Mercury pressurestat adjustments

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happytamper
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#1: Post by happytamper »

Hello to all.

Still using my faema urania and enjoying it. I am posting this as a new topic as I could not find any information about the adjustments on an older mercury pressurestat.

Recently the pressurestat has been cutting out a bit high at 1.5 on the machines pressure gauge. It comes back on at around 1.05.

I would like to know if anyone can tell me which dials on the pressurestat control the upper limit and which way to turn in order to lower the upper pressure limit. Here is a picture of the pressurestat and you can see the dials at the top. One dial has a large diameter and the other has a smaller diameter.

Thanks for any help.

Mitchell
LMWDP #77

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drgary
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#2: Post by drgary »

In that tiny photo it looks like a crumpled spider. Can you post a larger picture and point out the adjustment knobs so more of us can see it? If we get lucky enough some of us might eventually score a vintage machine with a mercury switch.
Gary
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Chad C.
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#3: Post by Chad C. »

Looks to me like the small nut on top is a locking nut for the round dial below it. Again, to me it seems like the top nut should be loosened, the dial adjusted, then the top nut tightened.

You might have a look at the bellows behind that ancient terminal block. They might be worn?

happytamper (original poster)
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#4: Post by happytamper (original poster) »

drgary wrote:In that tiny photo it looks like a crumpled spider. Can you post a larger picture and point out the adjustment knobs so more of us can see it? If we get lucky enough some of us might eventually score a vintage machine with a mercury switch.
Hi Gary, here is a better image. Adjustment knobs are on the left, image got rotated 90 deg counter clockwise.

Mitchell
LMWDP #77

happytamper (original poster)
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#5: Post by happytamper (original poster) »

Chad C. wrote:Looks to me like the small nut on top is a locking nut for the round dial below it. Again, to me it seems like the top nut should be loosened, the dial adjusted, then the top nut tightened.

You might have a look at the bellows behind that ancient terminal block. They might be worn?
Chad
Thanks for your suggestion. But why are there two dials, notice the large one and the smaller more to the left.

cheers
Mitchell
LMWDP #77

ira
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#6: Post by ira »

Well, I'd suggest playing with it. Likely the bellows presses on the lever which tilts the mercury capsule. When it gets to enough of an angle the mercury runs to the other side and the element turns off. From the pictures I can't see where the rod presses but if it's between the 2 knobs then the one that presses up on the lever sets the off pressure and possibly the other one sets the on pressure. But, the small spring pulling on the lever makes me think you can only set the off pressure.

Ira

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drgary
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#7: Post by drgary »

ira wrote:...the small spring pulling on the lever makes me think you can only set the off pressure.
And that's the way PSTATs work. When the machine is under the target pressure the circuit is closed. When it reaches target pressure, it opens the circuit. Pressure declines and the circuit closes. So target pressure is all you set.
Gary
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What I WOULD do for a good cup of coffee!

ira
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#8: Post by ira »

Yes, but it's not the way they have to work. There is no reason they can't be made with both adjustable set points and deadbands and given the quality of the pictures and the previous responses I didn't want to exclude that possibility.

I assume the big knob adjusts and the little one locks and the nut on top is to keep it from falling apart, that's how I would have done it, but given the pictures the best I could do was guess.

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drgary
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#9: Post by drgary »

Ira, the deadband and set point features are interesting. Can you describe how that's done with a mercury switch? Are any modern era PSTATs able to be set that way?
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

happytamper (original poster)
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#10: Post by happytamper (original poster) »

It looks like I will be taking Iras advice and begin experimenting with it this morning. I turned the big dial all the way down and let the machine heat up. The pressure went even higher and I shut it off. bled some pressure through the steam hose (made my morning coffee for my wife and I), and began to turn the large knob the other way, as it got closer to the small nut suddenly it became unscrewed. so there is a limit you can move the large dial upward.

I will move the machine way from the corner so I can get a better look at the whole thing and proper pictures can be posted for all to benefit from. It will also remove a bit of the guesswork out of this.

Thanks for all the input.
Mitchell
LMWDP #77

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