I'm having trouble with dosing and channeling.

If I dose around 18-19 grams, everything is Okay but when lower the dose to 16grams or lower, everything is falling apart. Lots of channeling, bigger and bigger channeling holes in the puck surface the less I dose, more soupy puck and uneven surface, looks like a little storm has hit the coffee.

When I dose 19grams, it looks good with no obvious channeling, and the surface is even and with a small imprint from the screen.

I thought it should be easier to dose low and get a nice extraction?
Im using a Isomac Tea with bottomless portafilter.

IMO, e61 machines have a no man's land of doses for various combinations of basket and screen.

My theory is that a large dose is held together by the screen and a low dose by water pressure. The no mans land occurs when the puck starts out against the screen and as the coffee is extracted, the pressure from the screen drops causing channeling.

IMO, it's also important to pull more that one shot after making grinder adjustments. The first shot seldom matches the second.

jonny2 wrote:bottomless portafilter.

If this also implies a gigantic triple basket, try a different/smaller basket when using a lower dose.

HB wrote:It also would not hurt to double-check the brew pressure; if it's higher than 9.5 bar against a blind basket, the margin of error is noticeably reduced.

The brew pressure gauge shows 11-10.5 when using a blind basket. Is this fine?

Puck analysis aside for a moment, how does the espresso's taste change with lower doses?

It get more sweet and bright but still cant get rid of lots bitterness.

not insult your intelligence, but

1.) how fresh is the coffee?
2.) do you normally get sweet shots from this blend/SOE?
3.) have you tried other blends/SOE's with similar results?

i also agree with the comment about headspace. it might be time to try out the old "nickel trick" - place a coin on top of your coffee cake and lock in the PF. you can adjust your dose volume until the coin just slightly makes an impression in the cake. this should help get your headspace in the ballpark. you may need a smaller basket to get good results with the desired (lower) dose.

~j

If your tamper is fractionally smaller than 58mm and fits fairly snug against the side of basket then try using a tamper that is 1 mm smaller. I have "57 mm" and a "58 mm" tampers. I can't get a tight tamp with the 58 mm tamper when my dose is below 17 grams and have experienced problems similar to yours (using Isomac Tea and LM double basket) when trying lower doses. If I switch to 57 mm then I have no issues.