Nason pressure switch shutting compressor off at 130psi

Our Nason pressure switch, connected to our Rev pneumatics hub, is shutting the compressor off at 130 psi instead of 120psi. We have updated the Rev PH, and tried two different pressure switches. Need help

How sure are you of the gauge(s) being correct? The pressure switches can also be a bit high or low, more often low in my experience.

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I’ll try swapping gauges

three different gauges, one brand new, are 127/128 psi

What’s the pressure switch part number? SM-2B-115R/443 is non-adjustable 115 psi and would be well out of spec if it’s not switching off until 130 psi (the tolerance is specified at +/-1 PSI or 5%, even if we use 5% (which we shouldn’t) that would only be 121 psi). It may be worth measuring the resistance of the switch in a controlled test as you raise the pressure to isolate whether the issue is with the PH or the switch.

Do you also have an analog pressure transducer hooked up to the PH, or only the pressure switch?

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SM-2B-115R/443. Do not have an analog pressure transducer

Can you verify that the pressure sensor / gauge that you are using to read the pressure of 130 psi is reading gauge pressure rather than absolute pressure? Does it read 0 or 15 psi when the system is de-pressurized?

If it is an absolute pressure gauge, then 130 psi makes sense (115 gauge pressure plus 14.7 atmospheric pressure).

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all three gauges, two of which we have used in the past, read 0 de-pressurized. Swapped out the PH, no difference. Confusing??

Crazy question, but how many air tanks do you have? I once ran into a team with 0 air tanks and the pressure switch didnt turn off fast enough and it always overshot the 120 psi.

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Hook up your multimeter - DC volts, across the pressure switch. Make sure you have good connections. The pressure switch will close when it senses full pressure, and the voltage will go to zero. See if there’s a latency between the switch closing and the PCM/roboRIO turning off the compressor.

If necessary, get rid of the PCM’s control of the compressor - see R812/A. Wire the Pressure Switch to a DIO port of the roboRIO with a PWM wire: Cut off the red wire and insulate its end. Connect black and white across the Pressure Switch. Which wire to which terminal doesn’t mater because the Pressure Switch doesn’t have polarity, it’s a simple mechanical on/off switch like a limit switch.

Look deep within the bowels of your shop for a “Spike H-Bridge relay.” If your team has been around long enough you’ll have a box of them gathering dust:
Look here for the Spike.
https://content.vexrobotics.com/docs/spike-blue-guide-sep05.pdf

Back in Ancient FRC History that’s how we controlled the compressor. Connect the Spike’s power input to the PDP/PDH, and its control input, with another PWM wire, to the roboRIO’s Relay port (which you probably never noticed :slight_smile: ) If you’re using the big compressor, consider replacing the Spike’s 20A fuse for a 30A one because the big compressor used to blow the 20s. Before you connect the compressor, use your multimeter to be sure you have the correct polarity (red is +) out of the spike to the compressor.

You don’t need to instantiate a Compressor object, just a DigitalInput(input #) to read the pressure sensor, and a Relay(port) to turn the relay/compressor on and off. It may help to find a mentor in the local Old Mentors’ Home.

At competition, bring a hard copy of R812, and the Spike page of the FRC Hardware Component Overview, because your inspector will probably have never seen a Spike.

Note you may have another problem, the Pressure Relief Valve should be opening at 125 PSI, see R811.

Please let us know what you find!

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Easiest test you can have - take the wires off the pressure switch. Enable the robot and then short the wires - the compressor should turn on. Pull the wires apart, the compressor should turn off immediately. If there’s any significant delay there, then the problem is in the pressure hub. If there’s no delay, then the hub is just fine and there’s something wrong with the switch. Easy enough to replace a switch!

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A little surprising that two switches would be set high unless somebody dinked with them. Maybe from the same maladjusted batch. They are adjustable, but that makes them illegal for competition. I would make sure the tubing going to it is not kinked. Being sloppy with Teflon tape and having a bit covering the fitting can do weird things.

No, they aren’t. It’s a common misconception that the little set screw on the side of the switch adjusts the pressure set point, but it doesn’t. They’re set at the factory and are non-adjustable.

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Thanks all for suggestions!. Thanks Jon for the quick test. Compressor shut off immediately, no delay. Multiple switches now on order

The technical support at Nason says different. :slight_smile: The set screw locks the barrel that you screw in and out to adjust the set pressure. I don’t recall ever setting one past 125 though. That was for an non FRC project. That switch has been working fine for 10 years + I have never adjusted one for a competition robot. Per rule once you have disturbed the adjustment, it is no longer legal for competition. But you know that. :slight_smile:

The data sheet specifies the part number schema, in which the “115” in the part number (SM-2B-115R/443) refers to a “Fixed Set Point”. Seems odd if they call a user-adjustable switch “fixed”.

If your support contact says otherwise, I’d love to see where they have documentation on the user-changeability of the set point - I haven’t been able to find any.

Also, if you look at their broader range of switches, you’ll find that they specifically note that ones that are adjustable.

It was a number of years ago. I talked directly to the factory support people. At the factory. I did not ask for official documentation. :slight_smile: I work in controls. I am used to calibrating things to NIST traceable standards. So I am used to thinking in these terms. These are inexpensive pressure switches. You order them in a range of set points. They have to be adjustable so the factory can calibrate them. Max for the SMs is 120PSI. So the OPs switches are either screwed down past the adjustment range or they are broken. Since it seems to be repeatable I lean towards the maladjusted. It doesn’t really matter, they shouldn’t use it on the robot.

Apologies for drifting so for off topic. It is a mute point.

Frank,
The adjustment is to insure that the microswitch internal to the pressure switch trips when there is 120 psi on the bellows inside. Moving the micro switch does not set the trip point. If you move it it will either work or not work. Inspectors will sometimes look at the switch to be sure that a device on the “do not modify” list has not been modified.

Any update?

At the Tallahassee regional we got called out on a non-approved pressure switch, so we got one from spare parts. Set above the relief valve, got another, same, got another, same. Got another from another team, same. Got another from another team, same… Pretty much anyone we got were ALL out of spec. When we got home we tested ours, also all out of spec. To be blunt, these are junk and are not set from the factory for what FIRST requires. Also if you get any moisture in them (on the pressure side), they’ll stop functioning. There are far better options than these Nason but until FIRST goes back to “or functionally equivalent.” go with the REV PCH and their analog transducer for finer control.

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