Blowing machine

[Tunborough]

Some measuring devices are quoted as being accurate to +/- x% of full scale. If your 20 L/min flowmeter is accurate to +/- 5% of full scale, this means +/- 1 L/min all the way down, which does look plausible. It reports 3 L/min at the same pressure that the lower-range meter reports 4 L/min. Earlier, it was reporting 4 L/min at pressures that I thought might be more consistent with 5 L/min. The manometer has its own challenges at the bottom end: you were seeing air flow while it was still reporting a pressure of zero, although that would be just +/- 1 in the least significant digit.

[Terry McGee]

Indeed, I was going to make mention of that 1L/Min zero reading on the manometer. I guess we're just up against the low-end limitations of these low-price gauges. So what's your feelings - can we live with those limitations, or have we hit a wall?

And I think I've given you all the first tranche of measurements you asked for. Where do we go from here?

[Tunborough]

Indeed, I was going to make mention of that 1L/Min zero reading on the manometer. I guess we're just up against the low-end limitations of these low-price gauges. So what's your feelings - can we live with those limitations, or have we hit a wall?

Nothing we can't live with. For serious work, I don't see going much below 6 L/min ... only to play around with the wispy sounds at the low end of the range.

And I think I've given you all the first tranche of measurements you asked for. Where do we go from here?

I'm still struggling trying to get the full set of measurements to line up. The old Generation is a real outlier. Could you please check the size of the windway exit on it. Is it really a rectangle only 1 mm high and 7.85 mm wide? The theory says that 244 mm H2O shouldn't be able to push more than 30 L/min through a 7.85 mm^2 opening, not 32 L/min, and that's without any drag through the windway. Any possibility of leakage happening, or irregularities in the windway exit?

Before we leave this entrirely, I'd appreciate a set of measurements on the 10 mm x 4 mm calibrator. If you have any whistles with different windway dimensions, especially higher windways, they might help me find my way.

If you're getting bored with this phase but not with the blowing machine, and want to apply it to meatier work, I've an idea how you can measure the flow vs. frequency relationship without us having to share a lot of whistle geometry. More about that shortly...

[Tunborough]

Are you game for more?

My next question is: For a given note, what frequency do you get for a given flow? For some flows, you can get two different frequencies, in different registers, depending on whether you are coming up from a lower flow or down from a higher flow. For every flow (at least those higher than the wafty aeolian flows), you will get some frequency.

It will make it easier for me to digest the numbers if, instead of a real whistle, you use a whistle head and a length of tubing around 30 cm or so that fits into the whistle head. I'm not too concerned about the I.D. of the tube as long as it's known, reasonably constant, and the tube fits the whistle head. If you can do it again with a tube of 20 cm or so, so much the better. The process would be similar to what you did for viewtopic.php?f=1&t=114079&p=1258742#p1258742, with measurement of flow, pressure, and frequency, paying particular attention to the flow just before and after regime transitions both going up and going down. It would be nice to get up to the third register, but I'm not fussy about anything more than that.

The geometry measurements I'd want are: height and width of windway at the entrance and exit, length of the windway, length and width of the window, length and I.D. of the tube, measured from the soundblade.

How does that sound?

[Terry McGee]

Just to mention I've edited the 30 by 4mm calibrator above to 30 by 3.94mm. Not that that is likely to matter much compared to the other sources of error!

[david_h]

Swapping the flow metres: Residuals from a straight line fit have a similar pattern. So whatever causes the pattern is probably not manufacturing differences between the meters.

20 l/min meter below 5 l/min: Points have a different trend to those above 5 l/min and it doesn't go through the origin. Points from the 5 l/min meter closer to overall trend, but still not going through the origin. Suggests meters not very good at bottom end of range and different trends below 5 l/min probably to do with the meters rather than flow dynamics.

Resistance. As Tunborough points out meter accuracy could be % full scale. That would fit what is seen. So pressure v flow relationship less reliable at lower flows. To summarise Resistance in one number either fit a line to the portion above 5 l/min or take the average of only those at higher flow rates - say 8 l/min and above. Fitting a line is dead easy in spreadsheets.

At Resistance of 0.45 (fitted as above) the 30x 3.94 calibrator is nicely in the windway range.

[trill]

. . . Here we go . . .

Terry . . .More when I get it done.

Well, I tried.

I was hoping to be able to tell whether the flowmeters are calibrated for air or oxygen from those 3 data points.

No dice. Beyond me.

Thanks for the indulgence. Sorry to have wasted your time.

FWIW: I have a query into the mfr of your flowmeters. Got an answer from their Amazon rep: "they'll get back". No word in a week.

[trill]

I am scratching my head about this correction equation now

*IF* Terry's flowmeters are calibrated for oxygen, then:
rho_scale = 1.331 kg/m^3
rho_new = 1.205
(https://www.engineeringtoolbox.com/gas- ... d_158.html)

The formula above gives a simple multiplicative correction of about 5%.

From that if the 'new' gas was denser than that for which the meter was calibrated the meter would be under-reading - the float would not be lifting as much. Is that right?

I think it is the reverse: the meter would over-read. The correction "FACTOR" above would be downward (less than 1.0).

*IF* Terry's flowmeters are calibrated for oxygen, the "new" gas (air) is less dense.

*IF* Terry's flowmeters are calibrated for oxygen, the true flow rate of air will be about 5% higher than indicated.
*IF* Terry's flowmeters are calibrated for oxygen, at 20lpm (indicated on meter) the true flow of air would be 21lpm.

The .3lpm difference I measured between air/oxygen at 6lpm fits right with that 5% figure.

[trill]

It appears to me that there are 4 of us participating in this thread that have a quantitative interest in flow, pressure, and sound (Terry, Tunborough, david_h, and myself).

Terry is generously contributing his shop, time, and expertise.

I would like to propose that the remaining 3 of us contribute to the purchase of a new meter for Terry, calibrated for air:
https://www.amazon.com/cjc-Digital-Test ... B0BG78ZDZH

Comes to about $90USD each.

I would happily contribute that.

[david_h]

From that if the 'new' gas was denser than that for which the meter was calibrated the meter would be under-reading - the float would not be lifting as much. Is that right?

I think it is the reverse: the meter would over-read. The correction "FACTOR" above would be downward (less than 1.0).

Yes that's what I was thinking - hence the question. If so that equation is giving a factor to apply to tell us what the meter would, erroneously, read. So divide by the factor not multiply. The stuff inside the square root sign is using the 'ideal gas law' to work out new_density/calibrated_density

My guess is if Terry's meter doesn't say what it's calibrated for then it will be air at 20C or not regarded as accurate enough to matter.

[Terry McGee]

Tunborough asked further up: "Any possibility of leakage happening..."

I wondered how I might check for leaks, then remembered that the so-far unused part of my rig is a Magnahelic Flute Leakage Detector. D'uh!

Now, I think I'm right in saying we needn't be concerned with leakage before the flow meters, as that is merely wasted energy, it can't influence the readings? (Unless it got so bad we didn't have enough pressure and flow, but we have heaps of spare capacity.)

So, I set up the Magnahelic in the usual way, and for interest's sake, plugged it into the 5l/min flow meter, whereup it read just over 1L/min.

I then connected it to the tube feeding the two 20L flow meters. And turned the second one on to make sure that it's included in the leakage test. At that point the Magnahelic dropped from reading 0.8 (it's usual set up point, don't ask me why!) to about 0.6. And the SCFH flow meter dropped from 1 to about 0.65. So they're both sensing some flow but with noticeable resistance.

(A reading of 0.6 on a flute would be regarded as extremely leaky flute, pretty hard to get a note out of.)

At this point the balls on the flow meters rise ever so slightly off their bases. Sensing the same flow as I had measured on the 5L, but way below the start of their scales.

Up to this point there was nothing plugged into the whistle connector, so the small amount of air coming through was wafting away harmlessly. And the Digital Manometer read zero.

I then covered the empty whistle connector output with a wet thumb. The Magnahelic and its SCFH flow meter both plummeted to zero. (Zero means zero leakage, ie a perfect flute.) The balls on the 20L/Min flow meters dropped to their bases, but sat there flicking a little. I think this is due to pressure waves from the aquarium pump still trying to shift air.

The pressure gauge (Digital Manometer) is now winding up, and settles after 5 or more seconds to around 850mm H20.

When I remove my thumb, there's a whoosh of pressurised air lasting about 1 second. The two flow meters jump up above mid scale, but then settle back to their virtually nothing. The pressure meter goes back to zero. And the Magnahelic meter and SCFH flow meter return to their previous levels.

I can pretend to be a "slightly leaky flute" (about 0.1 on the Magnahelic) by lightly covering the whistle connector output. The Pressure meter now reads around 240mm.

All of that implies to me that we have no leaks. But happy to run any other tests you can think of!

[trill]

. . . pressure waves from the aquarium pump still trying to shift air. . .

What does Auto-Tuner read on the fish pump ?

[Terry McGee]

I'm still struggling trying to get the full set of measurements to line up. The old Generation is a real outlier. Could you please check the size of the windway exit on it. Is it really a rectangle only 1 mm high and 7.85 mm wide?

I checked and confirmed that the windway exit height along the sides is 1.185, while the height in the middle is 1.13. We do have to keep in mind we're working with cheap moulded plastics from the seventies here. This is a whistle that I had trouble getting the head off, and subsequently had to sand out a fair bit to get the body tube back in. So we're seeing a shrunken product. Windway ceiling droop has set in!

We're now down in the levels of precision that the calipers don't agree, and I had to resurrect the micrometer to recheck my "feeler-gauge" drill bit diameters. This whole experience is a trip down memory lane!

The theory says that 244 mm H2O shouldn't be able to push more than 30 L/min through a 7.85 mm^2 opening, not 32 L/min, and that's without any drag through the windway. Any possibility of leakage happening, or irregularities in the windway exit?

As you will have seen above, I'm pretty sure leakage isn't an issue, although it remains a risk, particularly where whistle heads are involved. Do the new exit heights help explain? Keep in mind that the rest of this windway is much higher, so we are approaching orifice plate technology here!

Before we leave this entrirely, I'd appreciate a set of measurements on the 10 mm x 4 mm calibrator.

Here we go:

Code: Select all

10 x 3.93mm Calibrator			
Flow	MM(H20)	√A/P	Resistance
0	0	0.0	0.00
4	3	1.7	0.43
8	9	3.0	0.38
12	22	4.7	0.39
16	41	6.4	0.40
20	68	8.2	0.41
24	85	9.2	0.38
28	131	11.4	0.41
32	164	12.8	0.40
			
Average Resistance	0.40

Usual shapes. Interesting to compare the "average resistance" of the 30 by 4 calibrator (0.41 to 0.43) with that of this 10 x 4 calibrator (0.40). Three times longer but not a significant rise in "resistance".

If you have any whistles with different windway dimensions, especially higher windways, they might help me find my way.

I'll look into that.

If you're getting bored with this phase but not with the blowing machine, and want to apply it to meatier work, I've an idea how you can measure the flow vs. frequency relationship without us having to share a lot of whistle geometry. More about that shortly...

OK!

[Terry McGee]

. . . pressure waves from the aquarium pump still trying to shift air. . .

What does Auto-Tuner read on the fish pump ?

Heh heh, it hears the local mains power frequency, 50Hz, G1 + about 34 cents. From memory, it's one of those solenoid plus vibrating reed pumps. Enough to keep your aquarium aerated and to feed a Magnahelic, but not enough to blow a whistle. (I tried, just for fun!)

The pulses I mentioned are much lower, more in the metronome than the tuner realm. Except it would have to be a calypso band metronome.

Remember Philosophic Pitch? Sometimes called Scientific Pitch. It resurfaced in the 19th century to help confuse an already confused world even more. It was based on C = 256Hz, about A430. It's proponents, who included Verdi, argued that it was philosophically correct because it was a direct binary multiple of one second. And therefore could be traced back to the speed of rotation of the heavens. And you can't fight facts like that!

[trill]

. . . Heh heh, it hears the local mains power frequency, 50Hz, G1 + about 34 cents. . . just for fun! . . calypso band metronome. . .
. . . you can't fight facts like that!