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When steinway says 20 Celcius (68F) with 45% to 70% humidity.

That's between 6.58 and 10.23 in g(water) PER kg(air)

At 17.2C (63F) that would correspond to 53.5% to 83.5% humidity

At 26.6C (80F) that would correspond to 29.5% to 46% humidity


What is the manufacturer using for target when they give their recommendation?

When pianos were invented, central heating didn't exist most places, and the average room temperature in Europe was probably about 13 C.

Your hygrometer will show the relative humidity (RH) . Just keep the piano between 45% to 70% RH according to your hygrometer.

The absolute values are for HVAC people to calculate the necessary loads. You don’t have to bother about them.

Thx for the input hakki, I just wonder how they came up with that and what exactly are they targeting with those numbers, I assumed it's how much water is actually in the air, hence my conversion to g(water)/kg(air). But I thought I'd ask to make sure, because the temperature can greatly alter that range as demonstrated in the OP.

It is because the equilibrium moisture content of wood is related to relative humidity and not to absolute humidity.

Change in temperature has less affect on moisture content for a specific relative humidity.

See below for a detailed explanation:

https://www.woodproducts.fi/content/moisture-properties-wood

Another link particularly related with pianos.

https://pianomasters.com.au/the-environment-for-pianos/

Based on your first link, Hakki, do you know if anyone has a highlighted area in such a diagram specific for piano. Or are they saying Anything between the RH70 and RH40 line are OK.

Of course within human survivable range.

At any given RH, over the sort of range of temperature that is typical of a living room, the wood moisture content varies very little. That is why it is recommended to keep the RH as near-constant as possible, if a piano is to be happy. The actual value of RH is less important - it could be 45%, 50%, 55% - but the more constant it is at whatever value you have chosen, the better.

I am not sure what you mean by the "highlighted area"? An area is not being highlighted in this diagram. The red dashed lines are an example of how to use the diagram - i.e. if the air temperature was 22C and the RH 50%, then the wood moisture content will be a bit less than 10%. If the room temperature was 15C and the RH is 50%, then the wood moisture content would be almost spot-on 10%.

I don't understand why you are shading the region between 40% and 70% in blue?

I clarified with a second diagram, the first one was the example given in the website for some other purpose.

Well I would have thought that 70% is perhaps a bit high, but otherwise the RH values in this range are probably ok. (Hopefully we might get professional comment on this.) But you do not want the RH to go bouncing up and down within this range if you can help it. Aim to keep the RH as constant as possible - in other words, pick a value and stick to it.

I control the RH in my piano room to remain strictly within a narrow 5% band (i.e. not varying more than about 2.5% from the mean value), and as a result the pianos exhibit remarkable tuning stability. Even the 1804 piano with all-wood construction.

The thick lines are moisture content lines not relative humidity. Relative humidity lines are horizontal lines.

The below link might be better.

https://www.woodworkerssource.com/wood-moisture-content.html

Very good references Hakki, thanks.

Right, I am trying to understand what exactly are they targeting. So by that diagram, and their explanation had something to do with the tensile property of wood relative to wood moisture. So in that context, it could be that area I highlighted.

Is that the case or some other target?

RH is the range given, but What is it trying to achieve, what protection or property is the limiter.

No it is not the area you have highlighted.

What Steinway is saying that their wood will be okay with a moisture content between 8.5 and 13.1.

It isn't just the wood that is hygroskopic. Wool felt is as well. Wood takes on moisture much quicker than it gives it out to dry air. Same is true of wool, but wool takes moisture in even quicker than wood.

This difference in absorbing rates is why protecting against humidity spikes is the first line of piano defense.

Can you clarify? I am not sure who are "they", and I am not sure what you mean by "targeting".

To my mind when a piano manufacture is recommending a range of acceptable humidity levels they are thinking of the integrity of the soundboard rather than the tuning stability. As already stated in this thread tuning stability needs a constant humidity and temperature. I am afraid I don't know enough about the design of a soundboard such as the direction of annular rings, the effect of the crown etc to be able to calculate what change in moisture level could result in splits along the grain of the timber or parting of the glued joints.

Here is the formula some online calculators use to calculate EMC from relative humidity and temperature as input.

https://www.timberaid.com/calculator/fundamental/moisturecontent

So that is exactly what they are targeting moisture content ?

EMC for 68F and 45% RH is 8.5

EMC for 68F and 70% RH is 13.1

This is what Steinway says on their website.