It says to jumper the bypass of the pressure switch. What exactly does this mean, don’t quite get it. Thanks!
I presume this means to short the leads together on the pressure cutoff switch so it appears as though the pressure is always lower than the cutoff (~120psi).
This would cause the compressor to potentially run indefinitely. It is circumventing a safety mechanism. This is generally poor practice.
What set of instructions are you following that has this as a step?
The one on FIRST websbite. https://firstfrc.blob.core.windows.net/frc2017/pneumatics-manual.pdf
I am assuming you are trying to set the adjustable-setpoint relief valve, on your FRC robot, to start opening at 125 psi.
However, don’t do this. This is no longer considered best practice. Buy the $5 McMaster-Carr relief valve 48435K714 instead. The setpoint is factory set.
Found the step on page 15. Yes, my previous answer is correct - hold a wire across the terminals of the pressure switch.
If you do decide to do this with your current setup, rather than purchasing the no-cal-required valve:
Follow the instructions at step 3 carefully - do not overcharge the system. Don’t leave the jumper attached while running the bot normally.
I believe their intent is to get the system pressure just barely high enough to trip the relief valve into expelling some air (The jumper wire is required as the pressure switch should ordinarily prevent the system from getting to this point).
I generally use a screwdriver or other solid piece of metal to short the pressure switch, instead of a wire. The point is that if someone needs to actively hold the “jumper” against the terminals, there’s no chance of accidentally leaving it on the robot and permanently bypassing the safety switch. Really anything that conducts electricity will work as a jumper. I don’t remember if it’s still an inspection item, but I know for a while inspectors were required to bypass the safety switch to check the automatic release value, and many inspectors used this trick to make sure they remembered to remove the bypass when they were done.
Yes, it is still a required inspection item. When over-pressurizing the system to calibrate the relief valve, don’t go too crazy! I wouldn’t go over 130.
I prefer the newer McMaster valves because they don’t require this procedure. I’ll never go back to the other ones for my robots, it’s worth buying one from McMaster that’s already set for you. Plus, it speeds up inspection as the inspector doesn’t need to test the setting on the valve, just read the part number.
Yes you set the relief valve by jumpering or shorting the pressure switch. You need to monitor the pressure gauge while during this, but it is not particularly dangerous. I don’t trust the factory setting of any cheap relief valve. Of course you are using cheap uncalibrated pressure gauges to check your relief valves, that is a different can of worms.
A there are two critical reasons for performing this calibration.
- The relief valve does not come preset from the factory (except the preset valve that Jon talks about in his post above).
- Teams will often use the top hex shape to tighten the valve into a “T” or other threaded adapter. That will cause the adjustment to be tightened to the point where no relief is possible. In a system fault, this could raise the pressure in excess of 150 psi. The lower hex is the point that should be used to mount the valve to threaded parts.
I ran into a team at inspection this year that had (probably) done Al’s Item 2, AND had mounted their regulator with the high pressure going into an outlet.
That inspection… took a while. But they learned about common pneumatics issues (among other things…)