The end goal is a one ounce drink produced around 28 seconds. The diminished dose of a single requires a finer grind so an adjustment on the grinder is needed, but before you can adjust the grind you must find a starting dose. One of the critical points is to get the proper headspace (the air space between the top of the puck and shower screen). If you overdose a single you may not be able to lock the portafilter into the group or the surface of the puck will scrape against the shower screen. That will crack and disturb the puck that you spent so much time preparing.
One trick that can be used to gauge your dose starting point is to use a nickel as a gauge. Dose into your basket and tamp, then place a nickel on the surface of the puck and lock your portafilter into the group. Then remove the portafilter and look at the puck. If the nickel is pushed deeply into the pucks surface, or there is any cracking of the puck, your dose is too high.
To demonstrate this, and assist myself in finding a starting point for the single dose on the VBM, I decided to use this method. I started off with a 10 gram dose in the stock single basket. I distributed my dose by tapping my finger against the side of the basket. I also tap my basket down against my tamping stand to further level and settle the dose. Tamp, put my nickel in the center of the puck, lock it in and remove.
As you can see, the dose is too large. The shower screen did not contact the puck but the nickel was press deeply into the pucks surface causing radial cracks in its surface.
Using the results from the 10 gram test, I dosed down to 9 grams for the second try. Once again I prepared my dose and placed the nickel in the center of the pucks surface and locked in the portafilter. This time the nickel was slightly indented in the surface of the puck. I decided to use 9 grams as my starting point.
I continued to down dose without the nickel to see how low I could go. I continued to reduce the dose by .5 gram increments until my tamper was bottoming out against the concave sides of the single basket. At 8 grams my tamper was bottoming out against the sides of the basket. I was still getting a leveled tamp but it was not getting fully compacted by the tamp. You will feel when the tamper is hitting the basket instead of being stopped by the puck.
I decided on a 9 gram dose for the Ambrosia blend I was using. An 8.5 gram dose would also work but I was very, very close to bottoming out the tamper. You could also do like the Italians and grind extra fine and use a very light tamp just to level the puck.
I find the single shots from the Domobar Super to be comparable with other E61 vibratory pump machines. The singles tasted a little sweeter and brighter than the doubles with a creamier mouthfeel. A good shot and well worth the work.
