On Wed, 19 Dec 2007 21:33:59 -0800 (PST),
wrote:

Ok guys so I need some help. I have an aircompresser that is running
my waste oil burner. The burner needs very little air pressure. I
want to lower the start up pressure so that the compresser runs
longer when it is on and stays off longer. Does anyone know who makes
a pressure switch that is adjustable. Like maybe 35psi lower to 135
upper?

Thanks,
Zach

Two standard pressure switches, a little bit of relay logic - The
low pressure switch pulls up a "Run" relay that locks even after the
low-pressure switch has opened, and the compressor runs until the
stock high-pressure switch is satisfied and turns it off.

You will need a small DPDT relay for the latching start, and a plain
power contactor to start the motor. Or a motor contactor with an
auxiliary SPDT contacts.

A full-on motor starter is better - first it has the aux. latching
contacts built in for using "Start" and "Stop" buttons, so you could
wire it all by itself, no external logic or second relay. (Follow the
directions on the inside of the motor starter cover.) And you have
overload heaters in case of a locked rotor no-start - make sure you
know the motor full-load amps to select the right overload heaters.

And do NOT use the bleeder style unloader valve built into the stock
high-pressure switch with this setup - if you lose power and try to
start the motor on a high head pressure load, it might not start. Or
the pressure could build up on the compressor output side if you have
a leaky tank check valve - the high pressure switch's built-in bleeder
valve closed at 120 PSI, and has been sitting there closed till the 35
PSI start switch finally kicked in...

Get an 1/8" NO solenoid valve (closes on power), hook the coil
across the motor leads, and make it the unloader valve. Motor stops,
valve unloads.

-- Bruce --