I am in the process of rebuilding a Rancilio S27 single group HX machine. I have most of the project complete or on order waiting shipment, but while I am waiting for parts I thought it would be a good idea to give the Sirai p302.6 pressurestat a through going over.

I have pulled the main brass diaphragm housing and cleaned it, and verified that the PTFE diaphragm itself is in good condition and pliable. I also descaled the main feeder tubing from the boiler as well. But where I come to a problem is dealing with the contact points.

The P302.6 is a three pole pressure switch, of these three poles both outside pairs seem to be either corroded or burnt. Is there a recommended method for cleaning or restoring these contact points? I am hoping that total replacement is not necessary.

This is how the contacts look now, sorry for the poor lighting I do not have a macro light ring for that camera.



Any suggestions on how to deal with this problem are welcome.

-Cecil

I've previously suggested sanding contacts, but Ian (cafeIKE) informed me that this shortens the life of the contacts.

I'd suggest finding yourself an electrical contact cleaner, some type of solvent that removes oxidation/carbon buildup safely. I have never used them, but I think DeoxIT is the first thing I would try, based on my bit of research. Hopefully their product is better-thought-out than their name.

All the data sheet mentions are that they are silver contacts. I am guessing this would limit or at least inform what type of cleaner would be the best for them. But I am not in posession of that information so I am bowing to the alter of communial wisdom for help.

-Cecil

Can you check it with a multimeter and see if it's making good contact? If it is, I would wipe the contacts off a some alcohol and a Q-Tip and then run it until it dies. It might last 10 more years like that!

CRCasey wrote:The P302.6 is a three pole pressure switch, of these three poles both outside pairs seem to be either corroded or burnt.

One other thing I wanted to bring up: I do not think that I would call those poles. What you photographed are 3 independent pairs of switch contacts, all actuated by the diaphragm in the pressurestat. Just below the 3 contacts you should see six poles, arranged in a 3 x 2 matrix. When the switches are closed, there is continuity between the top and bottom poles of each column. I believe the machine is set up this way so that it can control 3-phase heating elements or simply multiple heating elements, but there are a lot of other potential applications of such an independent switching.

I am not trying to nit pick: in fact, I am fairly certain that I've used somewhat imprecise language in my above paragraph. But I did want to say that the usual reason why the middle switch is not corroded is because it's not in use. I can tell you from my own experience that it's quite safe to move the pairs of wires from the left or right switch into the center in order to "retire" one of the corroded switches.

Ahh, I see, this was set up for switching not only 110v but also for 220v where you would need to have the extra pole on the switch, but I am only switching two of them so the open pole is fair game. That makes sense.

-Cecil

To bring this to a bit different level...

On the pressure stat on my well, when the contacts would get degraded we would open the contacts manually, place an emery board between the contacts, then release the contacts back to closed with the board inserted. We would then rub up and down several times to unpit/clean the contacts, and then put it back into service.

But the whole sliver contact thing put me off from that.

Chemical cleaning first with something like deoxit and if that does not work, then head on to physical.

Is there a suggested post cleaning with No-ox or some other protectant?

-Cecil

How about you move the wiring from the most pitted actuator to the middle which seems to be in very good shape. It would at least be better. At least the contact isn't vaporized like it was in my used Elektra...

Mark

I've never been fond of high current switching through relay contacts. They eventually end up burned and pitted.

I'm really itching to take on a restoration project myself (hats off to all you who have done so!) and would seriously consider using a PStat controlled SSR to handle the high current switching.

George