On 2013-12-11, Cydrome Leader wrote:
Ignoramus31823 wrote:
Think some more, you will realize that what I want to do is the nicest
way of doing it.

probably not. The 90-120 psi stuff is just a distraction from the real
problem. you don't want to fill the large tank when there's a load on your
compressor from tools. Pressure on the system doesn't truly indicate if
you have a load or not.

This is completely untrue.

There is nothing wrong with filling secondary tank, and using tools at
the same time, as long as the system pressure is adequate (above 90
PSI for tools).

I don't deal with air systems, maybe somebody can answer this-

do they make simple flow guages or sensors for compressed air systems?

If so connect both tanks with a check valves in a Y, allowing flow out to
your load, but no flow between tanks. The connection to each tank from the
compressor is with a solenoid valve in the same Y configuration. The only
states are compressor connected to no tank, connected to the small tank,
connected to the large tank or connected to both tanks.

back to the flow detector-

When you detect a load on your system, disconnect the large tank from the
compressor. Only the small tank is connected to the compressor. It will
fill quickly so nobody is standing around.

If there is no, or low tool load, disconnect the small tank from the
compressor, and connect the compressor to the large tank. The air supply
to your load will be from the tank with the highest pressure, which is the
small one. Immediate usage needs are met.

When the pressure there (small tank) drops too much, you start the
compressor and disconnect the large tank.

Add a pressure sensor to the large tank. Once it hits minimum acceptable
pressure (90 in your case) leave both valves from the compressor to both
tanks open. You're now using the capacity of both tanks and the fill of
the large tank never interrupted with your immediate use requirements.

You need a little logic to run this, but it's nothing beyond a couple
relays.

Whatever you describe, is not hard to do with pressure switches and
relays.

i