Sena

It worked at first because it was new, and it stopped because it stopped being new. That is the whole thing, and it has a name: progressive overload, or rather the absence of it.

A muscle adapts to a demand it cannot currently meet. When you were doing 15 and pushing to 20, the demand exceeded your capacity, so you adapted. Once 40 became something you could do, the demand and your capacity matched, and there was no longer any reason for your body to change. Eight months of the same stimulus is one month of stimulus repeated eight times.

The uncomfortable implication: "40 feels about as hard as it did" is not neutral information, it is the diagnosis. If you had been progressing, the same 40 would feel easier by now. Constant difficulty at a constant workload means constant capacity.

Reading letters instead of words is uncomfortably accurate. I even do the pause-and-work-it-out thing on a chord I have played a hundred times in another key.

What works mechanically, having tried most of the bad options:

Tapered rubber reducers are right, tape is wrong. Not the plumbing kind — the stepped rubber cones sold for dust extraction, which you cut with a knife to the exact diameter you need. Cut it one step smaller than you think and stretch it on. That gives you a friction fit that seals and pulls apart deliberately.

Make the join at the machine, not at the hose end. A permanent adaptor screwed or taped to the saw's port, sized to accept your hose directly, means the connection that gets knocked is hose-into-adaptor rather than adaptor-onto-port. Only one of those two has to survive being kicked.

A short length of flexible hose as a tail. Ten or fifteen centimetres of hose permanently on the machine, hose clamped, so you are joining hose to hose. Two flexible things seal against each other far better than a rigid port and a flexible hose, and it takes the strain off the port, which on most saws is thin plastic that cracks.

Your model is exactly right, and it is the model most people never get to — the dial talks to the meter, not to the film. Everything follows from that.

So changing it mid-roll is not forbidden by the camera; it just underexposes or overexposes those frames relative to the box speed. Set 1600 on a 400 film and the meter gives you two stops less light than the film wants. Those frames are underexposed by two stops. Nothing stops you. The film simply records what arrived.

The reason it is usually described as "you must shoot the whole roll at one speed" is what happens next: development is a property of the whole roll. The film goes in one tank, in one developer, for one time. You cannot develop frames 1-12 normally and frames 13-24 for longer. That single constraint is where the rule comes from, and it is a darkroom constraint rather than a camera one.