Home-Barista.com

(This is CROSSPOSTED on alt.coffee; if you chose to comment on this post please do so on one but not both of the crossposted threads)


Due to questions raised on another thread in this forum, the issue of the impact of autofill on shot temperatures and pressure has been discussed. I have started this thread as a repository for any actual observations that might be made on this topic. The original issue raised was what would be the impact on a shot if the autofill was actuated during the shot itself, with particular reference to vibratory pump machines with autofill. There are two issues here, the first being the effect on the shot pressure, and the second being the impact on shot temperatures for both the shot in question and for subsequent shots in a consecutive shot series.

I do not have a vibe pump machine with autofill, although I do have one without it. Testing that machine would probably not produce useful information as the boiler is filled manually. I do have a rotary pump driven plumbed in machine, a Cimbali Junior D model which has been modified with both a PID for more precise boiler temperature control, and a (defeatable) delay timer for inducing preinfusion using regulated water mains pressure of about 3 bar for the initial 6 seconds of each shot. Because of concerns about the delay timer confusing these issues, I elected to bypass the delay timer (it is switched in my installation) leaving the machine in its native configuration other than for the PID.



As the first part of the test I installed the portafilter (PF) manometer and tested shot pressure, which remains as previously set at 9 bar. Then, as a second test I again tested the shot pressure with the PF manometer but after 10 seconds opened the water wand and dumped out 200ml of water which actuated the autofill almost immediately. The shot pressure did not change and remained at around 9 bar. Therefore, with at least this rotary pumped machine, shot pressure was not effected by the autofill coming on during the shot.

The next thing to test was the impact of autofill actuation on measured shot temperatures, both during the shot and in consecutive shots afterwards. I started with a PID boiler temperature setting that I normally use when I'm making milk drinks with the Harrar Horse SO beans I have recently been using for espresso. The Scace Device and an Omega Datalogger were used to record shot temperatures.

As a baseline I performed a 4-shot series without actuating autofill to show the consistency of shot temperatures observed:



Having obtained a baseline, and waiting 45 minutes to allow the machine to return to baseline condition, I performed a six shot series with autofill activating during the 2nd shot. The first shot was pulled, as before, after an initial 50ml cooling flush. Ten seconds into the 2nd shot, I drained 100ml of water out of the water wand while continuing to record the shot. The autofill ran audibly during the 2nd half of this shot and continued into the idle period in between the end of the 2nd shot and the beginning of the 3rd. The shot series continued as before in the baseline series, with 1 shot pulled per 2 minutes, equating to a pause of 90 seconds between the end of one shot and the beginning of the next one:



As you can see, the average shot temperature in the baseline series is a bit higher than that observed in the series where the autofill was actuated. The 2nd shot, during which the water was drained from the boiler and during which autofill was running the last half of the shot, did not appear to have its shot temperature effected by this maneuver. There was a trend towards declining shot temperatures with each subsequent shot, however within the range I have previously observed in this machine as set with this boiler temperature and without the autofill being actuated.

In summary, in my rotary pump driven Cimbali Junior D HX machine, I have observed the following with boiler autofill actuation during a shot series: (1) no effect on measured shot pressure during the shot when autofill was operating, and (2) no effect on shot temperature during a shot in which autofill actuates. Effects of autofill actuation that I DID observe were a trend towards overall lower shot temperatures in the shot series, especially in shots FOLLOWING the shot pulled simultaneous with autofill, and a fairly limited effect overall, all things considered, given the predicted perturbation of the system that I anticipated.

Finally, this is not a test of a vibe pump machine with autofill, and individuals owning such machines will need to test them in order to see how generalizable these observations might be.

ken

ill try to measure with my gauged PF, but i dont know how accurate it will be, as once there is pressure in it, and with no way for the water to release, i may not be able to sense the drop. i will try though.

in a 'non-scientific' approach, there is a dramatic difference in the pressure of the water when the autofill kicks on with an exposed grouphead..

If the pressure shown in your PF manometer does not go down when the actual pressure is reduced then your PF manometer is not working correctly. You could test this by seeing what happens when you stop the "shot" after the 3 way valve releases. If you are concerned that this is not a real test, you could make an actual shot while bleeding water through the water wand forcing the autofill to actuate. You will obviously get some visual cues as to flow rate from the PF when you do this, and if you would let us know what those were we would appreciate it. There is no difference in my rotary machine, and I can't test this theory in my vibe pump machine since it lacks AF.

ken

ok, just tested with the gauge, and it does drop.
put gauged PF on, turned on brew, pressure went to ~8.2bar. opened hot water wand, after a couple seconds the autofill activated, and pressure dropped to ~4bar.

HB wrote:Apparently Mr. Fox is not easily convinced, so I also generated a chart:

image: userpix/2_autofill_pressure_drop_1.jpg
For a vibratory pump espresso machine, the brew pressure craters during inopportune autofill

To keep it all in one place, I'll repeat the relevant explanation from the other thread:


The 14 or 15bar you refer to is a zero flow. The max pressure a vibe pump can produce drops off dramatically with an increase in flow rate:

image: userpix/2_ulka_procon_1.png
Thanks to Eric C for the diagram (Flow rate of a rotary pump espresso machine)

When the boiler refill solenoid opens, the water will flow into the boiler in a hurry as it's offering only ~1 bar of resistance, compared to the puck / OPV's ~9 bar.

No Dan,

It isn't that I'm not easily convinced, it is that when something can be easily tested to either prove or disprove a hyphothesis or a "hunch," there is no reason not to test it.

The online espresso venues are full of posts touting conventional wisdom which often (ultimately) is proven untrue. I have participated in testing some of these sorts of ideas, like the one that rotary pumps make better espressos than do vibe pumps, that HX machines can't be made (reasonably) temperature stable, stuff like that.

If all we are going to do is repeat the same stuff over and over without ever testing our assumptions, we might as well just make one big thread, put all the assumed garbage in there, and then make the forum read only.

ken

Dan,

As I indicated in my original posts on this topic, I have never owned a vibe pump driven machine with autofill, and have no experience with this phenomenon. Although I have helped friends to buy vibe pump machines and even set one up (an Andreja Premium), I've never operated it when the autofill kicked in. It seems like the wisest solution for mfrs. would be to program or wire the brain boxes so that autofill is prohibited during shotmaking. From the standpoint of current machine owners who have vibe pump machines with this "flaw," the cheapest solution would be to bleed off enough water from the boiler, via the water wand (assuming the machine has one) every day or two, at a time when the machine is not being used for espresso preparation. That way the boiler fill will be near the top of the range at all times and the autofill will not actuate due to such things as moderate amounts of frothing.

Another thing that one can do if their autofill comes on very often would be to make sure that the probe tip is positioned dead center in the middle of the boiler at its widest point from top to bottom. Any other position will will sense smaller drops in boiler volume and will turn on the autofill circuit more often. Another thing to check is that the probe tip is clean of scale and debris so that the contact is easily made.

ken

Ken Fox wrote:... bleed off enough water from the boiler, via the water wand (assuming the machine has one) every day or two, at a time when the machine is not being used for espresso preparation. That way the boiler fill will be near the top of the range at all times and the autofill will not actuate due to such things as moderate amounts of frothing.
...

ken

i empty a large coffee mug of water from the boiler every 1-2 days; mainly because of my fear of creating a water line from it sitting too long, but this seems like a good reason to as well.

This topic was discussed a few months ago on the How Autofill Degrades Shot Temperature Stability thread. On my rotary pump Vetrano, autofill clearly reduces group head pressure. I don't think it really matters whether the pump is vibe or rotary; when you open an alternate water path to the boiler, pressure is gonna drop at the brew head. My question is, what wizardry does Cimbali use to avoid this?
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John