Is this for the membrane or tank?

I have 3 of these on my RO system. 1 to measure pressure before the membrane, 1 after the membrane and 1 on the tank. They can be used where ever you need to know water pressure. These will have an air bubble visible through the glass, it's normal and relieves pressure in the gauge, so don't be concerned with the air bubble.

Where should this be installed? Input to ro system or output from tank?

TJK... There are varying thoughts on this subject. My research has led me to install the gauge directly after the third stage of my 7 stage "Aquasafe aquarium II". I think as long as you install the gauge after your pre-filters will give you the true reading of the pressure in your unit. The bottom line is how much pressure is going into your RO canister. The RO membrane is the only part of the unit that works more efficiently with pressures above 40 psi. (50-60 psi). I hope this helps. NOTE most manufacturers have tutorials on their web site.

What does this do? Does it tell you if you have a busted bladder in the tank, hence no pressure?

no its to monitor the water presure coming in out of the filters this way you can tell if filters need changing.. as far as the bladder in the tank being busted you really don't need a guage for that if that occurs you would be able to tell because there would be water coming out of the valve that you use to put air in it

ASIN	B00VUXD7ZG
Best Sellers Rank	#17,829 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #5 in Plumbing Testing Equipment
Brand	DIGITEN
Customer Reviews	4.4 4.4 out of 5 stars (808)
Date First Available	10 December 2015
Item Weight	191 g
Manufacturer	DIGITEN
Manufacturer Part Number	ROCON-24
Package Dimensions	8.79 x 7.39 x 3.4 cm; 191 g

DIGITEN Water Pressure Gauge Meter 0-1.6MPa 0-220psi 1/4" f Reverse Osmosis System Pump¡­

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DIGITEN Water Pressure Gauge Meter 0-1.6MPa 0-220psi 1/4" f Reverse Osmosis System Pump¡