Pouches and Bleeds
By Phil Dayson
In the past 18 months there have been numerous discussions on the relative merits of various sealing compounds, most of which were concerned with pouch subtleness/stiffness and long-term life. Although never mentioned, it was assumed that all of the compounds discussed were more or less equal in sealing ability.
Sparked by these discussions, I thought it might be interesting to actually seal a number of new pouches with different sealants, then measure the subsequent leaks over time.
A summary of my preliminary findings are listed below; all measurements are in cubic inches per minute.
Days
after Neatsfoot Silicone Mink Rubber
Sealing Oil Oil Oil Solution
0 52 57 59 53 (unsealed)
1 16 9 11 8
2 20 10 14 13
3 21 10 15 17
4 22 10 16 21
5 23 11 17 24
10 24 11 19 32
15 25 11 20 42
20 26 12 30 44
25 28 15 40 45
30 29 15 41 46
All measurements were taken using a pouch vacuum of 6 inches water gage. For comparative purposes a #71 bleed cup has a leakage rate of 70 cubic inches per minute under the same conditions.
I don't wish to draw too many conclusions yet, (30 days out of 20 years!), but the following does seem apparent:
- of the materials tested, a silicone-based compound appears to seal best;
- a rubber solution (at least with only a single application) appears to be almost pointless;
- whichever sealant is used, it seems necessary to let the sealed pouches "age" for several weeks before any bleed optimization is carried out.
Although only one pouch per sealant type was measured, I have no reason to believe that the results are abnormal. If these results generate any interest, further work could be done, e.g. application techniques, multiple applications, proprietary brands.
While on this subject, there is a related topic which I'd like to raise: Should pouches be sealed? A good theoretical argument can be made for not sealing pouches.
In a pouch/bleed situation, it is usually assumed that when a pouch and a bleed pass an equivalent amount of air at some stack vacuum then these two components can be considered to be identical at any other stack vacuum. This is far the case. The leakage flow in a pouch is laminar in nature -- the flow varies directly with the pressure drop. The flow through an orifice, by comparison, is turbulent in nature -- the flow varies by the square root of the pressure drop.
An example may help to get my point across. Compare Case A, where you have a "leaky" pouch and no bleed, with Case B, where you have a leak- tight pouch and a large bleed. At 6" stack vacuum the flows are set to be identical. Look what happens to the flows as the vacuum level increases (all flows in cubic inches per minute).
Stack Vacuum Case A Case B
inches water Leaky pouch/ Leak-tight pouch/
No bleed Large Bleed
6 50 50
12 100 70
24 200 100
30 250 112
Thus the simplistic approach of adjusting for a leaky or a leak tight pouch, by adjusting the bleed size, can lead to all sort of complica- tions. From a theoretical point of view at least, having no bleed but rather a very leaky pouch might have advantages in forte repetition. I think I remember reading somewhere that one piano manufacturer actually did this.
I hope the above is of interest.
Phil Dayson
[ The Stephenson paper mentions capillary bleeds of about 1.0 mm
[ diameter. Has anyone tried this method? -- Robbie |
(Message sent Fri 28 Mar 1997, 01:53:00 GMT, from time zone GMT-0800.) |
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