MC gage experiment

[Farrell]

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And you know, my limp expansion gauge hasn't recovered yet.

Having graduated from building measuring jigs to building building jigs, =
I haven't even tried to replace my failed endeavor. What I have done =
instead - and I'm quite happy with the results - is to simply measure =
the size of the panel, cross-grain.

To ensure accuracy of measurement, I tap in a very small nail into the =
panel near one edge where the panel is widest. I use one of my scale =
measuring tapes that David Sanderson or Jim Arledge issues for measuring =
string scales. I place the tape end loop around the nail (this ensures =
end placement accuracy) and measure to the nicely squared opposite panel =
edge (put a pencil mark to repeat measuring spot).=20

My shop is kept at a consistent 45% to 50% RH. So I know my spruce =
panel's EMC is real close to 8.5%. I put the panel in the hot box =
targeting around 6% EMC (adjusting heat to keep the RH just below 30%). =
I measure the panel daily. Within a week the panel stabilizes. I let the =
hot box RH go up to 30% and watch for a little bump upwards in the panel =
width as the panel goes from something a little less than 6.5% back up =
to 6.5% or so. That way I am doubly sure the panel's MC did indeed =
stabilize.

Across a typical width panel I will observe about a 2.5 millimeter =
reduction in width during the course of drying from 8.5% MC to 6% MC. So =
yes, you need to interpolate tenths of millimeters, but with a big =
magnifying glass this is doable.

Works for me. But I also think your idea is great.

Terry Farrell

----- Original Message -----=20
From: "Ron Nossaman" <RNossaman@cox.net>
To: <files@ptg.org>
Sent: Friday, November 07, 2003 12:03 PM
Subject: MC gage experiment


>=20
> I apparently never posted this to the list, intending to write it up=20
> and submit it to the Journal. Since I still haven't yet done that, and =

> someone asked, here's the gist of it.
>=20
> After the demise of Terry's differential expansion gage, I got to=20
> thinking - and you know what kind of trouble that can lead to. As=20
> usual, what I started out doing was different from what I ended up=20
> doing, which was different from what I should have done from the=20
> beginning. After flailing around doing it inside out and backward for=20
> about a week, here's what I decided I should have done in the first=20
> place, and how I would do it if I were building another one...
>=20
> I already had a $6 linear dial indicator from Harbor Freight that I =
had=20
> purchased last year for the inevitable dedicated tool I figured I'd be =

> building before long, or just for general purpose confusement. It was=20
> just too cheap to pass up, and this looked like a fine use for it. I=20
> made a maple frame of "U" channel for a slip fit of the panel scrap I=20
> intended to use. So far so good.
>=20
> Choose a well quartered 8mm plank, somewhat dense, with relatively=20
> tight grain that's uniform all the way across. If there's only a 50mm=20
> width that's uniform, rip it to 50mm. It should react slower than a=20
> light loose grain piece, so it won't get too far ahead of the wood in=20
> the soundboard panel when it's actually used as an MC gage. It=20
> shouldn't move as far as the loose grain stuff for a given MC shift=20
> either, and your gage can be longer for a given expansion scale to,=20
> hopefully, increase measurement accuracy. Now, what's the actual=20
> expansion/contraction rate and how to scale it to the dial indicator?
>=20
> Cross cut the chosen plank into strips of the right width (mine's =
50mm)=20
> to fit in the channels in  the frame pieces, leaving a couple=20
> millimeters free top and bottom so the strip won't bind. It's just=20
> supposed to slide easily, but not sloppy, in the tracks. Cut enough=20
> strips to fill the frame length. Stabilize them at some measured MC in =

> the hot box, house, or shop; wherever the temperature and RH is stable =

> enough for a couple of days until they quit moving. Somewhere around=20
> 10%MC would be good. Record the cross grain width of one of the pieces =

> as an indicator, and mark it as the test piece. Or do them all for=20
> comparison averaging. Record the MC from temperature and RH%. Dry the=20
> pieces in the hot box for a couple of days until the test piece quits=20
> changing dimension (and/or weight) at around 6%MC or so. Record temp=20
> and RH% with the same instruments you used for the high MC =
measurement,=20
> and figure the new MC. You now have a dimension @ a specific MC for =
two=20
> MC values that aren't near the less trustworthy high and low limits of =

> your RH% measuring device. I assume the error in an electronic=20
> hygrometer will likely be in the same direction and close to the same=20
> rate for both measurements, so the proportion of dimensional change to =

> computed MC% change should be valid whatever the numbers actually are. =

> This may not be the case, but it seems likely. If you have the=20
> facilities to do this with a sling psychrometer, that would probably =
be=20
> better. Now the scaling.
>=20
>  From the expansion figures I got, I figured I had room in my frame =
for=20
> a strip of the proper length to change 0.010" per 1%MC. That seemed=20
> sensitive enough to me to be useful, and simple enough to figure out =
on=20
> the gage without a conversion table. I can read it directly off the=20
> dial. From measurements taken on the test piece, find the expansion=20
> rate per inch per 1%MC increase, and glue enough pieces in line to=20
> nearly fill the frame, and mark the proper length to have a piece that =

> will expand 0.010" per 1%MC. Put a thin maple cap on one end of the=20
> strip for the dial plunger to ride on without denting, and mount it in =

> the frame with the gage so the gage reads what you've just measured =
the=20
> MC at. I mounted mine by driving a small wedge with a bit of glue=20
> between the frame edge and the panel at the mark I had made indicating =

> the appropriate length. That way, the waste end can expand and =
contract=20
> without getting in the way of anything, and My frame can be a bit=20
> longer than actually needed in case I want to replace the panel piece=20
> some day. Now, start testing it against as accurate a temperature and=20
> RH% means as you have available at different humidity levels to =
certify=20
> it's accuracy.
>=20
> The sucker seems to be pretty close, and since it's unconstrained, it=20
> won't suffer compression set either over time, or from extremes of=20
> humidity like most of the designs I've seen will. As long as I don't=20
> overheat it or contaminate it with something that affects it's =
moisture=20
> capacity, it should last a very long time. So far, I like it. Here's=20
> the photo.
>=20
> Photo:
>=20
> =
https://www.moypiano.com/ptg/files/attachments/28/96/b7/66/Dsc00001.jpg
>=20
> Alternate URL:
>=20
> http://tinyurl.com/u4ah
>=20
> _______________________________________________
> pianotech list info: https://www.moypiano.com/resources/#archives
> 
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