Is there any advantage in texture or any other quality to making microfoam with stronger steam power or does it just make the process faster (which may be important for cafes but hardly matters for home baristas). I've tried to work this out for myself by alternating steam tips, but I'm not consistent enough in steaming technique to obtain a satisfactory answer.

I've used many machines from anemic thermocoil through to 2-bar powerhouses. Yes, there's a difference. I find with 2-bar steam, it's easier to get microfoam, and the foam I get lasts longer. It's also more forgiving of tip placement. With my thermocoil machines, I had to maintain the tip in exactly the right spot. With 2-bar steam, the sweet spot seems to be much larger.

another_jim wrote:If you have lower pressure HX, use a nozzle with fewer/smaller holes to maintain maximal pressure and get easier microfoam.

But pressure is force per square area. Unless the tip holes are quite large, steam pressure should be the same from 1, 2, or 4 holes as long as the boiler isn't sufficiently depleted of steam under adequate pressure to require building it back up. More cross sectional area means more volume flow, which is why boiler pressure won't stay up for multiple drinks. But I think an ECM Synch at 2 bar should maintain the same pressure from 4 holes as from 2 for at least one or two capps. Even my Technika seems to do this with "only" 1.4 bar.

This morning I tried Jim's suggestion of using a much smaller tip opening to achieve better microfoam and it worked! Although it took a little longer, the final result was milk with much smaller bubbles, which allowed better defined latte art (and tasted sweeter too!). As to hole dimensions and location, I recently switched to a Weber Workshop Steam Dial, which essentially provides a curved slot facing straight ahead.

another_jim wrote:'m not sure about hole design; but I don't think it is only about ideal gas laws. The venturi effect and the relative orientation of multiple holes seems to create major differences in the steam exit velocity, and in the turbulence of the milk, so that different tips on the same machine can have a big effect.

I could be wrong, but I don't think that the number of holes can reduce steam pressure as long as the boiler is replenishing what's being expelled. See what you think of this explanation, Jim.

Orientation, bore diameter and shape, tip thickness etc all affect how the escaping steam interacts with the milk. But the pressure issue is a simple one because steam behaves as a fluid with regard to pressure. Steam pressure in a closed vessel is equal in all directions at all points, regardless of the shape of the container. So it's pushing with the same pressure against the inside wall of the wand as well as throughout the boiler space it occupies above the water level.......and at the tip holes.

Because liquids are incompressible, the surface of the water is as unyielding as the metal wall of the container in this regard - it's a solid resistance against which the steam is pressing evenly across it at X psi. The tip holes are just tiny leaks as far as the boiler is concerned. As long as the boiler keeps refilling and the heating element can make as much fresh steam as is "leaking", pressure remains the same throughout its entire contained volume as well as against every surface the steam touches. And whether there are 2 holes or 4, that steam pressure is still the same X psi as it is against the boiler walls, the surface of the boiler water, and the inner wand wall.

When it escapes through the holes, it behaves as a gas again and many things happen to it. As you point out, Jim, the venturi effect causes the pressure of the released steam to drop as its velocity rises - but this only occurs outside the wand as the steam leaves the enclosed space and is free to expand per the gas law. I've never seen a study of the escape patterns of steam from a tip hole, but I'm sure this varies greatly with the factors I cite above plus many more. But the pressure at the inner end of the tip holes has to be the same as it is elsewhere in the boiler regardless of the number of holes. If boiler pressure drops because the steam volume has dropped too low, pressure will still be equal everywhere inside the boiler and steam path. It's lower, but it's still the same throughout regardless of the # of holes.

If a low wattage boiler can only make a marginal volume of steam, too much escapes through multiple holes for it to maintain adequate pressure for foaming. So the pressure from a one hole tip will stay higher longer - but the volume of steam transferred to the milk will be lower in proportion to the flow rate. This means slower transfer of energy to the milk (thermal, mechanical etc). Adding more holes will just hasten depletion of steam.

David

another_jim wrote:...On the Silvia, same steam pressure, same 12 ounce boiler, but big holes in the steam tip, so you get a rush of lower pressure steam and mostly lousy microfoam.

Interesting you mention this. I've never used a Silvia but both popular YouTubers and coffee professionals Chris Baca and James Hoffman both have mentioned how good the Silvia is at steaming milk and it's the closest to a professional machine they've ever used on a consumer grade machine.

Early Silvia's used a tip that had "too many" holes. Rancilio switched to a single hole tip years ago. The single-hole tip is slower but works fine. However, the boiler's size means that the Silvia should only steam enough milk for two Italian-standard cappuccino's.

Plenty of people, particularly in North America, steam for longer as they use more milk, and plenty of Silvia boiler elements have burned out as a consequence. This was costly as (except for very early production) Rancilio integrated the boiler element into the boiler, so the whole assembly had to be replaced. In current production, Rancilio has switched to a boiler that has a replaceable element, and made the element itself tougher to handle North American (mis-)use.