Calibrate Pressure Switch?

[Mark Sheridan]

Originally Posted by Steve W

To quote Big Al "You may not modify pneumatic parts."

Originally Posted by Al Skierkiewicz

Mark showed me the response from Nasson and it appears to be more of a calibration of the switch actuator than actual pressure calibration. If you move to far in one direction the switch will turn off and too far in the other direction and it will never turn on. The effect is the pressure on the actuator caused by the spring in the switch I am guessing. The manufacturer states that this is only 2 psi. If you want your switch to operate reliably, I would not recommend any adjustment in addition to my previous statements.
Thanks to Mark for this in depth research.

Thank you for your response, I am gonna throw away that modified sensor, I have no idea if its set right anymore. Its not worth the risk.

[Tristan Lall]

Originally Posted by Mark Sheridan

Thank you for your response, I am gonna throw away that modified sensor, I have no idea if its set right anymore. Its not worth the risk.

Before you throw it away, see if you can cut it open and post a photograph. That might be useful for future reference.

[Al Skierkiewicz]

Or send it to me please. I love to break things to see how they work.

[Mark Sheridan]

Originally Posted by Tristan Lall

Before you throw it away, see if you can cut it open and post a photograph. That might be useful for future reference.

Sure. I will see if I can post it after my team's regional.

Edit: Or I can send it to AL

[lonnie lascelle]

I contacted Nason today and spoke with their technical support for the SM switch. They did indicate that it comes precalibrated/preset from the factory. They did indicate that if it is out of calibration you could contact them and they would be happy to repace it or recalibrate it by giving you an RMA. The technical support rep said that if the end user wants to "tweak" the pressure, that they will not warranty it. They said to field calibrate it, but only in a small band ~5-10psi, this is what they do at the factory if the proper base was assembled. The factory uses different bases that have different height diaphram pins for different PSI ranges. I asked that if the switch setscrew is loosened and silicone is removed is this a safety issue, and they indicated that the entire plastic top (limit switch) is removable without a safety problem. We did take one appart at our shop and it is a limit swith screwed onto a blind threded hub with a small diaphram pin that extends out of the ~. 0625 hole to push on the limit swith metal lever. They did indicate that if you desire an on the fly fully adjustabe switch that the CJ switch is the way to go http://www.nasonptc.com/pdfs/Switches_Catalog.pdf page 13.
I could post images of a disassembled switch possibly tuesday when we have our next meeting.

Edit- I see I posted this before seeing that Mark
contacted them too, the only discrepancy is the psi vs 5 to 10 PSI.

[Al Skierkiewicz]

For those lurking here, until directed by the Q&A, an inspector finding a pressure switch that has obviously broken the factory seal will be considered to be a modified pneumatic part. Fair enough?

[FrankJ]

Attached is a print out from the Nason switch catalog showing a cut away of the pressure switch. The SM commonly used is the diaphragm type. This is pretty much how most mechanical pressure switch work.

[Steve W]

Originally Posted by Al Skierkiewicz

For those lurking here, until directed by the Q&A, an inspector finding a pressure switch that has obviously broken the factory seal will be considered to be a modified pneumatic part. Fair enough?

Thanks Al.

[rsisk]

Originally Posted by Al Skierkiewicz

For those lurking here, until directed by the Q&A, an inspector finding a pressure switch that has obviously broken the factory seal will be considered to be a modified pneumatic part. Fair enough?

+1 for clarity! Thank you.

[Jibsy]

Originally Posted by Al Skierkiewicz

For those lurking here, until directed by the Q&A, an inspector finding a pressure switch that has obviously broken the factory seal will be considered to be a modified pneumatic part. Fair enough?

I felt like you turned and stared at me while you said that... I thought these bushes I was hiding behind were keeping me well out of sight.

[lonnie lascelle]

Agreed - consider pneumatic components sacred! Thou shall not touch thy pneumatics in a manner that changes their core use or function as intended by its creator.

[Mr V]

If you have an on board compressor you really don't want the compressor to shut off at a higher pressure, in fact you will be better off selecting a switch that turns the compressor on and off at lower pressures, assuming you have selected the proper stored volume.

Take a look at the compressor's specs and you'll see that the higher the pressure it is working against the lower the CFM output.

Some examples

80psi .45cfm
90psi .43cfm
100psi .39cfm
110psi .36cfm
120psi .34cfm

Note those numbers are based on a supply voltage of 13.8 v something that a FRC robot does not have the luxury of, and with a working voltage in the 11-12v range I would expect a steeper drop off of performance.

Personally if I were to select a switch I'd choose one that turns on at ~80 psi and off by ~100 psi to minimize the refill time, the heat build up and general stress on the compressor. If you've sized your storage system properly there should never be a case where the stored pressure drops below 60 psi with a 80/100 switch.

Now if you have an off board compressor there could be an advantage to selecting a switch that shuts off at 120psi but that would be minimal compared to the commonly used 115psi switch.

Either way a switch that has its calibration screw fixed in one way or another by the mfg should not be adjusted, doing so would be considered tampering with or modifying a pneumatic component and make it illegal.

[Mark Sheridan]

Originally Posted by Tristan Lall

Before you throw it away, see if you can cut it open and post a photograph. That might be useful for future reference.

Sorry for the delay. Here is pictures of the broken pressure switch I have. The silcone seal was very easy to break. The following shows the switch and the housing for the diaphragm. You can see that diaphragm is the piece that is loaded at the factory for the pressure setting and its only factory settable. As I try to pack this up for Al, I can't put this back together, the set screw used to hold the two halves together deformed the threads on the plastic piece and I can't get the treads to engage. So its really really only factory settable. I can see by breaking the seal, the top half can easily disengage (I did this all by hand) and thus keeping the sensor closed and unable to detect full pressure.

[Al Skierkiewicz]

Mark et al,
It is pretty obvious now that you have posted the pictures. The switch is using a standard snap action microswitch. These are not precision devices (the switch) and one must position the switch at a point where the lever will move enough to change the switch state. The hysteresis naturally occurs in the snap action mechanism of the switch. As the catalog page shows, the plunger moves when a specific pressure is reached or exceeded. It does not move a distance that is proportional for pressure. Mark, you can verify this action by applying pressure to the now open switch. I bet when it reaches ~120 psi, the plunger will suddenly move out.

[ArtemusMaximus]

Can pressure switch be replaced with this pressure sensor and then calibrate within the code?
http://www.digikey.com/product-searc...ds=734-1003-ND