[CAUT] temperature and pitch

[Fred Sturm]

On Dec 16, 2009, at 10:29 PM, Ron Nossaman wrote:

> Actually, someone else recommended the full length covers, more to  
> protect the case finish than anything else I suspect, but I've seen  
> no reason not to after the fact.


	The thing is, because the humidistat is always active (unless they've  
changed it recently), confining the moisture and isolating the piano  
and system from the room means it will constantly ratchet up the dew  
point and temperature. When it is at about 40% RH, the tank rod will  
activate and add moisture. When it hits 44%, the drying rods will  
activate and stay on until it hits 40%. And that cycle will keep  
happening, always increasing the moisture content of the air (dew  
point), since the moisture can't escape, until the drying rods can't  
get the temp hot enough to make the RH go below 40%. Eventually the  
drying rods will be on most of the time. So the MC of the wood will  
probably be just fine, as the RH will stay in that same range, but the  
temp will hit a maximum and pretty much stay there. The heat will be  
trapped to a large degree since it can't convect away (no air  
circulation), so all it can really do is radiate. The whole piano  
becomes hot compared to the room.
	A couple scenarios that could cause problems:
Heat and power go off during the winter. The moisture inside the semi- 
sealed system condenses on all solid surfaces as the temp drops.
Heat goes off, or is turned off, during the winter. Moisture condenses  
on the inside of the cover.
	And, of course, the problem described: when the cover is removed,  
there is a period of instability while the temperature adapts. I  
visualize the choir accompanist pulling the cover off, starting to  
play, it sounds bad, going to the church secretary and saying the  
piano needs tuning. Secretary says it was just tuned a couple days  
ago. Oops! Not good for the customer, not good for the reputation of  
the tech.
	Bottom line, I think if you put a full cover over a piano with a full  
D/C system, it should either be permeable (not vinyl) or vented in  
some way. I expect Roger Wheelock would say the same. The system is  
designed to be vented, not confined. Isolation is good up to a point,  
so bottom covers or back covers out of fabric are okay. But not vinyl  
or plastic, unless there is enough of a venting opening in a good  
location.
	BTW, if the humidistat weren't always active, this problem wouldn't  
arise, or at least not to the same degree. IOW, if it were inactive  
between 41 and 43%, and only cycled on when it exceeded those limits,  
there wouldn't be that ratcheting effect. I have asked Roger about  
this, and he claims that the always active model holds the MC more  
stable, but he said they were looking at possibly changing it. That  
was a few years ago.
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu