When I first steamed milk I did not think about what was really involved in the physics of the process. Water is heated in the boiler and vaporised under pressure. (In many common lever espresso machines the gauge pressure is usually not far from 1.0 bar.) This steam is passed through a valve and a tube and exits through an orifice or orifices. For optimal steam production the orifice or orifices need to be of such a diameter that the steam exiting the orifice(s) is equal to the amount of steam produced by the heater. If the diameter(s) be too small the pressurestat of the machine will turn off and on. If it be too large, the pressure in the reservoir will fall until the the amount of steam exiting matches the amount of steam produced.

There are engineering tables and programs to compute all of this. A convenient on line calculator can be found here:

http://www.tlv.com/global/TI/calculator ... ifice.html

The equations involved are non linear.

There is a SINGLE optimal nozzle diameter for a given heater power and pressure. For a single orifice nozzle, a 1000 watt heater, and 1 bar pressure the optimal diameter is about 1.4 mm. Optimised single orifice tips will deliver the steam at higher velocity than optimised multiple ones.

For a steam tip with n holes, each hole will take 1/nth of the steam produced. If the hole size were computed correctly in the first place plugging one or more will reduce the delivered steam.

The easiest way to measure the diameter of steam tips is with a set of number-letter drill bits. Their diameters are odd numbers in any measuring system because the series increases by a more or less constant fraction as you go through the series. One can find the largest drill that will fit into the opening(s) on the tip and look up the diameter of that drill. (Use the chuck end of the drill bit for this, not the drilling end.)

Optimised single hole tips with a 1000 watt heater at 1.0 bar will foam a lot of milk and heat it very fast. If one be careless one can also have milk flying everywhere.

Lots of of people have made single hole tips for the La Pavoni

the most used is 1,5mm (the reason could be as simple as most drill sets come in 0,5mm steps . like 1 1,5 2,,2,5 etc ,,so no 1,4)

If you calculations are right (or close enough)
that would mean that the boiler would run out of steam with a 1,5mm hole

BUT only after some time

The big Q is

Will the bigger hole steam the milk faster , but before the pressure drops too much

MORE energy must be applied to the milk ,with the bigger hole , in less time

BUT the velocity of the steam would sink slowly

Have you had this in mind when you made the calculations ?

not that i think a 7,1% bigger hole would make that big of a difference

If the hole be too large the pressure will drop a bit. The calculations are fairly complex, but I know from experience with La Pavoni's 1000 wat heater that 1.5mm does not result in too much of a drop from the 0.9 bar to which it is set. Higher velocity steam certainly speeds the process, and seems to make finer foam rather than large bubbles. With a slightly too large hole one gets initially higher pressure for the foaming phase, and then during the heating part of the process the pressure is not so high, and that is fine.

There are two types of drill sets available to make fine adjustments. There are some sets with small bits every 0.1 mm with the really small ones every 0.05. Then there are the bizarre number-letter drill sets that many like for drilling holes for tapping. REALLY small drills seem only available in this system.

That little online calculator is fun. Thanks for posting it, Robert. I'm also glad to see you point out the physics here - that wattage determines the rate of steam generation. It seems like conventional wisdom is to look at the steam boiler size, which I think would be of far less importance.

Cmtwgr wrote:If you calculations are right (or close enough)
that would mean that the boiler would run out of steam with a 1,5mm hole

Re the effect of going from 1.4mm to 1.5mm on the tip orifice - if you use the calculator you will see that 1 barG with a 1.4mm tip produces the same rate of steam as does 0.75 - 0.80 barG with a 1.5mm tip. So going up that small amount will cause the machine to settle in at a 20-25% lower pressure. Bigger effect than I would have guessed.

The Q is ,,WHEN does it settle ?
before or after we are done foaming the milk ?

IF you have 10 liter boiler . and have it at 1 bar , shut of the heating element ,
then it wouldn't be worried if a had enough steam power for 1,8dl milk ;o)
even without a heat source

in the Pavoni Europiccola , we only have 0,8 liter of water
and sins we just did flush and pull a shoot ,, we even have less when we want to steam

and the pressure is 0,8 to 0,9 bar

I think it will have more effect to chose When to steam
if you start steaming right before the light goes out , then you would have max pressure , and the heating element would be on , and most likely stay on during steaming

worst case would be right before the light wanted to go on again
here we would have minimum pressure , and the heating element would have to try to keep up with steam demand


but my Q is more about

is it better to have a 1,5 hole at the start , that gives a lot of steam to push air into the milk
and then later because of the pressure/temp drop , you would have more time to roll the milk before it gets to hot ?

OR if the 1,4 having higher velocity at the roiling stage , should be preferred ??

In large boilers there is lots of heat capacity to provide steam for a long time without having to have enough electrical power input to match the tip size. In machines like the Europiccola we are in the situation where there is little reserve. I like the way my MCAL is now, I bored out the first one hole one to 1.397 mm using a #54 bit. With the 1.321 mm #55 bit the pressure held indefinitely at 1.1 bar with the valve fully open. With the 1.397 one it falls over perhaps half a minute to about 0.9 bar and stays there. (Calculations from the table seem to be quite accurate here.)

I like it better with the 1.397mm because it lets me have a bit more steam power right at the start, and decreases the time to heat the milk at the end. The steam is travelling at extraordinary velocity as it emerges from the tip. When air is drawn into the steam tiny bubbles form. Protein denaturation also occurs during this process.

It is amazing how just tiny changes in diameter have substantial steaming effects. To make steam tips close to ideal one either needs to have letter number bits around #55, or else one of the sets that has millimetre sizes starting at 1.0mm and goes up by 0.05mm. One must be VERY cautious drilling with tiny bits to avoid breaking them off!

Small drills needs high rpm's

no Normal lathe has enough ;o)

if you go too slow (the feed not the rpm's) , some materials will work harden
too fast and you break the bit

keep the chip flowing , especially in stainless ,

the chip should carry away the heat

It is extraordinarily difficult to drill small holes in stainless because of serious work hardening! The bit must cut continuously or there is a serious problem. I suspect one reason why people use brass for steam tips is that it is relatively easy to drill small holes in it. Brass acorn (dome) nuts can be drilled to make steam tips provided the steam wand has the correct thread.

I did drill 5 Stainless acorn (dome) nuts using the same 1,5mm drill (HSS cobalt )
a small drop of cutting grease , and of i went
did mount the nut in the center chuck on the lathe with the hole outwards

did go into the threaded hole (M6) , and as soon as i felt contact , kept a steady pressure , that a bit tricky on 1,5 ton lathe , went through in one chip /didnt stop

afterwards i did debur it with a normal HSS 3 mm drill bit by hand /holding the nut and the drill with my hands
the reason for the 3mm was that it was easy to get into the threaded hole (minor 5mm) and wiggle it a bit

from the front i just barely removed the burrs , just so that a cloth would t catch the edge of the hole
a too chamhpfered hole would ruin the nozzle effect , and send the steam out in a cone shape instead of a jet,