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Minimum Vacuum for Stack to Work

> I don't think I've asked this question before.
> A customer has an expression piano, make escapes me at the moment,
> basically soft and loud or normal and accent.
> I've had to work in the valves rebuilt by someone else that used
> inferior leather "donuts" on the stems to adjust valve spacing.
> I'm testing the valve chest, with the pneumatics attached, by itself -
> not yet connected to the tracker bar.
> So my question is, what would be the minimum vacuum to activate the
> valve to shut the pneumatic ?
> Thanks,

That's sort of a loaded question, so I'll qualify it first.

What should the minimum vacuum level of a test pump be set 
to before it is connected to a stack to test the valves? 

The answer is 5 inches. 

Another way to phrase the question is: How many inches of 
vacuum should be required to seat all of the valves in the 

Again, the answer is 5 inches. 

BTW, if all the valves seat at 5 inches, each individual 
note should operate easily, or collapse the bellows when 
the trigger tube is opened. In fact, I normally keep 
uncovering note nipples to see how many notes will activate 
before the combined losses of the valves changing state, 
the open bleeds to the atmosphere, and all other losses, 
are so great that no more notes will activate. Normally, 
I can get to ten notes before things stop working. The 
most I've ever been able to get is 27, but the stack had 
a 1-1/2" supply.

Now, let's get fussy! Once the valves have all sealed at  
5 inches, how low can you turn the vacuum level and expect 
any particular note to function? 

The answer is 3 inches. 

While I have rebuilt stacks that could function on 1-1/2 
inch of vacuum, they had to be 100% air tight with as little 
as 3 inches of vacuum. And the only stack that has operated 
that well is the modern Universal stack. BTW, at 5 inches of 
vacuum, 41 notes could be operated simultaneously, but they 
had to be opened (or turned 'on') one at a time.

In every test I've ever run, it took at least 5 inches of 
vacuum to create enough energy to activate a note on a piano. 
Naturally, the more notes you activate simultaneously, the 
more "air flow" you need. Notice I didn't say more 'vacuum'. 
But this is where things get a little technical. Let's say 
you have a 3/8" vacuum supply line connected to the stack. 
Only so much air can flow through that hose at 5 inches of 
applied vacuum. Increase the diameter of the hose to 1-1/2 
inches, and it's a whole other ballgame. That's because you 
have four times more air flow just by increasing the diameter, 
even though the vacuum level remains the same. To do more 
accurate measurements (and figuring), you have to know the 
exact cubic area of the striker pneumatic, the size of the 
intake and exhaust ports, and the transient loss as the 
valve changes state (which is a function of the valve gap, 
the pouch size, the well size, and the bleed size).

Now that I've gone through all of that, let me tell you 
how I check a stack. I connect up a length of hose, put it 
in my mouth, and suck. What I hope to feel as soon as I 
start sucking is 'the valves all seating' and then a tiny 
amount of air flow. If it takes less than a few seconds 
of sucking before my lungs fill with air... there's a 
problem. What I aim for is being able to keep sucking 
for at least 12 seconds before my lungs are completely 
filled with air. Basically, it's the same 12-second test 
that's used to test a lower section. And that all relates 
to the old adage that you should be able to pump up the 
lower section, stop pumping, and walk around the piano one 
time, and the music will still be playing when you get all 
the way around and sit down to start pumping again. That 
is a well rebuilt (or built) player mechanism.

Lastly, the question arises: How can I tell how much vacuum 
it takes to seal all of the valves? If you have a variable 
vacuum source, that's pretty easy. Simply connect the stack 
to the vacuum source and slowly turn up the level until all 
the valves seal. How can you tell when the valves seal? Again, 
that's fairly easy. By placing your hand near the exhaust 
port on the vacuum pump, you can feel when the air flow drops 
significantly. (You can also hear a change in the way the 
pump sounds, but that's a bit harder to explain because it's 
a subjective word, i.e., struggling.) Then, disconnect the 
supply from the stack (leaving the pump set at the point 
where the valves all sealed) and measure the vacuum level. 
Also, as you might expect, valves that are horizontally inclined 
will require a higher initial vacuum level to seat than valves 
that are vertically inclined. However, once they have all 
seated, you should be able to reduce the vacuum level to at 
least 7 inches and expect every note to operate flawlessly 
(independently -or one at a time).

Hope this helps.


John A Tuttle

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