temp change=how much pitch change?

[John M. Formsma]

Regarding primarily large concert pianos, does anyone have data on how 
much the pitch changes with temperature fluctuations?

This is a question I'll no doubt answer in time as I get more experience 
servicing concert grands. But I regularly service only one Steinway D, 
and I haven't worked with it long enough to know how it reacts to 
temperature changes. It has a DC system, though, which keeps it very 
close - as long as it's plugged in and in its storage area.

Scenario:
Yesterday, piano was -2¢, so it was tuned to A440 in one pass since I 
was coming back today to check it again. Temp was 67.6°F, and RH was 26%.

Today, piano was about 3-4¢ flat (a little more in low tenor). Temp was 
72.9°, with the humidity at 25%. So I'm seeing the most likely cause of 
the change in pitch is the thermostat (and body heat from the orchestra 
as well). OK, I think I understand all the whys behind the changes, but 
I had no idea that 5° would make that much difference. After talking 
with a piano tech friend of mine who routinely takes care of several 
D's, he confirmed that these pianos are indeed that sensitive to the 
temp change.

For future reference, does anyone have data on how much change happens 
with temperature fluctuations like this? I could say, based on my first 
non-scientific collection of observation data, that it changes about 
3-4¢ for every 5° of temperature increase. <g>  Is that about right 
generally?

What do you guys do? Do you not worry about where the temperature is? If 
you think it will be different at performance time, do you set the pitch 
accordingly, and hope it changes in your favor? Or do you simply tune it 
to A440 and don't worry about it?

Another thing that's frustrating is that I tuned it yesterday with the 
stage lights on, so as to minimize the pitch change that comes from 
lights. But today, it was back in the little storage room and the stage 
is set up for the orchestra. Apparently, the piano is coming out after 
intermission. ?? I didn't look at a program, so I dunno. But anyway, it 
kind of stinks because today the unisons were all wobbly anyway - no 
telling what happened when the lights hit it tonight. Oh well, at least 
I have one thing in my favor: most people aren't as sensitive to pitch 
as tuners are, so I'm sure I'm overreacting to what was probably a very 
minor issue, if noticed at all. <g>

Thanks for any help with this.

JF