Breville Dual Boiler Volumetric Problems

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Postby lancealot » Sep 13, 2017, 9:47 pm

Hi thanks in advance for reading this and for any post that can help.

I think I am having problems with the volumetric shot control on my BES920XL. I am looking for experiences and feedback from anyone who has experience with using volumetric controls on this machine or knowledge of it that could help. Please don't use this an an opportunity to ask why I would want to use volumetric controls in the home. I have heard of other users of this machine having their controls work and I am interested in doing what it takes to make mine work reliably, or determining if I have a faulty machine, or learning that this feature just doesn't work on these machines as advertised.

Here is my issue. On my machine and another BES920XL that I get to play with infrequently, I cannot get volumetric controls to be repeatable. I will go through setting it up, grind, dose, weight, tamp. Set the machine to volumetric control using the special menu, long beep so I know the setting change was applied. Then go into the menu, select the control for the single shot button. Push the single shot button to let it do it's thing until I get the the desired weight, push the single shot button to shop the shot. Final weight, 32g. Hit exit. Long beep, setting change confirmed.

Then I will pull the exact same grind and dose 1 minute later. Machine stops at 25g. Another, stops at 25g. Another 37g. Another 42g. Next day, 25g. Reset to timed control and consider posting on HB. Try it weeks later same inconsistent result. Try it on the other machine that I have access too, same type of result.

I have this same experience with Josuma MG and Redbird. Both fresh. Grooming with the little knock-off grooming tool and tamping with the matching palm tamper. I don't have a bottomless but the shots are turning out good when i stop them at the right time (I like them, no major time or taste variances from shot to shot).

I looked into how the volumetric controls work on the BDB, and between posts and schematics I have seen it goes something like this. There is a flow meter at the pump feed line, measuring flow into the pump. There is another flow meter on the OPV's over pressure discharge line. The two are measured against each other to detect the flow to the brew head. In the background of this video you can see a diagram.

So, what has your experience been with volumetric control on your BES900 or BES920?
Do you have any tips or feedback for getting this to work for me?
Is this feature of my machine broken?
Does this just not work on all these machines?


Postby Bret » Sep 13, 2017, 9:57 pm

I think the volumetric "works" because you will get the same volume. I stopped using it because of exactly the issue you are experiencing: for my purposes, consistency of output weight is critical to to the extraction/taste. I have daily and sometimes shot to shot variation of the time needed to get the exact weight. It changes some with age of the beans, some with consistency of grind, tamp, etc. Plus, the time variation is informative: if it flows too fast, I can make an adjustment, etc.

I do this on manual, so the time variable is controlled by me directly. When I get it dialed in, the volume differences are negligible, even when I need to vary the time to get the weight. Since weight is what I want to be consistent, it works for me. But, like you, I found the volumetric to be accurate on volume output, but useless for my needs.

Basically, if you set to automatic volume control, but you measure weight, then you won't likely get much weight consistency. If you want consistent weight, you have to control for it with a scale. Time and pressure may vary a bit, and those variations can be informative relative to your dose, tamp, grind and bean age (all of those variables are at play) and if you fix weight by varying time under those conditions, you can make adjustments that inform the result. But if you want consistent weight output, but control volume, you'll end up frustrated.


Postby BobStern » Sep 13, 2017, 10:17 pm

The volume measurement scheme strikes me as inaccurate.

As you say, the volume is estimated by subtracting the measurements of two flow sensors: one at the input to the brew pump, and one at the overflow output of the over-pressure valve (OPV).

If the two sensors are calibrated slightly differently, subtracting their measurements will yield erroneous results. As an extreme example, if you insert the backflush disc to establish zero flow, the subtraction will yield a constant offset that may be either positive or negative and, depending on the polarity, cause the machine to erroneously think there is a constant flow and shut off after a certain time.

As a more practical example, if the sensor at the pump input reports a higher measured value than the sensor at the OPV overflow output for the same actual flow rate, subtracting the latter from the former will overestimate the brew volume. If the brew flow rate is very slow for a few seconds at the beginning of a shot, the error could be major, causing premature shutoff.

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Postby lancealot » Sep 14, 2017, 10:15 pm

Bret, thanks for your reply. I think you are off on this though. I will have to measure the volume but I do not think the volume will be consistent even though the weight is different. Here's why:
The flow meters take into account how much water is going into the pump and how much is flowing out of the opv. The opv value is subtracted from the pump input value the resulting value is the estimated volume of water going to the coffee cake. From there, if a consistent volume of water, flows through a consistent cake of coffee, then the volume of water coming out of the cake (espresso) will be the same and therefore weight the same.
The only way I see that I could be wrong about this is 1) if there is a leak - i don't think there is, 2) if the cake is absorbing more or less water - seems improbable to me, or if 3) the weight of water is changing spontaneously - hehe, or 4) as Bob thinks, there is a problem with the flow meters.

I think that Bob's thought that the flow meters are not calibrated the same makes the most sense. However, I wonder if these flow meters are even calibrated. I would bet that they are just manufactured and assembled into the machines. Does anyone know how much variation is possible from a (probably) inexpensive flow meter like they are using in these machines?

You can read about when they first started shipping the BES900's and the OPV's were not adjusted correctly. People had to send machines back. I wonder if this is the same kind of thing and they really should be testing the flow meters before they are putting them into the machines.

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Postby lancealot » Sep 14, 2017, 10:44 pm

Just putting this stuff out there...

Googling "encoder flow meter espresso" and selecting images, I got the following.

This person's work is cool in general. In this page is info and a link to a data sheet for a flow meter that looks just like the breville bes870 one on ereplacement parts. Don't know if the 920 uses the same meter as the 870 but I don't see why it wouldn't. I am not at a place to take my machine apart and find out, at least right now :D

Data sheet for the flow meter used, specs are there but I can't make sense of them:

Ereplacement part for bes870 Barista Express:

Looks like the same part in these pics of the insides of this BDB


Postby Bret » Sep 14, 2017, 11:27 pm

I'm not sure, but even if the two flow meters are calibrated slightly differently, wouldn't you get a consistent error? And wouldn't that consistent error/offset be washed out of the equation because you are manually setting an actual output volume that you measured in a marked shot glass that inherently 'accounted' for the offset?

I don't see how that gives you all the variation in your results from shot to shot, day to day.

When I have tried to measure volume directly, in a marked shot glass, I noticed that it is difficult to be consistent: do you choose the top of the crema, the boundary of the crema/liquid, etc. so you pick one of those as your standard approach only to see that from shot to shot and day to day and blend to blend that the ratio of liquid to crema varies. This is the reason many (most?) use weight as the metric for output rather than volume.

In any case, I agree with you that the volumetric measurement approach on the BDB does, for whatever reason, deliver an inconsistent result. It has been this way on two different machines for me. Since I have only seen 'recipes' provided in terms of dose weight and output weight, I didn't pursue the volumetric approach any further. It has been so long ago that I don't recall if I even got consistent volumes at all. I stopped using it and moved to weighing the output pretty quickly, so I don't know for certain how I determined to stop using it. It might have been because with crema variation I didn't see a clearly consistent result, or it might have been because I was weighing the shot and seeing inconsistencies. I'm pretty sure that I dumped the whole idea of fussing with the volumetric in one session, probably within a handful of shots from that session.

The subtraction of the three way valve amount seems pointless to me. The flow meter on the input side should be sufficient for the purpose: Since we don't dial in oz on the display for desired output, but simply press/release when we get the desired (visually measured) volume, the three way valve volume should not be needed for any calculation anyway. Whatever the flow was at when the shot was stopped should be consistent enough. The three way valve gets dirty, lifts some grinds off the puck which can affect the amount of water lifted off the puck, and the dosing itself can affect the amount of water on the puck to be lifted (and not all of it may be possible to remove via the three way valve). Volume on the input sensor at the moment the pump stops would probably be more consistent. The pressure can vary during the extraction, which can be affected by the grind/dose/tamp/blend which can also affect the pressure at the point of lifting liquid off the puck. Lots of potential variability there. If you have a dry puck vs a soupy puck, the volumetric subtraction will yield different numbers.

So perhaps it is not a calibration issue on the sensors (or not solely that), but a simple inconsistency in the amount of liquid pulled off thru the three way valve, which would make the fundamental approach inconsistent.

ETA: okay, I re-read your post: if the two flow measurements are in fact made across the OVP, then they don't take into account any variation in the flow at the three way solenoid at all, it is invisible to the calculation. So you could have a great deal of variation in what happens at the puck that will not be accounted for, and so that would explain the inconsistency, and I'd wager that this degree of inconsistency would be larger than it would if the measurements were made at the input side and the output side of the of the three way valve (though this would still be inconsistent).

I didn't see any specific reference to the flow meters in the video, but if the measurement is made only that far upstream, then I think that is why the volumetric approach can be so inconsistent. If everything else we do remains consistent shot to shot over time, including accurate grind changes to account for bean aging, the approach might work well enough to be reasonably consistent.

From my perspective, for this to really work, there would need to be a flow meter between the basket and the spouts in the portafilter :-)

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Postby lancealot » Sep 15, 2017, 12:05 am

The locations of the meters are in the diagram on the chalk board behind Phil in the video.


Postby Bret » replying to lancealot » Sep 15, 2017, 12:12 am

Yep, thanks. I watched again (the first time I thought he was actually going to talk about it 8) )

I think that without any accounting for variables with the puck, at the puck, with the amount lifted off the puck by the three way valve, there is no way that one measurement that far upstream will ultimately be consistent in the cup. I think it will be roughly as good as just setting a shot time to always be, say, 30 seconds. If there was a flow meter was measuring bi-directionally between the puck and the three way valve, it would be more consistent, but still subject to puck variations (e.g. a soupy or dry puck).

They still need the flow measurements where they have them, for boiler filling, etc. I think the idea to use them as located ALSO for shot output volume regulation is flawed, but then I don't remember them actually saying that specifically. I think they describe it (without digging out a manual) as a choice to control by time or volume. I think they are both probably similarly consistent, which is why many of us end up weighing input & output, adjusting grind to get the desired combination of output/time and worrying somewhat less about the time (particularly if using variable preinfusion approaches, etc.).


Postby martinngyh » Sep 20, 2017, 12:40 pm

If it is the Dual Boiler's fault, you can check the weight of the water it dispenses without the portafilter. If it is consistent, you can change the volumetric setting and see if that setting is registered or not. That way you can prove whether or your machine is operating as intended.

If volumetric function on your machine is not malfunctioning, there are a lot of variables besides a constant the volumetric setting. The variables are your grind distribution, ambient temperature and freshness of your beans.

When you think you grind and tamp properly, sometimes it is not the case. The easiest way to check is to use a bottomless portafilter. The shot to shot variation is usually due to that. I use an OCD-like grind distribution tool and a puck shape tamper. It decreases my inconsistency a lot. With these tools, I have less channeling, which makes coffee flow faster, increasing your yield.

I notice, in a hotter morning, like 75-80F in my apartment, the yield is greater then when it is about 70F. Higher temperature, faster the flow. Therefore, the grind setting you use in the morning should be different in the afternoon given the same volumetric dosing setting.

Finally, coffee beans are less fresh in a week, so whatever setting on the grind size you use this week should be changed next week for the volumetric dosing to work.

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Postby slipchuck » Sep 20, 2017, 12:54 pm

I don't have the BDB but have a breville that works the same way. I tried and tried to get the volumetric right but there are so many variables that everything had to be perfect.
I finally just gave up, bought a scale and manually dispense the shot.
Been happier ever since :)

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