I have two 85 gallon bladder tanks. With the 2nd tank valve shutoff - so I am only operating on one tank - and an outside hose bib running wide open I would time how long it takes for the pressure switch to go from 60psi to 40psi. Typically 125 seconds. When I valved in the 2nd tank and ran same test - it was the same 125 seconds. I expected it to take twice as long.

I'd check volume (using a five gallon bucket) rather than time, even though you would think it would be about the same result. But aside from that, I'll take a stab at this.

When you first use a bladder tank, there is air above and below the bladder, which means the tank's reserve volume is not what it is designed for. But as the weeks go by, the air below the bladder dissolves into the water and eventually is gone. When that happens, you'll see the tank performing as you expected. And as you suspect, adding a second tank should just about double your reserve.

Just a minute here! Are you attempting to convert two bladder tanks into storage tanks? They don't work that way, (see image).
Let us know exactly what you're attempting to accomplish and we can help. What was the original thoughts behind two bladder tanks? Were you running out of water? Let's have more details and informatiuon. Back to you, Tom

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Originally Posted by ;

how long it takes for the pressure switch to go from 60psi to 40psi.

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Sounds like he's just adding a second pressure tank to his well system to increase the interval between pump cutting on/off.

Are the tanks actually 85 gallons or are they "rated" at 85 gallons?

Are both tanks set at exactly the same precharge pressure? I am curioius though, why you would need an extra tank. 125 secs seems more than reasonable with a hose bibb wide open. With two tanks the pump will have to also run twice as long to pressure the tanks back up to 60 psi. Please let us know what you are trying to accomplish.