I claim no inside knowledge of the logic board, but as an electronics guy I can tell you that if you wanted to measure capacitance, you would need to excite it with an ac signal, such as the square wave observed. A simple resistive measurement would have no need for an ac excitation.

You COULD get the job done with a resistive measurement, however I suspect capacitance is preferred since water is not particularly conductive, especially the relatively soft water preferred for espresso use.

Either method would be sensitive to deposits on the pcb between the probe terminals, so a good cleaning in that vicinity as recommended is a good idea.

Hello again,
My comments are based on the control boards that I have physically had in my hand and have done the circuit analysis to determine just how the auto-fill circuit works. And for the La Marzocco machines I deal with primarily the 3D5, 4D5, and the older "mask" control board manufactured by Gicar. The simpler non logic autofill boxes for the semi-autos work on resistance where the voltage drop due to the change in resistance causes a transistor to be biased to turn on or off. I don't have a GS board in front of me to look at or I would be more sure of my comments. But I don't expect that Gicar strayed to far from what they had been using in other applications. So far I haven't seen any level controllers that use capacitance, but rather a voltage drop due to grounding via the water. And I guess I am getting old enough and cranky enough that I believe "I saw" trumps "I heard."
Cordially as always,
jpboyt

The board from an early GS/3

More hello,
Seeing as how I have totally hijacked this thread, My apologies, (Original poster, you are still free to contact me if you like) and now that Jacob has posted a photo of the 3D5 "Deluxe" board, it appears that someone with more spare time than I could/should trace the wiring from the probe wire to the micro-controller. Once all the components in the circuit are identified then we should be able to "follow the electrons" to see how this circuit works. One would also need to evaluate the form, voltage, and current of the signal wire going to the probe. Maybe someone wants to hook up an o-scope to their machine?
Beyond that I guess we are all speculating or depending on second hand information. GS/3 uses the same probe as a Linea (LA052) so my educated guess is that Gicar is using the same type of circuitry. But until I have a GS/3 control board in hand to play with I will retreat from my adamant position of the GS/3 using the technique of pulling the logic signal low to signal a full boiler.
Cordially,
jpboyt

Marshall wrote:For all the above reasons (and probably more), I think it is best to leave the GS/3 on 24/7.

From my 1 year experience with the GS/3 I concur!

NO reason to leave the GS3 on 24-7...it's a significant waste of energy, has a built in timer & after 3 years of full time usage turning on for 1 hour in the morning...no adverse affects!

I removed the side panels yesterday to do my monthly maintenance of draining the steam boiler & due to the concerns about the triangular plate attachment nuts "not maintaining sealing torque..." I put a socket to them to inspect & found no looseness whatsoever. My unit was produced Feb/2011 and LM has most likely addressed this also.

I have had no leaks or failures from the Vacuum valve or boilers...and other than the MP group o-rings seeping (once) they are still sealing after 2 years!

You are lucky!

Marshall wrote:For all the above reasons (and probably more), I think it is best to leave the GS/3 on 24/7.

Hard to say if this is true. There are a lot of factors to consider, including but not limited to thermal expansion/contraction, rated operating hours for the (many) components in the machine (the SSRs would be a concern), mineral deposits in the vacuum breaker, energy consumption, potential danger of leaving heating equipment unattended, etc.

I'm skeptical that cycling the machine on and off on a daily basis increases the likelihood that the steam boiler seals will fail. Many GS/3 owners cycle the machine, and we don't hear about steam boiler seals failing. As far as I know, only three owners (including me) have reported such a failure, and I know for a fact that two of us had the infamous "fire sale" machines that had numerous problems. Bear in mind that my steam boiler seals were leaking from day one, though I didn't realize it was happening until much later. It wasn't a failure that occurred over time, though it did get worse. My best guess is that the seals on a few of these star-crossed machines were defective, or they were not the correct seals, or the end plate nuts were not properly tightened at the factory, or the nuts came loose in shipment.

I think if an owner is worried about the nuts on the end plates coming loose over time, the best course is to make sure they're tight now (don't over-tighten!), and add lock nuts to prevent them from moving in the future.

The vacuum breaker is another story. I used to leave my machine on 24x7 because each time it was turned off, evaporating water inside the vacuum breaker left mineral deposits on the metal side of the valve seat. Cycling the machine once a day eventually caused the valve to not close completely. Also, over time the o-ring on the valve shaft disintegrates and the valve won't seal properly. In older machines, the partially open valve spews steam and water inside the machine, which can accumulate and find its way into the brain box. This common problem has been extensively discussed here, and the original consensus was to run the machine 24x7.

However, beginning with serial number 1170, LM redesigned the vacuum breaker so that it would discharge through a plastic tube into the drain box. This modification is available from dealers and LM USA as a retrofit kit for older machines (if you don't want to drill a hole in the chassis rail above the brain box, you can use a tee connector to share the tube coming from the steam boiler OPV.)

I installed the new vacuum breaker and now power cycle the machine on a daily basis. I don't worry about the vacuum breaker anymore. It does fail to close completely after 6-12 months of operation, but it's easy to remove and clean. I bought a spare vacuum breaker to make that even easier (i.e., I just swap-in a clean valve that's ready to go.) Rather than cleaning the old valve, I bought several maintenance kits, each of which consists of a new valve shaft, Teflon seat and o-ring. The kit is very inexpensive -- just a few dollars plus shipping.

Peppersass wrote:Rather than cleaning the old valve, I bought several maintenance kits, each of which consists of a new valve shaft, Teflon seat and o-ring. The kit is very inexpensive -- just a few dollars plus shipping.

For what it's worth, That conical teflon piece can almost always be cleaned as good as new with a quick scrub using a non-abrasive scrubber and a quick wipe of a cloth. The pin usually can be similarly cleaned, or potentially dropped in a small citric acid bath to fully clean it. The primary disposable part is the o-ring. Eric Svendson kindly posted the correct o-ring for replacement some time ago. You can find that here (in silicone, McMaster-Carr P/N 9396K12). A lifetime supply of these is in the $10 range-A comparatively great value to be sure.

If you're not bothered by the trouble of performing this 'modestly challenging' maintenance, it is more economical to replace that o-ring and the group gasket a few times a year than it is to run 24/7. Assuming that La Marzocco really has resolved the risk of the catastrophic leak from the steam boiler element/heat exchanger o-rings, that ought to cover the main risks of shutting the machine down on a daily basis.

I have had my GS/3 for six and a half years and it must have been turned on and off something like 1.25 times a day in average.
I'm on my third vacuum breaker (both failures were potentially dangerous, had I not been present).
My second steam boiler OPV.
My second steam boiler drain valve.
(my third group gasket and screen)
And never had any leaks!