have never done a fill with my dive tanks so I do not know. For typical use you fill each chamber part way (per the manual) and the baffles keep the pressure in the individual tubes separate. But as each tube is completely 'topped' off for pressure each adjacent tube affects the pressure of the next tube. So in normal situation (no compressed tank fill) having the OP valves in lead tubes 2 & 4 they protect pressure overfill in all 5.
I do not have my Nautica or other manuals handy anymore but there is an excerpt from a Novurania manual below...
If anyone wants to get some real great feedback on RIBS and inflatables by folks that are fanatics and very seriously involved post over at Rib.net - great stuff over there.
Below is straight from the Novurania owner's manual:
RECOMMENDED PRESSURE 3 PSI
Inflate all tube compartments to maximum pressure of 3 PSI. If a pressure gauge is not
available, check by depressing the tube with your finger. When the tube cannot be
depressed by finger pressure, the approximate working pressure has been reacted.
CAUTION
The foot pump provided with your Novurania will not allow over-inflation of the tubes.
NEVER use dive tanks,
air compressor etc., to inflate tubes; over inflation can occur
causing damage to the tubes and seams. Under-Inflation can cause excessive stress on
the tubes and hull, always maintain proper inflation pressure while operating your boat.
IMPORTANT
If the boat will be exposed to direct sunlight for a long period of time, slightly deflate the
tubes to compensate for expansion due to heat.