Well, you can solve half the problem with just one of the simple gauges that fits onto your pump. We sell a couple of these (one from Slingshot, one from PKS) and they're both only $20. I used to only recommend them to beginners, but I started noticing that when I was landing/launching kites for proficient riders that their kites were under-inflated. Now, I recommend them to everyone. It just takes the guesswork out. And, if you KNOW that you pumped it to 8 psi and check it later b/c it feels *squishy* and discover it's less, you may have identified a leak before you find out the hard way.
I always tell my students, "Remember, limp is NEVER good!"
I do admit, it would be cool if they came up with a monitoring cap-gauge, but I have also heard those tend to leak as well.
Aaaah, seriously ?
C'mon, I assume you are talking about the typical pressure gauges ?
Everybody knows, they are worth NOTHING in terms of absolute values
When you know exactly how much YOUR kite has to be pumped, according to your gauge, well, fine then.
But for an absolute measurement, they are IMO totally useless whatsoever
(I am working every day with measurement equipment and calibration)