Stating the steamingly obvious...
Thank you to those customers who responded to my milk steaming survey... there aren't many data points in the sample, so I'll try not to draw any conclusions that seem to be a bit of a stretch (I know there are actuaries and statisticians and general maths nerds on my mailing list).
The most obvious observation is that there appears to be a clear relationship between steam pressure in the steam boiler and speed with which a given volume of milk is steamed. There are many potential confounders when it comes to making such a simple conclusion... the factors which might change the speed of steaming for the same apparent steam pressure are:
1. Accuracy of the steam manometer - aka measurement error;
2. The length of the pathway from the steam boiler to the milk being steamed - the longer the pathway the less output steam pressure there will be;
3. The diameter of the lumen of the steam wand - the bigger the steam wand the more steam pressure will be applied to the milk; and
4. The nature of the steam wand tip - the easiest to spot difference is the number of holes in the tip which generally ranges between two and four. In general, the more holes the tip has the more steam hits the milk. The glaring exception to this is the Sproline Foam Knife... a tip with a single massive "hole" which is in the form of a long thin line across the tip. The Foam Knife produces ridiculously good textured milk.
The less obvious observation is that the machine with the highest apparent steam pressure of 2.1 bar steamed milk just over six times faster than the machine with the lowest apparent steam pressure of 1.0 bar. Comparing these two machines shows a six-fold increase in steaming speed for a doubling of steam pressure, the higher steam pressure machine is three times as efficient. That list of steaming speed confounders looms large!
Looking at all of this from another perspective, coffee machine price demonstrates a strong relationship to steaming efficiency.
One way of thinking about this might be that good (read expensive) engineering might line up all of the confounders in the right direction towards steaming efficiency.
Your mileage may vary... please let me know if you think of any other steaming speed confounders.
To read about lever espresso machines click here
To learn about the Scace thermofilter click here