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Hello Terry,
The switch in our Humidistat has a synthetic polymer band. You can see =
it if you look through the vent holes on the side of the Humidistat. It =
will either be a clear polymer band or a white woven polymer band.
As moisture flows into the Humidistat the band absorbs moisture and =
swells. Eventually it swells enough to trigger a mechanical switch =
which energizes the Dehumidifier outlet and shuts off the Humidifier =
outlet. Then drier air enters the Humidistat and the band begins to =
shrink. Eventually it shrinks enough to trigger a second mechanical =
switching action which causes the Humidifier outlet to be energized and =
the Dehumidifier outlet to be shut off.
In our humidity controlled test rooms the cycle generally takes 3-4 =
hours with the Humidifier being on two thirds of the time and the =
Dehumidifier being on a third of the time. Say we have a system =
installed in a vertical piano and we have a backside cover on the =
instrument. A typical calibration for the Humidistat would be a lower =
set point of 39% relative humidity and an upper set point of 45%. If we =
place a good hygrometer probe on the Humidistat and record the readings =
during the cycling we can see a range of 35% to 65% relative humidity. =
So we can get up to 20% "overshoot" on humidification and a few percent =
on "dehumidification". Now after cycling has been established we bring =
the test room relative humidity up to 70% and hold it there for several =
weeks. The soundboard stabilizes after a few days at an equilibrium =
moisture content of 8.0% and then remains there +/- .1%. Then we lower =
the relative humidity in the test room to 20% and again hold it there =
for a few weeks. The equilibrium moisture content of the soundboard =
restabilizes at 7.2% and then remains there +/- .1%. Then we conduct =
the test again, but without a Climate Control System. At 70% relative =
humidity in the room the soundboard stabilizes at 10.8% equilibrium =
moisture content. At 20% relative humidity the sounboard stabilizes at =
5.0% equilibrium moisture content. So the System reduces the =
differential from almost 6% to 0.8%. With a Backside Vertical System or =
a Grand System with an undercover we can get down to a differential of =
0.2%. My point is that even though the piano is seeing a controlled =
repeatable cycle of relative humidity which is larger than one might =
expect, the soundboard equilibrates and provides pitch stability to the =
instrument.
Electronic sensors measure relative humidity by evaluating changes in =
resistance or capacitance in the air. Capacitor based systems are =
generally more accurate and more expensive. All electronic sensors =
react within a few minutes to a change in relative humidity. The least =
expensive equilibrate within 30 minutes and these are what you find in =
the inexpensive hygrometers. Very expensive ones equilibrate very =
quickly. While they are called electronic sensors they really are in a =
sense measuring electrochemical events. The responsive of the sensors =
deteriorates with time, as I understand it, due to films building on the =
sensor points resulting in a variable response to measuring the =
electrochemical event. Thus they drift over time, so they require =
replacement or recalibration. The very expensive hygrometers we use are =
not immune to this. We have to return them to the factory for =
recailibration annually. What we are looking for at Dampp-Chaser is an =
affordable electronic sensor that has minimal drift and a fast response. =
So far we haven't found it.
I hope this helps.
Roger
ps - Terry, I am copying some of my staff on this as Tammy and Mitch =
help me with technical customer service. I hope you don't mind.
----- Original Message -----=20
From: Farrell=20
To: roger@dampp-chaser.com=20
Sent: Tuesday, January 16, 2001 8:36 PM
Subject: Re: Climate Control Promotion
Thank you Roger for your very informative response. You stated your =
position well, and I think I have a much better feel for the role these =
inexpensive hygrometers should be allowed to play.
However, having a bit of a background in wood technology, I really try =
to thoroughly understand climate control on pianos. You wrote of =
humidistat "overshoot". I do not understand what it is you are referring =
to. See below -
"The sensing band in the Humidistat is slow to respond to a rapid =
increase in humidity, and one observes "overshoot" where higher than =
expected relative humidity levels are recorded by a hygrometer placed in =
the vicinity of the Humidistat."
"We experiment with electronic sensors that respond quickly to avoid =
the "overshoot"."
Could you please expand on this a bit to clarify?
Additionally, or specifically, I am curious as to how fast do the =
Humidistats respond. Is there anything you can say about that without =
Fed-Exing a 120 page report to me (I realize a good explanation might =
require just that)?
Thanks again for your input. I really appreciate it. If you think your =
response would be of interest to the pianotec list, please post it there =
- I would think it might.
Terry Farrell
Piano Tuning & Service
Tampa, Florida
mfarrel2@tampabay.rr.com
----- Original Message -----=20
From: Roger Wheelock=20
To: pianotech@ptg.org=20
Cc: Gayle Mair ; mitchell_smith@dampp-chaser.com ; =
tammy_bradley@dampp-chaser.com=20
Sent: Tuesday, January 16, 2001 4:53 PM
Subject: Re: Climate Control Promotion
Dear List,
I offer the following in hopes of shedding some light on relative =
humidity readings measured with inexpensive digital hygrometers inside a =
piano when a Dampp-Chaser Climate Control System is operating. I will =
use the hygrometer we distribute as an example. Please note that we =
only distribute this hygrometer as a tool to use in showing a piano =
owner the extremes of relative humidity he or she might experience in =
the home. We have never promoted it as a tool for technical or =
scientific studies. In our testing we use hygrometers that range in =
cost from approximately $500 to $1500. We believe that one needs to =
invest over $100 to get a reasonable tool to use in testing System =
performance.
=20
We have switched hygrometer manufacturers and do provide a slightly =
better hygrometer than we initially distributed. But even these are =
only moderately accurate, reasonably precise and the sensor will drift =
significantly with time. (This is simply the nature of the inexpensive =
electronic sensor in these instruments.) So kindly understand our =
intention was to provide a tool where you could show a piano owner a =
40-50% relative humidity swing had occurred when the hygrometer was left =
in their home for a period of time.
The other source of some confusion is in how the Climate Control =
System operates. The Humidistat contains an electromechanical switch. =
We measure the calibration of each switch in one of our humidity =
controlled test rooms. Here we bring the humidity up to about 65% and =
down to about 30% very slowly over a 20-hour period. Computers record =
the upper and lower set point for each switch and our regular =
Humidistats now have a six percent range with the average of the two set =
points falling between 42% and 44% relative humidity.
When a Climate Control System is functioning within the piano the =
relative humidity rises and falls much faster than the twenty hours of =
controlled humidity change the switch experienced in our calibration =
test room. The sensing band in the Humidistat is slow to respond to a =
rapid increase in humidity, and one observes "overshoot" where higher =
than expected relative humidity levels are recorded by a hygrometer =
placed in the vicinity of the Humidistat. We observe these in our =
product development humidity controlled test rooms. Here, however, we =
are also measuring soundboard equilibrium moisture content. What we =
find is that we get good stabilization of the moisture level in the =
soundboard. We use relative humidity as a surrogate for moisture =
content in wood. Unfortunately they do not always directly correlate at =
any given point in time. The soundboard tends to equilibrate at =
constant moisture content while relative humidity is cycling in a =
controllable, repeatable cycle. The equilibration of soundboard =
moisture content is the driving force for all our product development =
research, and this is reflected by the pitch stability which our product =
can provide to the piano. We discuss our product in terms of relative =
humidity, only because it is a concept that a layperson can easily =
grasp.
=20
I am one who believes in under promising and over delivering. I =
only recommend the hygrometer as a sales tool. I only claim the Climate =
Control System will help to provide pitch stability.
=20
Maintenance of pitch stability is directly dependent on how confined =
the space is in which the Climate Control System operates. We have =
introduced a back side cover for vertical pianos that provides better =
confinement. We continue work on a grand undercover to provide the same =
benefit. We are pleased to have received patents last year for both of =
these systems.
=20
We also continue work on a better Humidistat. We experiment with =
electronic sensors that respond quickly to avoid the "overshoot". =
Unfortunately, the ones we have examined all drift significantly. (The =
electromechanical switch we now use does not drift.) We cannot imagine a =
technician going in every year to replace a sensor on a Humidistat. We =
do monitor R&D on humidity sensors and hope that the ideal unit will =
come along in the future.
=20
I would be happy to answer questions any of you on the list might =
have if you would like to email me privately. We at Dampp-Chaser are =
genuinely thankful for your interest in our products and appreciate the =
diversity of opinions offered in this forum.
Roger Wheelock
Dampp-Chaser Corporation
roger@dampp-chaser.com
www.dampp-chaser.com
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