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This is a 'how long is a piece of string' question. I don't use a dehumidifier as Scotland is not Florida. That said, as one who abhors out of tune choirs, I often wonder if I should let a sudden change in the weather pass before another fine tuning? Is there a rule of thumb concerning whether to tune or not if the weather suddenly changes? e.g. do nothing until x days have past during the weather change? Perhaps my asking RPT professionals is unfair as I realise that you have regular commitments regardless of short term weather changes.
Ian

Ian,

I believe you tune your own, so I would think touching up the unisons would be relatively easy for you. For the average person, it would take several days to a week of consistent same direction humidity change for it to get objectionable for most. Rain one day, then clear and dry the next is little to worry about. However heavy playing in conjunction with humidity change definitely can accelerate the process.

Peter Grey Piano Doctor

Just to say... I find my dehumidifier invaluable. I am in London - I am not sure that Scotland is so different.

An additional thought:
Temperature usually always moves simultaneous with relative humidity and temperature changes can have an almost immediate effect on the tuning.

I do touch up weekly and its not a chore for me. I was just curious from a technical viewpoint how quick the soundboard absorbs and releases moisture. As I recall reading, it takes much longer to release moisture.
Ian

As far as I know (I read a technical paper on the subject by a Swiss scholar), a piano soundboard which is rather thick compared to a harpsichord or a guitar, will take ages to really dry up to the point where cracks may appear, and only in conditions of relative humidity below 25 %. It is telling to read how all piano manufacturers claim they age their timbers for years before use. But I may be an optimist. I think the danger is in well insulated big city appartments that are overheated in the winter, and people living in Arizona. It also depends on how many other humidity absorbing elements (furniture, books) are present in your room.

I do use a humidifier during the central heating season.

I think it depends on the piano and the player.

For instance, I tune a Steinway M at a school. Prior to this they had a Steinway L. The L wasn’t in great shape but the tuning pins, bridge, and soundboard were fine. No issues tuning it. But it was exposed to large humidity shifts and went out of tune in a short time. Then they got a used M to replace the L. That piano stays in very good tune in the same room and conditions. At my own house, my M&H BB stays in amazing tune compared to my former Steinway L. Same room and conditions.

And of course everyone has their own tolerance for what is out of tune.

The point is that, other than the generically recommended 2 times a year, there’s no way to have a ‘standard’ for when to tune. It depends on your desires and the specific piano.

I understand this is because the string length and tension that affect pitch are only related to metal objects (that is the string themselves and the cast iron frame) and metal responds immediately to temperature change, but practically never to humidity. Humidity only affects wood. It is a big issue when tuning wooden organ pipes.

Wood takes on humidity much quicker than it gives it up. A few hours of fog and the pitch will start to rise. A few hours of arid air and nothing changes. It takes several days for drier air to cause the tuning to change.

That is why the damp chaser dehumidifier rods controlled by the humidistat really help reduce pitch movement.

One could always come up with 100 grams of Sitka spruce and a gram scale.
Maybe two pieces, one unfinished the other with shellac coating and do some experimenting.

This has absolutely been my experience. If, after my air conditioning has kept the RH somewhere in the 40s for weeks, I decide (against my better judgement) to open my windows after a rainstorm, it just goes way outta whack in a big hurry. Upon closing the windows and turning the AC back on it gradually -over a period of days - goes back to (more or less) where it had been.

Especially the longest strings on the long bridge are affected more. It might make the intervals across the long and short bridges sound unpleasant.