Mercury pressurestat adjustments - Page 2

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ira
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#11: Post by ira »

drgary wrote: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?
I'd guess it's never been done as until the advent of PIDs it doesn't seem like anyone cared about really minimizing the deadband. The tighter the deadband, the more the element turns on and off and the shorter the life of the device. In a device like this one the more tightly the lever/mercury tube is coupled to the bellows the smaller the deadband until you hit the limit of the mercury tube itself. If you replace the spring with a tiny bit of friction then you can have the bellows both push and pull and widen the deadband. If you want to make it narrower, you'll probably need to make the pivot adjustable so any movement of the bellows causes more or less travel of the lever.

It would not be hard to rebuild that to make the deadband adjustable, but probably better to leave it as a safety and hide a PID somewhere. Works like new and looks original, best of both worlds.

ira
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#12: Post by ira »

happytamper wrote: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.
It dawned on me during this discussion that you should make sure everything moves smoothly and the pivots are lubricated. Excessive friction might also cause the problem you're seeing and a through cleaning might also improve it's operation.

Ira

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drgary
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#13: Post by drgary »

ira wrote:It would not be hard to rebuild that to make the deadband adjustable, but probably better to leave it as a safety and hide a PID somewhere. Works like new and looks original, best of both worlds.
This is what I've done with my Conti Prestina. The PSTAT is now a backup safety device.
Gary
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D'Laine
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#14: Post by D'Laine »

These bellows operated mercury switches are very old technology which tend to drift off calibration yearly as the metal, springs stretch and the pressure charge in the bellows leaks out. They were low-tech and cheap to make in the old days, and pretty well can be left in the old days, museums and such.
Yes, if you think you like it. Polish it up and have it on display. There's a multitude of electronic controls which can do a better job for less money. Pid being at the top end of those controls.

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drgary
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#15: Post by drgary »

And PID is cheap and straightforward. Here's how I did it. You can hide it better for your machine.
Gary
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happytamper (original poster)
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#16: Post by happytamper (original poster) »

I agree with you D., very old technology and device. But there is a romance to using these old mechanical pieces and keeping them in operation when possible. There is also the cost of replacing.

I adjusted the dials some more and the top dial definitely controls the high end of the pressure. Turning the lower dial but did not give any definite results which tells me it is either not working or dirty or just old and worn out.

I will be changing the group springs in the next little while and at this time I will do a thorough cleaning and lubrication of the Mercury pressurestat as suggested by Ira and report back to the forum.

I am considering Garys and d'laines suggestion of the pid which I also consider the best way to go. However that may involve opening the boiler and on an old machine if I open the boiler I may be inviting all kinds of problems. I remember when I last opened the boiler I had to replace the gasket, which I could not source and had to cut out of a material I bought from a boiler company.
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#17: Post by drgary »

I was able to PID my Conti lever with a thin wire thermocouple tucked under the group to boiler gasket and attached with a couple of o-rings to the dipper tube. So yes, I had to remove the group to do that but the machine is unmodified by the PID.
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ira
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#18: Post by ira »

And probably a thermocouple tucked between some insulation and the boiler would do just fine if you use enough heat sink goo and a big ty-wrap under the insulation to get a good thermal connection. Not perfect, but probably way better than the pressurestat.

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#19: Post by drgary »

ira wrote:And probably a thermocouple tucked between some insulation and the boiler would do just fine if you use enough heat sink goo and a big ty-wrap under the insulation to get a good thermal connection. Not perfect, but probably way better than the pressurestat.
This was the other option I was considering and it should work well too. PIDs can be programmed with an offset temperature so once you correlate temperature within the coffee cake to what the PID is reading you can have it reflect your brew temperature pretty closely.
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