I'm not an 'expert" by any means...just a regular user of vacpots over the last 3 or so years
FWIW, if you have a smooth-top electric stove, then you do not need to use a diffuser. I have used my little 20oz 5 cup Yama without any problems at all. I also collect usable vacpots from the '30's - 50's, and use the same stove with them. Again, I have not experienced any problems.
The 1st bit of advice I'll share is to use just water in it, without any coffee grounds a couple of times. I use a cloth filter with my Yama about 60% of the time, and a Cory brand or Silex glass filter when the mood strikes me.
When you are 1st getting used to 'how' the vacpot process works, you 'should' start timing how long the process takes. Your goal is 4 minutes + or - 15 seconds, Total Extraction time.
I begin by filling my Yama lower globe to the 5 cup ( 20 oz ) line with high quality H2O, wipe the bottom dry, and place it on the stove and set it to level 8 on a 10 scale. Assemble the upper globe with the cloth filter assembly, and marry the upper globe to the lower globe. Keeping a close eye on everything, when the water starts to push North into the upper globe, I cut the heat to level 3, and slide the vacpot 1/2 on and 1/2 off of the burner. Again, I use a smooth, glass top stove, it's not going to fall or go anywhere...
What happens next is that 80% of the water is now in the upper globe, and @ 200F + or - 5F. If you remove the vacpot from the burner entirely, within 30 seconds the Southern Pull will occur, and the water will vacuum into the lower globe. The goal here is to observe that everything is working (sealed), without adding in the coffee grounds.
Cloth filters are a little more forgiving than glass filters when it comes to grind quality. A glass filter will 'bounce' if the water is too hot which means that you have too much heat, and you need to control that, otherwise, you will experience a stall and a bunch of fines in the end cup.
I use my trusty little Rocky grinder to fill up 1/2 cup (volume) of ground coffee, the grinds are approximately the size of beach sand. It won't do you any good to know my grind setting #. (it's somewhere between espresso and drip) This is the variable that you need to work out for yourself. It's the one factor that determines the 'contact' time.
A typical time for me is (once the water is up into the upper globe and the heat is cut to say level 4) to then add in the grounds and start the timer, after 2 minutes, remove from the heat source, and time the Southern pull until the vacuum is finished. ( the bubbles that occur after the coffee is vacuumed dry) if your bubbles occur at the 4 minute (total extraction time), you are good to go. If not, then you need to make an adjustment, let the mix go to 2.5 or 3 minutes, if the southern draw is too fast. The coffee grounds and water are still in contact with each other the entire time. if the total extraction time is only 3 minutes, the result will be a weak brew, if the time goes to 5 minutes, it will be bitter.
Oh, yeah, another thing to watch out for is the roast level, a darker roast ( FC++) or Vienna roast is oily, and this will affect the time as well ( slower extraction times). The fresher the coffee, the more the coffee will bloom, add in the grounds slowly, stir them in with a spoon if your coffee has rested less than 5 days.
Once you taste the exceptional coffee that a vacpot can produce, it will all be worth it.
It took me around a month to really 'get it', and all the 'trouble' to get there is really an enjoyable experimental journey that will reward you for years to come. The guide that Mark Prince put together on Coffee Geek is worth printing, and using. And he explains things better than I do
Gary
