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Thread: Coffee machine experiments
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20th Sep 2019, 06:11 PM #76
Hi Bob,
A case of de ja vu ?
I sense success here
Seriously though those all metal confectionery tins if deep enough would be perfect. A couple of small self threading screws will ensure electrical continuity between the box and lid.Best Regards:
Baron J.
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22nd Sep 2019, 11:39 PM #77Member: Blue and white apron brigade
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Now that the noise problem has been been reduced to something manageable Ive been playing around with various temps and pressures.
The setup is a magnet for testing and experimentation - I'll be spending many days futzing around with it if I'm not careful.
One issue I've uncovered is the usual 10s water flush before pulling the shot is WAAAAY too long and drains too much heat from the heat exchanger.
This is because the NEW pump has a nominal flow rate of ~10x more than the original pump! In practice it turns out to be about 6x more than the original pump but it means only really short (2s) flushes are all that is required.
The Graphs shows the temperature following 3, 5, 10, 15 and 20s flushes.
The green line is for a 20s flush after a coffee has been extracted.
Flushes longer than 5s take many minute to recover anything close to the starting temps (ie >90ºC)
Screen Shot 2019-09-22 at 5.35.04 pm.png
The next issue is - despite the supposed ideal placement of the Thermocouple, - it may still not be in the optimum place.
Below is a cross section schematic of the machine
The TC is inserted as shown by the red arrow an lays along somewhere along the red line - if its too far back from junction "2" it may be sitting in a dead zone and not really measuring the actual water temp.
I will need to remove the TC (for the ump-teenth time) and do some probing with a fine wire.
TCposition.JPG
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25th Sep 2019, 10:53 AM #78Member: Blue and white apron brigade
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I probed the depth of the hole where the TC sits with a piece fo wire and was able to determine the TC was about 10 mm back from junction "2" in the schematic above so then made a new TC that sits just inside the junction and tested it out A small consistent difference was observed for a 10s flush but not enough to explain the "over cooling" of the group head reported by the probe.
I then measured the flow during a flush which for 10s was ~250 mL which is about 6X more that the original ULKA vibe pump.
This is equivalent to flushing the entire water content of the heat exchange finger ~1.7 x so the fact that this volume overcools the whole head doesn't surprise me.
I could stick to 3s flushes, that’s about 1/2 the volume of the heat exchange finger which should be more than enough but really wanted to understand why this was happening.
Then I checked this flushing flow rates against the flushing flow rates of machines that use external pumps and found out that such machines use a flow restrictor with a 0.5 - 0.6mm diameter to the group head to reduce the flow by, guess what ? - ~6X!
Just a reminder that this is not a problem when extracting coffee as the coffee grounds generate a back pressure which restricts the flow to expected flow rates, it's only a problem when flushing.
I could turn the flow down using the VFD but that would mean running the motor at ~8Hz which I don't really want to do because that would mean making continual adjustments to the pump speed in between shots.
The high normal flow of the pump also has a valuable use in rapidly refilling the boiler when water is withdrawn from it.
To investigate this I made a pseudo restrictor with a brass disc and fitted it to a portafilter and the following temp profile was observed.
The short term temp drop during the 10s flush time is dramatically reduced - the longer term temp drop of 2ºC is probably more related to the TC placement still being an issue.
Screen Shot 2019-09-23 at 7.07.24 pm.png
The flow restrictor needs to be further back in the system, somewhere between where the water is diverted to the boiler and before it enters the group head. Looks like I'll be taking the machine again for the umpteenth time.
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25th Sep 2019, 08:07 PM #79Philomath in training
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Not sure how you would do it in this situation, but once I had to measure pipe temperature with a thermo couple and the suggested method was to mount a small piece of tube to form an oil pocket and then put the TC in the oil. Theory being that the heat transfers better with the liquid interface.
As for the flushing, have you considered a diverter valve so that you get say 2 seconds of flush and then the rest of the water is diverted back into the tank - heat up the pipework without losing so much hot water.
Michael
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25th Sep 2019, 09:20 PM #80Member: Blue and white apron brigade
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Thanks Michael. I've also seen that method in use.
However, now that over flushing has reared its head for the moment I'm not too concerned about the TC placement although I might come back to it later..
As for the flushing, have you considered a diverter valve so that you get say 2 seconds of flush and then the rest of the water is diverted back into the tank - heat up the pipework without losing so much hot water. l
Another approach I have thought of is via the micro controller using a a pot (to set a flush time eg between 0.5 and say 4 secs) and a button that activates the flushing.
However this afternoon I found a readily accessible and suitable spot on the machine (where the fresh water copper pipe/brass flange enters the heat exchanger) where I could easily insert a restrictor
I turn up an 1.0mm thick x 11.1 mm diameter disc from some phosphor bronze (PB).
It’s beautiful stuff to turn but is still considered quite hard and has better corrosion resIstance than brass.
Even though it was easy to turn II could tell it was much harder than brass because I broke 3 drill 0.5 mm bits drilling the orifice.
Unrestricted flow rate with the PB restrictor is now ~19 L/Hour - was aiming for 18 L/hr but is MUCH better than the 90+ L/Hr without the restrictor.
The other thing I have noticed is the temp of the head at the TC is significant effected by whether the machine is fully assembled or not. ie its SS panels enclosing the machine are in place.
With the top and side panels off the head temp only gets to 85ºC as measured by the TC in the new position.
Putting the panels back in place increases the TC temp to 92º
Am toying with the idea of adding some insulation between the brew head where the TC is located and the SS cup rack tray on top of the machine.
The insulation cannot be any more than about 5mm thick and nothing exposed that frays or releases fibres it needs to be able to cope with about 100º.
Anyone have ay ideas?
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25th Sep 2019, 09:53 PM #81Philomath in training
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How about some self adhesive EVA foam sheet from Clark rubber? I bought some recently for a project at work and for a 3mm thick piece I think 500mm sqr was around $10. Other thicknesses are available (4.5mm, 6mm, ...)
Michael
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26th Sep 2019, 12:33 AM #82Member: Blue and white apron brigade
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Thank Michael.
I got excited when I read this as somewhere I have a roll of EVA I purchased a few years back for some reason I cannot remember.
Then I looked up the max temp EVA can handle and it appears most types of EVA can only cope with 70ºC with some special ones going to 80 and I really need something that can cope with at least 100º.
However the mention of Clarke Rubber has got me looking on their website and one option would be strips of neoprene rubber which can handle 100ªc
Even better would be Silicone rubber as this can handle more than 200ºC
A couple of issues I can see with this are - it usually comes as a very thin (~1mm) sheet - which would be better than nothing - maybe join a couple of sheets
Chinese Ebay vendors sell 5mm thick 500x500mm sheet but it costs $50!
AND
how to stick it to the SS panels - maybe with something like HT silicone?
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26th Sep 2019, 12:43 AM #83
Hi Bob,
Silicone baking sheet from Ikia !
26-06-2019-006.jpg26-06-2019-002.jpg
Lovely colourBest Regards:
Baron J.
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26th Sep 2019, 08:27 AM #84Philomath in training
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You may have trouble sticking silicone sheets in place. Sealant may do it, but I don't know how good the bond will be.
Neoprene will stick with a contact adhesive, but the nice thing about the EVA foam was the foam part. I'm wondering whether you get a sheet and put it in, and perhaps put some foil tape over the places in close contact. You might find that it works well enough - after, what you have works without insulation so even if the insulation is not 100% successful, you still gain.
Another thought (although more complicated) is to get some thin sheet metal (Al, SS?) and using double sided tape, add a layer inside the housing. It will give you an air gap that should provide some insulation.
Michael
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26th Sep 2019, 09:37 AM #85Member: Blue and white apron brigade
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Thanks, some good ideas there.
I have just found one of SWMBO's silicone baking sheets - it's <1mm thick but it fits the inside top of the machine near perfectly so I can try it out as insulation (at least for the top part) without cutting and sticking it on. I will just lay it on top of the inside of machine and see what happens. This will also form a small air gap in between the Silicone sheet and the the SS top.
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26th Sep 2019, 10:19 PM #86
Hi Bob,
Further to Michael's suggestion, thin perforated alloy sheet could be used to hold the silicone sheet, forming a sandwich.Best Regards:
Baron J.
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26th Sep 2019, 11:52 PM #87Member: Blue and white apron brigade
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I've been using it all day with the thin silicone sheet.
Best estimate is the silicone sheet increases the temp at the TC measuring point by about 0.7ºC and it appears to help stabilise temps during flushing a little better
Hard to say exactly how much as the pulsing of the boiler element on and off contributes varies the temperature by about +/- 0.5ºC over a period of a few minutes minutes.
When I get round to it I will add another One or two sheets and see what that does.
The in line restrictor is the main improver in stabilising temps and appears to have reduced temp variations especially during flushing.
I'm now pretty happy with where it's at - Now I have to practice my coffee making.
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28th Sep 2019, 12:07 AM #88Intermediate Member
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Bob i can drop off some high temp rated neoprene. It is in the form of pipe insulation, approx 80mm od 20 id. You will just need to trim it up to what you need.
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28th Sep 2019, 08:58 AM #89Member: Blue and white apron brigade
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28th Sep 2019, 02:01 PM #90
Hi Bob,
Have you thought about the insulation that is used on the walls of camper vans ?
Its a 5 mm thick synthetic foam rubber that is self-adhesive on one side and a 2 mm thick metallised plastic bubble bonded onto the other. Its rated to 150 C. I've loads of it from when I did my van, I'll take a picture later.
One thing that I will say is that once you have stuck it on it doesn't come off easily, even with the heat of direct sunlight on the van walls.Best Regards:
Baron J.
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