Temperature and brew pressure of second shot

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mathof
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#1: Post by mathof »

I've noticed that if I prepare two pucks at the same time and pull them in succession on my LP Europiccola, the second pull requires more force to push the water through the puck than the first. The water pressure in the boiler (c. 0.7 bar) is the same, as is the pre-infusion time (8 seconds). However, the group temperature is higher, 77C when I lift the lever on the first shot; 83C on the second. Therefore, the brew temperature will be higher on the second shot.

I assume what is happening is that the hotter brew water causes the coffee in the puck to expand more and this provides more resistance to the water as it is pushed through the puck (and consequently requires more force). Perhaps the coffee grinds expand further and this closes up the gaps between them. Or maybe something else is going on. Any ideas?

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drgary
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#2: Post by drgary »

There's been a lot written about this, so ...

If the machine has been idling at full temperature and you've flushed water through the group and wand you'll have gotten rid of pressurized air. If not, your first shot will be cooler and you'll be pushing air through it first, for a spongy feel. In machines equipped with a PSTAT they stop heating when they come up to pressure. With an air pocket the water won't be fully heated. If the machine is at full temperature, though, locking in a cool portafilter can cool the group so the second shot is the same temperature as the first.
Gary
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mathof (original poster)
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#3: Post by mathof (original poster) »

drgary wrote:There's been a lot written about this, so ...

If the machine has been idling at full temperature and you've flushed water through the group and wand you'll have gotten rid of pressurized air. If not, your first shot will be cooler and you'll be pushing air through it first, for a spongy feel. In machines equipped with a PSTAT they stop heating when they come up to pressure. With an air pocket the water won't be fully heated. If the machine is at full temperature, though, locking in a cool ksportafilter can cool the group so the second shot is the same temperature as the first.
Thanks for replying. I use Robert Pavlis' method of expelling air from the group through the steam wand while venting "false" pressure, which seems to eliminate spongy pulls. My question is not about temperature control of the machine, but about the effect of different brew temperatures on the resistance offered by the puck to pressure exerted by the piston. It seems that higher pre-infusion and/or brew-water temperatures increase resistance.

I am curious to learn if this observation valid. And if so, what explains it?

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drgary
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#4: Post by drgary »

Matt:

Just to double check, do you flush water through the group itself or only through the steam wand? If you're not flushing water through the group you're not using rpavlis's method.
Gary
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mathof (original poster)
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#5: Post by mathof (original poster) replying to drgary »

What I do is after the pressurestat switches off the element, I open the steam valve and lift the lever while steam is coming of the wand and allow a small amount of water to come out of the group before closing the steam valve. After that, measured by the digital thermometer I have attached to the group by a K-type thermocouple, there is a rapid rise in temperature which I use as a guide as to when to raise the lever to pull the shot.

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drgary
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#6: Post by drgary »

Thanks.

I wonder what would happen if you were to put the same amount of identically ground coffee in a glass, then fill one glass with hotter water than the other. Use just enough water that the grounds entirely absorb it. Time how long it takes for the coffee to absorb the hotter versus cooler water. That might answer your question.
Gary
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mathof (original poster)
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#7: Post by mathof (original poster) »

This morning, I accidentally flushed extra water through the grouphead just before inserting the portafilter. I then cooled the group with a bowl of water to my usual temperature. But subsequently I found the pull tighter than normal. I suppose that there is not a linear co-ordination between how hot the cylindar is that contains the brew water and the readings I take on the surface of the bell.

The initial venting of air from above the piston allows steam to heat the top of the group. The second helping of air comes through the dry coffee puck when the lever is lifted for the pull. This air is below the piston and is apparently vented by the first drops of water vaporising as they enter the cylinder and thereby driving the air out through the puck before it is sealed by the rest of the water passing through it.

I now suppose that what accounts for my initial observation is that the hotter cylinder on the second pull - which does not show up on the surface temperature reading when I cool the group - vaporises the brew water more efficiently and thus drive more air out from below the piston.

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drgary
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#8: Post by drgary »

Good thoughts, Matt. It's one of the reasons why I discussed what happens when a machine has been idling at full temperature. These days I take advantage of the warm-up phase to pull a cool shot of a dark, chocolatey roast for my first morning capp. Then I move on to the lighter roasts.
Gary
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mathof (original poster)
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#9: Post by mathof (original poster) replying to drgary »

Sounds tasty, Gary. Now that I've got the LP-8 more or less tamed, I intend to move on to find out what works best for various coffees.