Why do my second brews fail?

[metaljong]

Almost every time I make a second brew too soon after the first, it fails.

Hardware:
Wega Mininova Classic
Wega 5.8 grinder (same as Compak K3 Touch I think)
Bottomless portafilter basket, maybe a Synesso

I make the first brew, ~18g of coffee for ~18-25g of coffee depending on the grind and brew time.

If I wait for long enough, maybe 10 minutes, the second brew is fine. If I make a second one immediately after the first, it's as though I've ground much finer or tamped much harder or something's blocked. None of those are true. What I usually do is clean out the basket and put it back in place to stay warm then drink the first ristretto then straight onto grinding the second. All the variables are the same. The machine should be more than powerful enough to make a second cup and the pressure is straight back up there as soon as you shut the water off again.

The only thing I can think of is that the first grind goes into a dry basket and when I put a second grind in it's usually into a wet basket and maybe the first dry grinds hitting the wet basket block the filter holes? But I'm not sure because I think that if I wait the 10 minutes it's sometimes also still wet (maybe?, I can't say that for sure).

Any tips to eliminate this annoying trait? I failed twice this morning. First brew great, second brew about 5g of coffee from ~18g of grind after 30 seconds, third brew 18g of coffee from ~18g of grind after a minute and it sputters and drips and just doesn't come out.

I've had the machine for a year and it's always done this. If I have guests for coffee I can't do a run of them in a row and it's not really feasible to wait 10 minutes between brews.

[gibberblot]

Probably the wet basket. This is a known cause of channeling. I just dry my basket between shots.

[lucasr53]

From my non technical background or knowledge I'd suggest it's a pressure problem. A wet basket would cause channeling ie run faster not slower. So maybe the pressure in the machine needs time to build up again? Not sure with ur machine but might suggest a problem there.
Another option. .. grinder. Does your grinder retain a lot of grinds? If so you could actually have your grinder dialed in for old grinds. So when you make one straight away it's 18g fresh grind whereas before it was say maybe 14g fresh n 4g old. To test this theory, Before your first shot, throw out the first part of the grinds and then grind your 18g for your shot.
Off the top of my head that's what came to mind. Hope that helps.
Also dry your basket before dosing so you can take out that variable.

[metaljong]

I'll start with drying the basket, because that's the only thing that changes.

My grinder stays the same. It does retain grounds but I don't change the setting at all except to "ride" the burrs a bit like other Compak users do (there's a small amount of play in the grind scale setting that you can use to finely adjust on the fly without actually changing the grind setting). This has a very small but useful effect but I can ignore it for the purposes of this problem because it doesn't affect a brew after a long enough pause.

The machine should be more than good enough but yes, it feels like a pressure issue. Like even after a minute it still is struggling to produce anything besides a few drips and maybe spurts, as though I've suddenly ground waaay finer and the machine can't handle it. But the machine should handle handle it no problem. It wasn't cheap, has a good rep. and is a quality brand.

[MrJack]

Dry you're basket ( I'm really surprised you don't actually). Out of curiosity, you you clean the shower screen between shots?

As to what lucasr53 said - do you purge the grinder when you finish, or before you start? I always start with an empty grinder.

[MrWhite]

Try rinsing the basket then drying before the next shot. If I only wipe between shots I get the same effect.

[Bames]

How do you know your dose is that same? Are you weighing your dose?

[Journeyman]

Is it a single boiler machine? Apart from the wet basket, I'm wondering if, even though the pressure climbs again straight away, it is showing steam pressure not water? i.e. the water isn't enough for a while to give a proper shot and the top of the puck gets clogged before enough water comes through?

I guess you need a mininova user to comment - is there a specific mininova thread you can post your question?

[smokey]

My thoughts are that the water is not hot enough, do you have a thermometer to check? There may be a fault in the boiler.

[metaljong]

I tap out the puck, rinse the basket then just put it back.

I don't clean the shower screen until the end of the morning, by which time two or three double shots have been run through it. If the time has been long enough since the last shot I do purge the superheated water out of the head.

I don't purge the grinder, so my first shot of the day will have some stale grounds but the next one or two will be fresh.

I weigh with some 0.1g kitchen scales that sit under the machine. Weigh the grinds, weigh the shot. I aim for ristrettos with 18g of grind and somewhere around 18-25g of shot in 25-30s depending on what I feel like and how the pour is looking. When I get annoyed and things aren't working, like this morning, I will occasionally let it go for a minute and have a nasty shot just for the sake of it. But that's what motivated this thread - it means things are really bad if it takes longer than 30s.

Machine is a single boiler HX E61 machine, tank, vibration pump, RRP ~$3000 so a fairly substantial machine.

The gauge might well be measuring steam pressure (0.9bar I think) but it does decrease noticeably (not heaps, but it's not steady) somewhere in that 0.8-1.0 range when pulling a shot. I never look at it since it's "steady enough". E.g. it might go from 0.95 to 0.85 bar, I don't know. I would be confident in saying it doesn't go below 0.8 bar. Maybe I'm wrong. It heats up quickly in the morning so I don't think the boiler has a problem. It might reach temperature and pressure in a couple of minutes, but I normally leave it for about 20 minutes until the head is superheated.

No thermometer but if drying the basket doesn't work I will think about getting one.

[Journeyman]

You could try grinding a shot that gushes, then do another one at the same too-coarse grind. If THAT blocks then it would kinda have to be the basket wouldn't it? i.e. with a gusher you're not asking so much in terms of pressure from the machine. Just an idea...

[MrWhite]

I don't purge the grinder, so my first shot of the day will have some stale grounds but the next one or two will be fresh.

A good purge before your first shot should help the consistency between the first and second shots.

[MrJack]

You don't need to wait for pressure to build up - it is provided by the pump...

My money is on the dose/grind bei g ok for stale coffee, but not for fresher coffee.

Personally I flush the group and wipe the shower screen (if required) every shot.

[metaljong]

By how much should I purge the grinder? If I'm grinding three 18g doubles a day my main concern is throwing out say 10g, which is a fairly substantial cost over time.

The thing with your logic MrJack, is that if I wait say 10 min+, it's all good. It's just if I brew, drink then immediately make another that it fails. Say 2 min.

[Journeyman]

Purging might be cheaper than what is happening now - how much do you lose on choked shots?

As for old grinds, I keep a wooden kebab skewer beside my grinders. At the end of grind I stick it in the spout and wiggle to get the last of the grinds. (I wiggle the STICK, not me, OK? ) It reduces the amount left in there considerably and it only takes a little practice before I could stop the grind short, get the extra and still be at 15g - 16g in the basket.

Not sure if you have a doser grinder or not but maybe it could reduce the waste?