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Don Taylor

Constant pressure source for a tuning table

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On another thread Dana and Chris refer to constant pressure tuning tables/devices that they both built out of 6" duct blower fans.

 

This sounded like a great idea but I thought that I would try using a computer fan (an Arctic P12) in the hope that this would pull enough air to do the job and yet be quiet and not need an A/C connection.  It is also cheap and I already have a controllable voltage D/C power supply.

 

The Arctic fan is rated at over 50 cfm but it does not do the job.  I am now looking at other fans, including duct fans, but before I buy another fan can anyone tell me what sort of air flow is needed to sound the reeds?  A particular product recommendation would be great.  How about this one?

 

A picture of one of these constant pressure tuning tables would also be nice to see.  I am envisioning simply duct taping the end of a duct fan directly onto the bottom of table top of a tuning table.

 

I have made a regular tuning table using a cheap eBay Chinese 20B as the bellows donor and while that works to sound the reeds I am having difficulty getting a reproducible tone out of the reeds.  I am not confidant enough to actually tune anything yet.  I suspect that my problem is because the air flow varies enough to push the reeds tone about by several cents.

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I don't have an electric fan, but I'm getting much more consistent results since I added a vacuum gauge to my tuning bellows so that I can measure the pitch at (close to) the same pressure every time. The one I have is a 0-500 pascals (that's about the same as 0-2" of water) Dwyer Magnehelic gauge.

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12 hours ago, Don Taylor said:

The Arctic fan is rated at over 50 cfm but it does not do the job.

Hi Don, the key specification is exit pressure, not volume flow (cfm).  None of the axial fans you link to can push air with enough force in order to develop the pressure you need.  For tuning, you'd like at least a couple inches of water pressure, and those axial fans can deliver only tenths of that.  You need a centrifugal blower, or a regenerative blower for tuning.  If you want to power the reed with a full range of possible bellows pressure, get one that has a spec for maximum exit pressure (at zero flow) of at least, say 6 inches W.C..  The axial fans you link to do not even have a spec for exit pressure.  So look at blowers with a spec that specifically tells you what maximum pressure they can deliver.  

 

If you want an elaborate setup, you can install plumbing from pvc pipes and two three-way ball valves, which will allow you to supply air flow in two directions, if you're tuning accordion style reeds that have two tongues, one for each air flow direction.  And you might also look into a speed controller, like what Dana uses.  

 

Best of luck,

Tom

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5 hours ago, ttonon said:

the key specification is exit pressure, not volume flow (cfm)

Will the exit pressure depend upon the size of the hole made for the reed holder in the table top?

 

If so then this is a very small hole compared to the size of the vents on any of these fans.  If I could buy a fan with a defined exit pressure then would I not have to calculate the change in exit pressure between the size of the fan vent and the size of the reed holder hole?

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My blower is for six inch ducts and is relatively quiet at the speeds I run it at, certainly not close to the volume of the reed.  I have a larger one that can reach a much higher pressure, but it is really overkill.  The squirrel cage type blowers freewheel at too low a pressure unless they are quite a bit larger than the centrifugal one here.  They are designed to move high volumes at low pressure.  On mine, I attached it vacuum side up beneath a hole in the table that I clamp my tuning jigs over.  These blowers are quite commonly used in long runs if ductwork, and should be available from places that stock equipment for plumbing and heating companies.  I got mine from MSC  more than 20 years ago.  ( mscdirect.com). I set a date check tuning pressure with a digital manometer, though the  magnahelix gage did well for me for years.  Since keeping the pressure relatively constant is the important thing, a gage of some sort is more important than the air source since like Alex, you can use ordinary bellows or other manual things as long as you can see the results of what you are doing.  I tune so many reeds that just leaving the blower on and flipping the lever operated flap valve if I need to temporarily shut off air flow,  make for a lot less effort over a day of tuning.

Dana

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Hi Don, pump performances are given by their "pump characteristic curve," which all have a characteristic shape shown in the attached figure.  You see that at zero flow (cut off), they produce maximum pressure.  That's because the power output is almost constant, equal to the product of pressure difference times flow rate.  As one increases, the other decreases.  That's a rough guide.  Even at zero flow, the pump is still putting out power, only it's all going into dissipation and heating the fluid.  

 

For a single reed, the air flow rate is extremely small, down around 10E-5 cfm.  Take that as zero.  Thus any fan you hook up to the reed will produce it's maximum pressure.  Even if you powered more that one reed, or more than a few reeds, it will still be essentially zero, compared to the capacity of most any of the types of fans commercially available.  

 

Bottom line is, yes, in general, the flow rate through from a fan will determine the pressure output, but in your case, the change in pressure for different size vents or number of vents is negligible.  

 

Best regards,

Tom

 

PS I did forget to advise to look for surplus or used equipment.  Many years ago, there was a huge supply of all kinds of fans and blowers for a fraction of full price cost.  I know that market dried up quite a bit, and I suspect a lot of it now goes overseas, like so much equipment that industries here in the States needed; that is, until so many of those industries went kaput.  But still, it's worth a look.

Pump Characteristic Curve.gif

Edited by ttonon
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Thanks Tom and Dana

 

I have a much better understanding of fan technology now and can see why an axial fan will never cut the mustard.

 

Being a cheap b****** I am reluctant to spend much on any product unless I know it is going to work for me.

 

So in the pursuit of cheap I tried an old vacuum cleaner (the type with a hose) fed by my router's speed controller set to minimum.  The router speed controller did not lower the speed enough but, in principle, this works.  I need to reduce the speed of the vacuum much further which will hopefully also reduce the noise from the motor. 

 

I let the vacuum bleed air so as not to over draw the reed, the reed sounded and I could read its frequency on my meter even with the vacuum blaring away.  The frequency was fairly steady considering I was just holding the nozzle of the vacuum up to the underside of the tuning table.

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