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Thread: Giotto Rocket steam

  1. #1
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    Giotto Rocket steam

    Hi guys I feel that my Giotto could do with a little more poke in the steam wand when I make a single portion of milk it is fine but needs more umph to steam two coffees worth........has anyone successfully changed the tip or will that just make me run out of puff too early?

  2. #2
    Senior Member Gavisconi007's Avatar
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    How many holes does the tip have? When I had a Giotto I blocked one hole up with a toothpick trimmed down to be flush with the end of the tip. Worked a treat.

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    Quote Originally Posted by Gavisconi007 View Post
    How many holes does the tip have? When I had a Giotto I blocked one hole up with a toothpick trimmed down to be flush with the end of the tip. Worked a treat.
    It has two so you got more steam by closing off one hole? I was expecting to add holes?

  4. #4
    Senior Member Gavisconi007's Avatar
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    Block one off and the pressure out of the remaining hole will increase. Should solve your problem.

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    Just took the tip off gave it a good clean made a huge difference all good now guys

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    Quote Originally Posted by Gavisconi007 View Post
    Block one off and the pressure out of the remaining hole will increase. Should solve your problem.
    Erm, no.

    Block one hole and your *flowrate* will roughly halve (as the flow area has halved, but the pressure remains much the same). It will also possibly be slightly wetter, as the steam will spend longer in the wand.

    Steaming will definitely take longer.
    level3ninja likes this.

  7. #7
    Senior Member Gavisconi007's Avatar
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    I thought Pressure=Force divided by Area

    Force should be fairly constant due to boiler back pressure. The steam exits a smaller area, hence wouldn't the pressure at the tip exit point be greater given that we are dividing by a smaller (area) number?

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    Senior Member Barry O'Speedwagon's Avatar
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    I think the point is probably that 'steam pressure' is probably not quite the issue. It's a matter of how much steam is blasted into the milk in a given time (and the moisture content of that steam). So while the pressure within the steam wand may be doubled when you halve the aperture, the amount of steam exiting the wand isn't double. I may be wrong.

  9. #9
    Senior Member Gavisconi007's Avatar
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    The OP cleaned his steam wand tip in any case and my brain hurts. Plus it was 30 years since I did Physics. I'm off to have a coffee!
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    I have the same issue with my Giotto. Will have to give it a clean in the morning and see how it goes.

  11. #11
    Senior Member Gavisconi007's Avatar
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    Quote Originally Posted by mkerr2308 View Post
    I have the same issue with my Giotto. Will have to give it a clean in the morning and see how it goes.
    If cleaning doesn't work try my high tech pressure profiling mod. Definitely worked for me.

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    Quote Originally Posted by Gavisconi007 View Post
    I thought Pressure=Force divided by Area

    Force should be fairly constant due to boiler back pressure. The steam exits a smaller area, hence wouldn't the pressure at the tip exit point be greater given that we are dividing by a smaller (area) number?
    The force in this equation is due to collisions between molecules and a surface. Halve the area and you also halve the number of collisions (and thus the halve the force) - no change in pressure. Not an equation that is very useful when we are dealing with a flowing fluid.
    Quote Originally Posted by Barry O'Speedwagon View Post
    I think the point is probably that 'steam pressure' is probably not quite the issue. It's a matter of how much steam is blasted into the milk in a given time (and the moisture content of that steam). So while the pressure within the steam wand may be doubled when you halve the aperture, the amount of steam exiting the wand isn't double. I may be wrong.
    Pretty much. The flow through a hole is proportional to its area. When you open a tap, more water come out...

    How does this affect the pressure?

    The steam pressure in the boiler depends on how densely packed together the water molecules are (and their temperature). More molecules = more frequent collisions = greater force = higher pressure. Higher temp = faster molecules = more frequent collisions with more force = higher pressure.

    The pressure *in* the tip is only slightly lower than that in the boiler (due to energy lost from friction between the flowing steam and the piping). Halving the hole area will reduce the flowrate of steam, and thus slightly reduce the amount of energy that is lost = slightly less pressure drop = closer to boiler pressure (but always lower).

    The pressure outside the tip is essentially always the same as the surrounding air. What people think of as higher pressure is often just greater flow or higher velocity flow - which is all we care about with respect to steam (and temperature of course).

    Clear as mud?
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  13. #13
    Senior Member Gavisconi007's Avatar
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    Behmor Brazen - $249 - Free Freight
    Mr Jack

    Yes, clear as mud.

    In the second last paragraph of your thesis you say that halving the area will reduce the flow rate of steam. In your last paragraph you say that what people think of as higher pressure is often just greater flow. How can there be greater flow if the flow rate has reduced?

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