Grind gets coarser as grinder warms up

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donkeykong
Posts: 2
Joined: 11 years ago

#1: Post by donkeykong »

Firstly - I want to say this is an amazing resource which has helped me deepen my understanding of espresso machines significantly. Despite regularly using the site this is my first post as this is the first issue I have encountered which has not already been considered/discussed/resolved on these forums.

I have an Expobar Office machine with built in Cunill flat burr grinder (the Tranquilo I believe). The grind consistently becomes coarser as the machine heats up - and it is now driving me crazy.

This has been an excellent machine for me to get up the learning curve but I am well aware of its many limitations and the grinder in particular not the greatest. Because it sits right beside the boiler it absorbs some heat and I am aware of the effect that can have on coffee etc. To avoid this, I try to grind the coffee I need before the boiler warms up fully (if I am only pulling a couple of shots) or I only dose the weight of beans I need to grind - so the beans are never stitting exposed to the heat of the grinder for longer than absolutely necessary.

I recently replaced the burrs as I found the grind was becoming a bit dusty. When I examined the old burrs and the new ones side by side I realised that I probably should have done this a while ago but - as I say - I have been learning the ropes and had been achieving reasonably consistent results with the old burrs. With the old burrs my sweet spot for espresso was approximately one full half turn of the dial back from the 'zero point'.

So - I installed the new burrs and the first thing I noticed when dialling it in was that the sweet spot was MUCH closer to the zero point of the grinder (4-5 clicks) than with the old burrs (possibly an issue in itself?). I was also having a nightmare dialling it in - even when I finally got a good shot I found it very difficult to replicate consistently. After much trial and error I have now observed the following:

1. At any given grind setting the grind becomes coarser from the first grind after the machine is switched on (GRIND 1) to around the 10th grind (GRIND 10). After GRIND 10 the grind level stabilises and I can turn the dial to a finer setting which then allows me to get lovely consistent pulls. This translates to around 5 clicks finer on my dial from where I get a good shot when the grinder is cold.

2. One of the reasons that this is so easy to measure is that espresso sweet spot of the 'warmed' grinder is actually AT THE ZERO POINT of the cold grinder. So in the evening I will be happily pulling consistent shots - if I then leave the (empty) grinder at that setting (and check the lower burr rotates freely), the next morning the lower burr will be touching the upper burr when I adjust it manually. At that point it is back 5 clicks to start the process again.

3. As I mentioned above I do believe the boiler imparts extra unwanted heat into the grinder (or at least prevents it from cooling as quickly between each grind) but I have tested this with the boiler switched off and I have achieved the same results - it takes around 10 grinds before the grind setting will sette down to a consistent grind level, after which it is fine.

4. I have been repeating this for several weeks and have observed this fact pattern consistently. In that time I have removed, clean and refitted the burrs 5 times and this has made no difference. As far as I can see the burrs are both seated correctly and the lower burr spins evenly.

5. The only conclusion is that something is happening between GRIND 1 and GRIND 10 that is causing the coffee to grind more coarsely and the only obvious factor which is changing in that time is that the burrs are warming up.

6. Each time I have tested this issue I have used the same batch of beans for each test so level of roast, level of oils, freshness etc should not be an issue. I have also used various batches of beans an eliminated the possibility that a particular bean/roast is at fault.

7. Worth mentioning is that dust issues I had with the old burrs is gone - each time I grind I get lovely and consistent grounds.

So it is that the physical properties of the burrs change when warm such that they somehow move further apart? (if anything I would have expected the burrs to expand when warm and therefore move closer together = grind finer grind).

Or is it that putting room temperature beans in a warm grinder somehow causes them to be ground more coarsely?

I should note that, whilst it is perfectly possible that this was happening with the old burrs (which were quite dull), I can't test that as I have thrown them out. I had been using the machine with the old burrs for a couple of years without ever noticing similar consistency issues.

I have been in contact with Expobar directly (who supplied the new burrs) and whilst very helpful they have not seen this before and have been able to offer any suggestions other than to check that the locking pin on the dial is not slipping (it is not). They also confirmed that they do not believe the batch of burrs are defective as they have had no other issues reported.

Any suggestions gratefully received. I already know the obvious one - to buy a better grinder. Unfortunately not an option at the moment.

hamish5178
Posts: 187
Joined: 13 years ago

#2: Post by hamish5178 »

Not helpful, but, there's a reason you don't see any commercial machines with built in grinders. And yes, heat is known to screw with grind settings, even in stand-alone grinders. I think Schomer maybe did some writing about this. Just google around for 'burr grinder heat' etc.

donkeykong (original poster)
Posts: 2
Joined: 11 years ago

#3: Post by donkeykong (original poster) »

Thank you for your response - after some further research there's not much out there about the specific effect of heat on grind level - just that heat is universally recognised is BAD...

I am still stuck on the fact that it is not (just) that the beans which are being affected by heat: the physical distance between the grinder burrs is increasing significantly from cold to warm. As described above the zero point moves as much as 5 clicks on my dial as it heats up. That can't be normal.

The same burrs are used on nearly all Cunill grinders - has anyone else experienced this?

coffeesnob1
Posts: 63
Joined: 12 years ago

#4: Post by coffeesnob1 »

Your observation about the change in flow rate is correct, unfortunately your reasoning isn't. As the burrs heat up the heat from the metal is transferred to the beans which accelerate the oxidation of the grounds causing a "Staling" effect. The more heat that is transferred to the grounds, the more CO2 is given off in the grinding process, leaving less to be released during the extraction. CO2 has the effect of slowing the extraction rate - it repels the surrounding liquid and leads to an increase in flow resistance and thus slows the "speed" of your extraction. Hope that makes sense.

Greenbud
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Joined: 11 years ago

#5: Post by Greenbud »

Yes! I have the exact same condition with my Tranquilo. I wanted to grind each of my two 18g doses for my Classic as I needed them, but for the second shot, the grind was always too coarse, and inconsistently so, depending on how long I waited before making my second cup. Now I grind enough for two, stir it up with a spoon and weigh it into two portafilter baskets. The second basket sits on the counter with the tamper on top until I'm ready for it (usually under an hour). This seems to work just fine. I expect that if I need to grind an amount significantly more than 38g though, I'll have to move the wheel a notch finer.

My best guess is that as the machine gets hot, the grinder and its internal parts expand in such a way as to cause the burrs to move farther apart, resulting in a coarser grind. This is borne out by your experience and mine that when hot, the burrs touch significantly down the dial from their cold touch point. For me, that's about 1/5 of a turn!

I found your post because after following my routine so long I forgot about the temperature thing. Yesterday I tried to dial in a good grind after taking the Tranquilo apart and cleaning out the gook. It took me 3 or 4 diarrhea shots, jumping a few notches each time, before I remembered. Actually it didn't hit me until I stuck my finger in the hole and felt the hot burrs.

As far as the hot burrs causing any observable staling of the coffee, I think the coffee blasts past the burrs too quickly to pick up much heat. I'm no epicure, but as fare as staleness goes, it's easy to tell the difference between my own freshly roasted coffee (any variety) and beans bought from the store when I've been too lazy to roast and have run out. Besides, the burrs get hot to the touch, but not frying pan hot, or even boiling water hot. Just pain in the butt change the grind size hot!

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RapidCoffee
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#6: Post by RapidCoffee »

Greenbud wrote:I have the exact same condition with my Tranquilo. I wanted to grind each of my two 18g doses for my Classic as I needed them, but for the second shot, the grind was always too coarse...
How are you dosing the grinder? Do you grind with a hopper full of coffee beans? Dose 36g of beans, and then grind the two shots? Dose 18g of beans and grind, then repeat?
John

Greenbud
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#7: Post by Greenbud »

I had tried to do 18g at a time, but the two grinds were always different. Now I weigh out 37g (the grinder gods usually take a bit for themselves) into a little Tupperware glass which fits perfectly upside down into the hole on the top of the tranquilo. This prevents coffee from shooting out the top. I stir the grounds up a little with a spoon, then weigh them into 2 baskets.

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RapidCoffee
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#8: Post by RapidCoffee »

Greenbud wrote:I had tried to do 18g at a time, but the two grinds were always different.
I don't believe burr temperature has much, if anything, to do with the coarser grind in the second shot. IMHO it's all about bean load.

The first 18g is ground with a bean load in the hopper. The second 18g has no bean load. Even a small bean load makes a noticeable difference in grind setting.

In general, single dosing the hopper requires a much finer grind setting, compared to the grind setting when the hopper is full. That is why your second shot is ground coarser.

Mixing the grinds obviously works, but you may get better consistency by either maintaining a bean load in the hopper, or single dosing each shot. If you choose the latter, be prepared to adjust the grind setting significantly finer.
John

Greenbud
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#9: Post by Greenbud »

The two 18g grind sessions were performed exactly the same way: 18g beans in the top-grinder on-grinds fall directly into portafilter. The only variable is the grinder temperature.

The stirring is part of the new routine, which works very well.
In general, single dosing the hopper requires a much finer grind setting, compared to the grind setting when the hopper is full. That is why your second shot is ground coarser.
The size of the ground coffee cannot possibly be any larger than the distance between the two burrs, whether there is 18g or 1800g of beans pushing from above, at least in this universe. Perhaps a greater force from above might reduce in some small way the amount of fines produced by those last few beans bouncing around a bit before going through, but my observation is that the coffee feeds itself steadily into the grinder right up to those last few bean fragments.

Thinking about it, it's probably not just the burrs, but also the motor shaft and the aluminum housing that holds the stationary burr that are participating in the expansion.