Pressure cut off switch

[William.c.Jones]

We're rookies and we wired our pressure cut off switch in series with the supply voltage after the spike relay. Now we've read that this is not correct and have to rewire the switch to the digital side car. what happens if your code is wrong and it does not read the switch won't the compressor say on.

[Teched3]

Originally Posted by William.c.Jones

We're rookies and we wired our pressure cut off switch in series with the supply voltage after the spike relay. Now we've read that this is not correct and have to rewire the switch to the digital side car. what happens if your code is wrong and it does not read the switch won't the compressor say on.

I believe the Spikes can be used in both normally open or closed state, and can be controlled through software. The compressor switch will turn off the compressor and is installed between the Spike input and the Digital sidecar. If your programming is wrong, the pressure switch is a control device to ensure it will turn it off when the system reaches 125 lbs. Sorry I can't help you with the code, but I'm sure someone else on this forum can, or I can get the info from our student programmer.

See R72, which verifies that the pressure switch must be wired to the digital sidecar input port and ground.

[William.c.Jones]

So if I read your post correctly then we have it right and the pressure switch goes between the Spike relay and the Compressor. does that mean the manual is wrong where it says " Do not put the pressure
switch in series with the power supply to the compressor.""It must be wired directly to a digital input and ground port on the Digital Sidecar."
The way we have it seems to work although the lab view code is set up to read the pressure switch from the side car. I'm just worried about inspection I don't want to rewire this later.

[keericks]

The code is correct as is the manual. The pressure switch must be wired to Digital input and ground on the side car. Then you wire a relay port to your spike. The software reads the value of the pressure switch and commands the spike relay to turn on/off based on the pressure switch reading.

[William.c.Jones]

Thanks I guess I will change my wiring to wire the pressure switch to the side car. All though this will create an unsafe condition. with the switch in series with the supply it will always shut off when the switch opens. While software may be correct if the switch wire breaks or the code miss reads the switch the compressor will stay of until it blows a gasket.

I'm a hardware guy I don't have a lot of faith is software.

[William.c.Jones]

After some more thought I guess this won't be to bad. since the pressure switch is normally closed if a wire breaks the software will read that as pressure being reached and shut off the relay. That just leaves an error in the software that may cause the compressor to remain on. That's not likely.

[Racer26]

You are correct, it DOES create the case where an error in the code causes the compressor to run continuously. This is however protected by the pressure relief valve (set between 125-130psi). If the compressor over-fills the system because the code didn't shut it off, the relief valve will open and dump the excess to atmosphere.

This rule has been in place since at least 2003.

I agree, the simpler, software-proof solution of series wiring the pressure switch with the relay would make more sense, and be less prone to problems like that, and it will work.

I believe the reasoning for the rule being the way it is, is due to the pressure switch not being rated for the ~25A startup current on the compressor. This means that while it does work, it will dramatically shorten the life of the switch, and one day, the switch will fail, and potentially fail closed.

[EricH]

If your software guys use the stock code, only changing exactly where the compressor and switch are reading, and it fails and nothing else does, then I will be greatly surprised. And I do mean greatly surprised, as there is code that says, "Turn the compressor on" and "Turn the compressor off" and does nothing else.

I can think of a number of hardware failures that would leave the compressor running all match:
--tubing comes out
--major leak in tubing or part
--somebody forgets to close the drain valve
--the relief valve is improperly calibrated, resulting in a leak there
--the regulator is on wrong, causing a vent at 60 psi.

[William.c.Jones]

Wow Thanks, I'm convinced I'll have this wired up correctly in the morning. I didn't consider the current rating on the switch and all the other things that could keep the compressor running and of course the relief valve.