The Impact of Autofill on Shot Temperature and Pressure in an HX Machine

[Ken Fox]

(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

[LeoZ]

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..

[Ken Fox]

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

[LeoZ]

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]

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


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:

If the vibe pump would get up to 14 or 15bar in its unmodified state, and especially if the OPV is located distal to the input solenoid, I think it is very possible that the pressure at the puck would remain unchanged as the pump is capable of producing almost twice as much pressure at the group as one has the OPV set up to deliver.

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:


Thanks to Eric 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.

[Ken Fox]

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

image: http://www.home-barista.com/forums/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: http://www.home-barista.com/forums/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

[HB]

I understand your skepticism of so-called conventional wisdom. However, in the case of an inopportune auto-fill on vibe pump machines, the negative consequences are so stark that actually quantifying them was an exercise in confirming the obvious. Your measures of the consequences on a rotary pump are not so self-evident, and your results put the debate to rest. In recognition of your diligence, I measured the same effect on my Valentina so others would benefit from similar hard data.

[Ken Fox]

I understand your skepticism of so-called conventional wisdom. However, in the case of an inopportune auto-fill on vibe pump machines, the negative consequences are so stark that actually quantifying them was an exercise in confirming the obvious. Your measures of the consequences on a rotary pump are not so self-evident, and your results put the debate to rest. In recognition of your diligence, I measured the same effect on my Valentina so others would benefit from similar hard data.

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

[LeoZ]

... 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.

[RapidCoffee]

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?
________
John

[Ken Fox]

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?
________
John

Hi John,

I have been giving this matter some additional thought.

The other thread wasn't really so much about autofill occuring INTRA-shot as it was about autofill occuring during a session when many shots (with frothing) are made. The autofill would have been triggered in the earlier test scenario by doing a lot of frothing, however each simulated frothing session would obviously expel less water than what would be needed to trigger the autofill. In the current series I abruptly removed 100ml of water from the boiler in several seconds, which obviously would trigger the autofill.

I should add also that it is a bit difficult in a typical machine to be absolutely certain that the autofill is filling the boiler at the same time as the shot is being made. The obvious reason is that the rotary pump and motor are going to be running to make the shot, and in the absence of a sight glass one is looking for additional noises like maybe something coming out of the input solenoid to try to confirm that the boiler is being filled at the same time. Since autofill machines aren't made (or at least aren't made anymore) with sightglasses, you have no visual cues to guide you either.

It is certainly possible that changes in sounds that I heard after the 100ml of water was removed from the boiler could have been something other than the boiler filling and since the rotary pump remained on after the shot was completed it is definitely possible that no actual boiler filling occured during the shot even though the autofill circuit was indicating a need for a refill. If so, then both test scenarios, as reported in this thread and the other one, were actually similar with the major difference being the volume of fresh water put into the boiler by the autofill during the test series. The earlier series probably introduced a lot less water as the simulated frothing sessions would have almost certainly expelled less than 100ml of liquid water equivalent.

As to how Cimbali would have managed to refill the boiler simultaneously as the shot was being made, without effecting the shot pressure, I guess the first question would be whether the boiler is actually refilling intra shot even though the boiler circuit is calling for it. Possibly, with a trained ear and multiple tests, after taking the side panel off and listening with your ear close to the boiler it would become obvious. Perhaps Michael Teahan or TJ or someone else who really understands how these things work can explain it since I'm not sure of a good way to convince myself whether the boiler is actually refilling intrashot or not. IF the boiler is actually refilling intrashot, then there must obviously be enough power in the pump and motor to accomplish both tasks simultaneously, or the plumbing must be set up in a way to preferentially give the pressure to the shotmaking circuit in favor to the boiler refilling circuit.

ken

[HB]

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?

IF the boiler is actually refilling intrashot, then there must obviously be enough power in the pump and motor to accomplish both tasks simultaneously, or the plumbing must be set up in a way to preferentially give the pressure to the shotmaking circuit in favor to the boiler refilling circuit.

I haven't given this serious thought since an inopportune auto-fill during an extraction is a very rare event. Maybe once or twice in the last few years? I don't know the inner plumbing details of your Cimbali Junior, but a restrictor (orifice) for the boiler refill leg of the circuit similar to the grouphead gicleur would impede the inflow enough such that the pump pressure would remain steady. It shouldn't strain given that it can put out 900cc/min at 8 bar.

[Ken Fox]

I haven't given this serious thought since an inopportune auto-fill during an extraction is a very rare event. Maybe once or twice in the last few years? I don't know the inner plumbing details of your Cimbali Junior, but a restrictor (orifice) for the boiler refill leg of the circuit similar to the grouphead gicleur would impede the inflow enough such that the pump pressure would remain steady. It shouldn't strain given that it can put out 900cc/min at 8 bar.

I agree and think that an inopportunely timed extraction would be mostly an issue when one tried to bang out shots for a large number of people many or most of whom requested milk drinks. Then, I think you would get what I demonstrated in the earlier thread with a wider degree of shot temperature variation than one would get normally. Of course, my system has its temperature controlled by a PID and I have the boiler temp set lower than most people would use it for repetitive shot series with mostly milk drinks, so quite probably in a stock system this all would get lost in the "background noise" of a pstat driven system operating at a higher boiler temperature, albeit with more temperature variation than I normally see in my system.

As an aside from the earlier thread, I had given serious consideration to modifying my machine, yet again, to try to prevent inopportune boiler fills, using a relay and perhaps other electronic parts. After a little bit of reflection I decided not to bother with it as in my actual usage pattern large runs of drinks, especially large runs of milk drinks, happen seldom or never. Having now done this second "experiment," where at least 100ml of cold tap water were quickly dumped into the boiler during a shot series, and seeing how little difference it made in the shot temps obtained, I'm even more inclined to leave well enough alone.

For those with upcoming parties where they anticipate pulling long runs of milk drinks, it would not be a bad idea to prepare in advance by bleeding off enough water from the boiler to turn on the autofill. At least that way you will know you are starting out with the boiler at the high end of its fill range, and you will reduce the likelihood of an inopportune autofill cycle. It would not be a bad idea at all to bleed off a few ounces of water every couple of days in regular domestic use, which will have the benefits of both reducing the likelihood of scale buildup, and of reducing inopportune boiler fills during your normal espressomaking routine.

ken

[jesawdy]

As to how Cimbali would have managed to refill the boiler simultaneously as the shot was being made, without effecting the shot pressure, I guess the first question would be whether the boiler is actually refilling intra shot even though the boiler circuit is calling for it. Possibly, with a trained ear and multiple tests, after taking the side panel off and listening with your ear close to the boiler it would become obvious. Perhaps Michael Teahan or TJ or someone else who really understands how these things work can explain it since I'm not sure of a good way to convince myself whether the boiler is actually refilling intrashot or not. IF the boiler is actually refilling intrashot, then there must obviously be enough power in the pump and motor to accomplish both tasks simultaneously, or the plumbing must be set up in a way to preferentially give the pressure to the shotmaking circuit in favor to the boiler refilling circuit.

Ken et al.

Finally got a chance to read this thread.... and I looked at the plumbing diagram and electrical diagram for the M32 Bistro (which is not the M21 Junior, but hey, it is Cimbali). I am not an expert in reading said diagrams, but I believe on the M32 at least, it is electrically wired such that the Coffee solenoid and the Boiler supply solenoid cannot be active simultaneously. My suspicion is that the level sensor is detecting low level, LED indicates that boiler refill is needed, but not necessarily occuring. The wiring diagram indicates that pump motor relay is tied to the coffee solenoid and the boiler supply solenoid, but there is a switch that limits it to one or the other.

Ken, if you are so inclined, you could put a meter on the boiler supply solenoid, and see if it is actually opening intrashot or not when a refill occurs. I think it is actually opening immediately upon completion of the shot when the boiler level sensor is calling for refill.

I can attempt to post the electrical diagrams if you'd like.

The hydraulic diagram also shows an "Injector" on both the Coffee plumbing circuit ahead of the heat exchanger, and on the boiler supply plumbing ahead of the Boiler supply solenoid valve. There is a third "Injector" ahead of the water supply tank solenoid valve as well (The M32 1-group has a pour over type reservoir, even though it can be direct plumbed). I am not sure what is meant by an "injector", but based on the three locataions, I wonder if they are some sort of flow restrictor or orifice. Is this consistent with a gicleur (I'm too lazy to look right now)?

UPDATE - For this one, it is simpler to link to a PDF of the diagrams that I created. Get a copy of the electrical and plumbing diagram here.

-Jeff

[LeoZ]

while we seem to be drifting onto the subject.. is there any benefit to upgrading my pump from vibratory to rotary? would it impact the machine internals in any negative way?

[Ken Fox]

while we seem to be drifting onto the subject.. is there any benefit to upgrading my pump from vibratory to rotary? would it impact the machine internals in any negative way?

Based upon blind taste testing done TWICE over ~2 years by Jim Schulman and myself, I/we would say that if your goal is to make better shots, you would be wasting your time doing a rotary conversion as there is no evidence that a properly set up vibe machine makes less tasty shots than a rotary machine. I am assuming that you have an overpressure valve and it is set to some sort of reasonable value such as 9 bar. In fact, if your machine lacks preinfusion, the slower pressure ramp up of a vibe pump is much more forgiving for shotmaking than is the almost immediate ramp up with a rotary, in a sense making a vibe machine superior for all but the very best baristas who may not need "forgiveness" in their machines.

Rotary pumps are quieter, of course, so one might desire to have a rotary pump driven machine for reasons other than shot quality. Converting a machine from vibe to rotary is not a simple matter of swapping out the vibe pump and replacing it with a rotary pump and motor, however. A long time ago I considered doing a rotary conversion on my old vibe Cimbali Junior and later, with the testing we did, I decided that this was not worth doing, at least for me.

ken

[Ken Fox]

Ken et al.

Finally got a chance to read this thread.... and I looked at the plumbing diagram and electrical diagram for the M32 Bistro (which is not the M21 Junior, but hey, it is Cimbali). I am not an expert in reading said diagrams, but I believe on the M32 at least, it is electrically wired such that the Coffee solenoid and the Boiler supply solenoid cannot be active simultaneously. My suspicion is that the level sensor is detecting low level, LED indicates that boiler refill is needed, but not necessarily occuring. The wiring diagram indicates that pump motor relay is tied to the coffee solenoid and the boiler supply solenoid, but there is a switch that limits it to one or the other.

Ken, if you are so inclined, you could put a meter on the boiler supply solenoid, and see if it is actually opening intrashot or not when a refill occurs. I think it is actually opening immediately upon completion of the shot when the boiler level sensor is calling for refill.

I can attempt to post the electrical diagrams if you'd like.

The hydraulic diagram also shows an "Injector" on both the Coffee plumbing circuit ahead of the heat exchanger, and on the boiler supply plumbing ahead of the Boiler supply solenoid valve. There is a third "Injector" ahead of the water supply tank solenoid valve as well (The M32 1-group has a pour over type reservoir, even though it can be direct plumbed). I am not sure what is meant by an "injector", but based on the three locataions, I wonder if they are some sort of flow restrictor or orifice. Is this consistent with a gicleur (I'm too lazy to look right now)?

UPDATE - For this one, it is simpler to link to a PDF of the diagrams that I created. Get a copy of the electrical and plumbing diagram here.

-Jeff

What I'm curious about is whether the input solenoid will even permit the water to go in two directions simultaneously, e.g. to the boiler AND to the group. If this is not possible than the question answers itself as the water can only go one place at a time and not two, which would mean that no matter what the autofill circuit is calling for, if it is subordinated in the solenoid, what it wants and what it gets are two different things The situation might be different with double boiler machine, so this comment is directed only at HX machines.

Unfortunately I cannot test anything right now as I am in France and will not return home for almost a month.

ken

[TroyR]

Topic merged to prior thread by Moderator

Other than the obvious of course, which would be pumping reservoir temperature water into the boiler probably lowers boiler temp.

So temperature is managed via pressurestat in my Hx boiler, what effect does pumping additional fluid volume into the closed system have?
Will temperature actually rise via a magic water-pressure relationship? Or will my temperature fall due to the fact that the pressure was increased by adding fluid volume instead of adding more heat?
Whichever way it happens, is a temperature rise or fall compounded by or negated by the addition of reservoir temp water entering the boiler?

Man, I hope that made sense.

BTW, I have a Bezzera BZ02.

Cheers,
Troy

[Ken Fox]

Topic merged to prior thread by Moderator

Other than the obvious of course, which would be pumping reservoir temperature water into the boiler probably lowers boiler temp.

So temperature is managed via pressurestat in my Hx boiler, what effect does pumping additional fluid volume into the closed system have?
Will temperature actually rise via a magic water-pressure relationship? Or will my temperature fall due to the fact that the pressure was increased by adding fluid volume instead of adding more heat?
Whichever way it happens, is a temperature rise or fall compounded by or negated by the addition of reservoir temp water entering the boiler?

Man, I hope that made sense.

BTW, I have a Bezzera BZ02.

Cheers,
Troy

Thanks for resuscitating this long dead thread that deserved to have died a long time ago

I do not understand your question, to be honest. If the question involves what happens when you pull a shot and water is introduced, that water is of course in the heat exchanger circuit and merely passes through the heat exchanger which is located in the boiler, but has no impact on boiler pressure other than to slightly lower it since cooler water entering the HX will have some impact on boiler temp.

If the question concerns autofill or manual boiler fill, hence increasing the volume of water in the boiler, then what is happening is that cooler water is introduced into the boiler which of course increases the amount of water vs. air/gas/steam space in the boiler, but reduces the pressure because it cools off the system. Later, a few seconds later, after the pstat has called upon the element to heat up the water, the element will then be turned off when the pstat cycles off because the pressure is back at the top of the range, hence there is essentially no change.

If on the other hand you were to introduce very hot water into the boiler, that might have other effects such as increasing the boiler pressure but the pstat would be off at that point and would wait for the temperature to fall before calling upon the element to fire on again. Of course that is not going to happen as the water inflow does not come from pressurized heated water but rather from room temperature or even cooler, water.

Hope this makes sense.

ken

[TroyR]

If the question concerns autofill or manual boiler fill, hence increasing the volume of water in the boiler, then what is happening is that cooler water is introduced into the boiler which of course increases the amount of water vs. air/gas/steam space in the boiler, but reduces the pressure because it cools off the system. Later, a few seconds later, after the pstat has called upon the element to heat up the water, the element will then be turned off when the pstat cycles off because the pressure is back at the top of the range, hence there is essentially no change.

Thanks Ken, this is the scenario I was thinking of.
When the machine autofills and pumps water into the boiler wouldn't there be a pressure increase due to the fact that it is pumping into a closed system? This alone should de-activate the heater.
However, as you mentioned, if the addition of cool water should cause the pressure to drop then would you expect that these two occurences effectively negate each other?
IF they did in fact negate each other, then couldn't we be left in a situation where our heater is off because the pump bumped up boiler pressure even though our boiler temperature has been lowered by the addition of fresh water?

How differently would you expect the machines you tested and datalogged to behave if they were controlled by pressurestat as opposed to PID?

Troy