I just took my p302-6 apart after the machine blew off a bit of steam (pop-off valve testing complete, along with a water-tight test of my own at 3 am...) and discovered some major pitting. I disassembled the switch, took out the contacts and filed them (almost) smooth, and then burnished them with a bit of 600x sandpaper. After re-assembly, the p-stat works well. I'm still getting a bit of a blue sparkle at opening and closure of the switch, and I'm wondering how much life I have in this thing.
Anyone done that before?
Is it possible to get switch parts, or am I going to have to drop a hundred bucks on this?

Arcing is a fact of life, though the proper sized capacitor across the points will help minimize the arcing. Or just put in an SSR and the points will last forever.

Ira

ira wrote: the proper sized capacitor across the points will help minimize the arcing.

It's 220V with a draw of 4KW on the heating element. Both conductors are interrupted. Which points get the cap across them, and how does one calculate the value of 'proper size'?

Sadly, I don't know. I do know the capacitors need to be AC rated and will likely be a few dollars. And I just realized I night be wrong. It would work switching an inductive load like the points in a distributor which I why I thought of it but I'm not sure what happens with an resistive load. Here are two pdfs that discuss this.

http://www.illinoiscapacitor.com/pdf/Pa...ession.pdf

http://relays.te.com/appnotes/app_pdfs/13c3236.pdf

Ira

I do not rebuild Sirai p-stats, and generally only "manicure" the contacts to get a customer through a couple of weeks if I'm out of stock.

Some arcing is totally normal for these. That said, the effects of years of arcing and pitting are the major cause of pressurestat failure. In fact, I tell my customers that every Sirai pressurestat will fail at some point - either welding closed or carboning open. This is why annual replacement is sometimes recommended by manufacturers for their high-volume commercial clients. I don't do annual replacement, but will sometimes suggest preemptive replacement if the contacts look super gnarly. I always replace when doing a refurb.

Though the capacitor thing caught my attention, I'm a little doubtful that it would be an effective fix. I've seen several alternative pressurestat configurations, but never a capacitor. If a capacitor was effective in preventing one of the biggest common reliability problem in commercial machines, I'd think that one of those clever Italian engineers would have done it?

The main reason that I replace p-stats instead of dressing the contacts is that, over time, the pliable diaphragm at its heard will become stiff and brittle. This increases switch hysteresis which introduces wider swings in boiler pressure. It also will eventually rupture if left in long enough.

What does the large cap on your p-stat look like? Older ones (10ish years) have blue-green painted metal caps, somewhat newer ones are black plastic with a yellow "Sirai" sticker, and the current one has the name "Sirai" molded into the plastic cap. If you're dealing with the first style, I'd think replacement would be in order. If the newer styles you may find that the manicure is appropriate for a while longer. Just keep an eye on the contacts and watch for signs of sticking.

Hope that helps.

Chris - some light reading for you - http://relays.te.com/appnotes/app_pdfs/13c3236.pdf .

As a point of interest, the Bezzera Strega uses an RC circuit in their heating element mechanical relay (16 amp rating) which is controlled by the pstat. Their part number is 7003665R in the event 1st-line or another Bezzera dealer has one in their parts bin that someone can give you data from.

BradyButler wrote: I tell my customers that every Sirai pressurestat will fail at some point - either welding closed or carboning open. This is why annual replacement is sometimes recommended by manufacturers for their high-volume commercial clients.

The main reason that I replace p-stats instead of dressing the contacts is that, over time, the pliable diaphragm at its heard will become stiff and brittle.

I did inspect the diaphragm closely, ans do far is show no signs of deterioration. OF the three sets of contacts, one is pristine (and unused, go figure) an the other two are now manicured, and maintaining the .2 Bar swing. I was thinking that the third unused set of connections would be very easy to move and if I could find another that had only burned up two, I'd be in clover. Any of your customers willing to toss a used P-stat with one good set of contacts my way? Even if the diaphragm did start to show some wear, the $45 to replace that is still less than half of a new p-stat. The contacts are an obvious failure point, and are easily removed and replaced. As is the diaphragm. I can't help but wonder why there is a diaphragm replacement kit, but the contacts are unavailable...

Another way to make the pressurestat last is to use the contacts to switch an external contactor or SSR. That way the pressurestat contacts are only switching a small amount of current and should last essentially forever.

Ira