Test For Air Tightness

[Don Taylor]

Is there a test for how air tight a concertina should be?

 

I have only ever laid hands on my own concertina so I don't have a comparison to judge it against.

 

If I hold it vertically and fully compressed and then let go of the bottom end then it takes about 12 seconds to slowly open up to mostly open - that is it stops expanding, but it would open more when playing it.

 

This is a Lachenal Excelsior EC treble with 5-fold bellows.

 

I would like to not have to work the bellows so much when playing, but I don't know if that I am being reasonable in that expectation.

[david robertson]

The usual source of mild leakiness in an unrestored concertina is the chamois seal around the bellows ends. With age, the chamois dries and shrinks a little, and if the reedpan does the same, air creeps round the edges. See if there is any slight movement of the reedpan within the bellows. If so, lift the chamois seal, and stick shims of light card under the chamois. Try shimming every other edge to start with - the aim is to achieve a tight fit, but if you overdo it, there's a chance of damaging the corner joints of the bellows frame.

[Chris Ghent]

Air moving around the edges of the reedpan due to chamois compression or reedpan shrinking will not lead to your bellows opening when the concertina is held up by one end. The air is still in a closed system. It is more likely to be bellows leaks or pads not sealing.

 

Also, shimming every other side of the seal between the reedpan and the bellows frame just creates high and low spots. Better to shim three consecutive sides to take up the slack.

 

Some concertinas use a lot of air even when there are no leaks, it is all down to the quality of the reeds.

 

12 seconds is a very fast drop, check the pads seal and the bellows. Hold the instrument up to your ear and listen at each end while putting push pressure on the bellows without a key depressed. If pads are leaking you will hear air rushing past them. To check for bellows leaks again put pressure on them and run your tongue around every valley just above the leather, also the joint between bellows frame and action box, feeling for puffs of cold air allowed by leaks. Allowing your tongue to actually touch the leather is very ugly, a glass of wine removes the taste but not the memory.

 

Do not attempt to cure air leaks by winding harder on the end bolts.

[gcoover]

Of course, no movement or dropping at all is ideal, and some come pretty close.

 

Pads not seating well would be a major suspect.

 

You can also take the bellows off, go to a totally dark room, and if you put a light bulb inside you might be amazed at the number of pinhole leaks that sometimes show up in older bellows.

[Don Taylor]

'Allowing your tongue to actually touch the leather is very ugly, a glass of wine removes the taste but not the memory.'

 

Shudder. This 'tina smelt really bad (mildew and tobacco) when I first got it. It is much better now, but I think I would need the wine before licking the leather and brandy might be needed afterwards.

 

The chamois looks pretty compressed to me so that is certainly possible, and I did notice that at least one of the pads does not cover its hole at one edge. It was the only one I really looked at so there might be others. These are older bellows - original to about 1870 - so I will try the light bulb test.

 

So several things to test now that you have confirmed that my drop test time is not good.

[SteveS]

'Allowing your tongue to actually touch the leather is very ugly, a glass of wine removes the taste but not the memory.'

 

Shudder. This 'tina smelt really bad (mildew and tobacco) when I first got it. It is much better now, but I think I would need the wine before licking the leather and brandy might be needed afterwards.

This isn't necessary and I agree unpleasant.

 

I suggest you hold up the suspect part of the bellows to close to your lips and the open/close the bellows - actual contact with the bellows isn't necessary - the lips are sensitive and any rush of air out of holes or gaps in the bellows/chamois should be easily felt.

 

If the chamois isn't fluffy but is hard and smooth, it's time to replace it. You may also need to shim the chamois around the bellows frame to compensate for any shrinkage in the reed pan.

[Theo]

Bob Tedrow's padchecker is superb.

[david robertson]

Air moving around the edges of the reedpan due to chamois compression or reedpan shrinking will not lead to your bellows opening when the concertina is held up by one end. The air is still in a closed system.

Closed only if the bellows end seal is effectively sealing the joint between bellows and action box, as well as between bellows and reedpan. If one seal is poor, the other is likely to be just as bad.

[Chris Ghent]

Its not quite the same, the reedpan/bellows frame joint is an light interference joint from which both sides can be retreating, the actionbox/bellows frame joint is under active and adjustable compression from the bolts.

[Greg Jowaisas]

 

Air moving around the edges of the reedpan due to chamois compression or reedpan shrinking will not lead to your bellows opening when the concertina is held up by one end. The air is still in a closed system.

Closed only if the bellows end seal is effectively sealing the joint between bellows and action box, as well as between bellows and reedpan. If one seal is poor, the other is likely to be just as bad.

 

 

Its not quite the same, the reedpan/bellows frame joint is an light interference joint from which both sides can be retreating, the actionbox/bellows frame joint is under active and adjustable compression from the bolts.

There have been occasions when, with considerable effort, I have not been able to detect overt air leaks to improve the air tightness of an instrument and a change to new bellows chamois has proved the winning trick.

 

Greg

[gcoover]

Bob Tedrow's padchecker is superb.

 

I built a simple one using an oversized round concertina pad - just cut a hole in it and glue it to a plastic tube. Works great!

 

Gary

[Don Taylor]

There have been occasions when, with considerable effort, I have not been able to detect overt air leaks to improve the air tightness of an instrument and a change to new bellows chamois has proved the winning trick.

 

Greg

Replacing the bellows chamois looks tricky.

 

This is probably a silly idea, but ...

 

If this is my problem (and I have not confirmed that it is yet) then I wonder if it would be possible instead to add a chamois gasket to the inside of the pad board. The gasket would be hexagonal shaped and glued to the pad board with holes punched in the gasket for the pad holes and the end bolts.

 

This gasket would also cover all of the chamber edge tops so it would essentially re-gasket the whole face of the reed pan as well. I would leave the existing old chamois in place.

 

Since the edges of the yellow chamois would show between the pad board and the bellows frame I wonder if anyone has tried dying the chamois black - or even used a black marker pen on the outside edges where they would show.

 

This is maybe not a permanent fix, but maybe it would do until I can afford to have someone install a new set of bellows and to re-gasket the ends properly.

 

Thx. again.

 

 

Don.

[Geoff Wooff]

I think that covering the whole pad board with chamois will have a deadening effect on the tone .

 

Replacing the chamois gasket is not really that difficult ( there is an essay on the subject here on Cnet somewhere) and yes you can stain the exposed edges black with a marker pen... perhaps it is not that perfect but it is simple.

[Don Taylor]

Thanks Geoff - good point. And also thanks to Theo for pointing me at Bob Tedrow's pad checker.

 

I made a pad checker and I do have two pads with slight leaks - not enough to give ghost notes but they have been noted for fixing soon.

 

I taped over the leaky pads with some low tack masking tape, reassembled the concertina and tested again. I did not see an improvement, maybe be a little slower on opening but I am not sure. My problem is more than just these two pads.

 

Next I thought about how to eliminate possible leaks around the ends. I cut two pieces of chamois leather an inch or so over the size of the ends, punched 6 small holes in each of them positioned where the end bolts go and sandwiched one in each end between the bellows and an end.

 

I tested again. I still do not really see much change. I am now pretty sure that the pads and the ends are not at fault. When I removed the chamois gaskets that I made, both show clear imprints top and bottom. The seals looked good.

 

So I guess it must be the bellows, but I have yet to find anything.

 

Just to be sure that I am not chasing wild geese, I have video taped a drop test:

 

Bellows drop test

 

It is only 20 seconds or so, would you be so kind as to give me your opinion as to whether I should pursue this.

 

Thx. Don.

[Geoff Wooff]

Well, you are loosing air somewhere, it would be a shame if your bellows is the problem because they do look nice. My 48 EC with a new bellows does drop too but very very slowly. I think that good compression is a very usefull thing for best control of dynamics and crispness in delivery of chords.

 

Your bellows could have the pin holes in the leather, a porosity due to age.. check the corner gussets using a lamp as suggested above .

 

Another thing to check is the flatness of the end boards (pallet boards) , if they are warped it is possible that air is escaping between the bellows frame and end boards.

 

These 'Soft' leaks can be difficult to trace.

Edited April 5, 2013 by Geoff Wooff

[Randy Stein]

I think the best way is to completely submerge the instrument in water and move the bellows back and forth and see where the air bubbles come from.

Of course there are other issues with this method but it does work.

rss

Edited April 6, 2013 by Randy Stein

[Lester Bailey]

I think the best way is to completely submerge the instrument in water and move the bellows back and forth and wee where the air bubbles come from.

Of course there are other issues with this method but it does work.

rss

I recommend the alternative method of filling the bellows with butane gas and inspecting for leaks with a candle

[Theo]

I'll leave the comedy bellows testing for others!

 

Bellows can develop numerous minute leaks through many different routes. Some of these are:

 

wear on the outer corners

pinholes in gussets from flexing of the creases in the gussets

gusset leather becoming permeable

tiny gaps in the glue that holds the gusset in place, usually right at the bottom of the bellows valley

splits in the hinges at the bottom of the bellows valleys

 

etc, etc

 

A safe method of testing if you can't feel or hear individual escapes is to apply a dusting of powdered talc to the suspect area, then apply pressure to the bellows and look for little puffs of talk. You have to be very observant because the powder is so light that any visible movement of the powder only lasts for a very brief moment.