This thread discusses adding Slayer-like flow profiling and Strada MP/EP-like pressure profiling to a GS/3 AV. It's possible to do these mods to a GS/3 MP, but with the new conical valve it's probably not worth the effort.
Note that in my case a couple of necessary projects preceded the work described here -- outboarding the GS/3 pump/motor and replacing the rotary pump/motor with a variable-speed gear pump for manual pressure profiling. I plan to post descriptions and photos of those projects and will insert the links here when completed.
It doesn't require a military-style pot. There's plenty of precision in the off-the-shelf pot I use. Personally, I don't think an EP approach is necessary. Manual profiling is more than adequate and I find it quite repeatable.AssafL wrote:Given Dick's analysis I would think it is a rather low reward exercise to convert a AV to an MP. It is probably cheaper to buy a used one.
A better exercise is to mimick a strada. Apart from the look is a paddle a strada is a GS/3 with EP control. One can use a massively better quality potentiometer (e.g. A NOS military pot), and get a much better ROI.
Dick is 90% of the way there, and has the added advantage of AV control.
A low profile, small footprint 1 group strada EP. I dig it.
Edit: the more I cogitate over this the more I like it. The GS3 MP has a very substantial compromise to allow the use of the 9 bar pump. A very elaborate valve (with bypass), giving up the AV ability, expensive O rings, etc. Meanwhile a gear pump, a sensor and a controller can make a far better solution. An added needle valve (or an expensive proportional valve Yay!) would extent the capabilities of the system even more.
I'm only 90% of the way there because I can't reduce the flow rate below line pressure, and the minimum line pressure to keep the pump from cavitating only allows pre-infusion for a max of 10-15 seconds.
I can add a needle valve in front of the pump to mimic the MP action, but that puts the pump in danger. I'm currently running some tests with a cheap needle valve at the group to determine what kind of pump speed(s) are necessary to prevent damage when a needle valve is in front of the pump, and what max pressure that speed produces when the needle valve is removed from the circuit.
Even if the pump speed doesn't have to be reduced when the needle valve is inline, as you know my setup isn't quite safe because the expansion valve doesn't actually protect the gear pump and the machine from excess pressure. For that reason alone, I need to add a proper external bypass or electronic feedback loop. Not rocket science and not very expensive, but either one is a bit of a project.
With the bypass I have to find the right device. Not sure where to start with that. Then I have to find a location (in or out of the machine) and way to plumb it so the excess flow is dumped into the drain box. Probably have to drill the rail over the drain box or create a tee with the exhaust. Fittings between the pump and bypass have to withstand high pressure.
I already have a pressure transducer for a feedback loop. Easiest place to install it is at the pump output, ahead of any needle valve I might install later. Might need a fitting or two to make that happen. My guess is that a feedback loop could be constructed with discrete components -- i.e., something that regulates the 5VDC signal to the motor speed pot in inverse proportion to the voltage from the transducer. Probably an op amp circuit adjusted with pots. Have to think about that. I have an Arduino available to do it in software, but that takes just as much circuit construction and wiring, and seems like overkill unless I'm going to move towards an EP-style solution, which I'm currently not motivated to do.
Location of the needle valve is certainly a matter for debate. Slayer puts it ahead of the pump and leaves the pump running during pre-brew, which requires tubes and fittings that can withstand high pressure. This results in a gentle ramp to max pressure that's determined by the needle valve. Think of it as a second gicleur. Once max pressure is reached, the needle valve is removed from the circuit and there's no interruption or bumpy transition because the pump has been running at the same speed the whole time.
Putting the needle valve behind the pump, as you have it, requires turning off the pump during pre-brew. Otherwise, the pump will cavitate due to the flow restriction. After pre-brew, the needle valve comes out and the pump is turned on. If it happens to be at the speed that results in 9 BAR at the puck, the ramp will be determined by the gicleur. The alternative is to do a slow manual ramp with the speed control, which is what I do now after slow pre-infusion without the needle valve. It's not at all clear to me that there's anything substantially different with this method. The argument I've gotten is that with the needle valve in front of the pump it's automatic and repeatable. You don't have to think about it.
Yes, if I go the microprocessor route the ramp could be automatic, but it would have to be selected to match the needle valve setting. Again, it feels like overkill.
Like I said, though, with or without the needle valve in front of the gear pump, it's high time I added a safety mechanism to avoid excess pressure. Right now, it's my responsibility to not forget to reduce the pump speed when backflushing, etc.
Finally, the one flaw in converting an AV the way I have is the superheating that takes place in the heat exchanger due to the low flow (or pressure as you say.) There's no good way to fix that without a way to regulate the flow after the heat exchanger and dump the excess (i.e., in the group, like the MP.) Well, there's a way, but it's not satisfactory: run hot water until the boiler autofills, which reduces the temperature in the boiler enough to avoid the superheating effect. Sometimes I forget to do that!
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