MC gage experiment

[Ron Nossaman]

I apparently never posted this to the list, intending to write it up 
and submit it to the Journal. Since I still haven't yet done that, and 
someone asked, here's the gist of it.

After the demise of Terry's differential expansion gage, I got to 
thinking - and you know what kind of trouble that can lead to. As 
usual, what I started out doing was different from what I ended up 
doing, which was different from what I should have done from the 
beginning. After flailing around doing it inside out and backward for 
about a week, here's what I decided I should have done in the first 
place, and how I would do it if I were building another one...

I already had a $6 linear dial indicator from Harbor Freight that I had 
purchased last year for the inevitable dedicated tool I figured I'd be 
building before long, or just for general purpose confusement. It was 
just too cheap to pass up, and this looked like a fine use for it. I 
made a maple frame of "U" channel for a slip fit of the panel scrap I 
intended to use. So far so good.

Choose a well quartered 8mm plank, somewhat dense, with relatively 
tight grain that's uniform all the way across. If there's only a 50mm 
width that's uniform, rip it to 50mm. It should react slower than a 
light loose grain piece, so it won't get too far ahead of the wood in 
the soundboard panel when it's actually used as an MC gage. It 
shouldn't move as far as the loose grain stuff for a given MC shift 
either, and your gage can be longer for a given expansion scale to, 
hopefully, increase measurement accuracy. Now, what's the actual 
expansion/contraction rate and how to scale it to the dial indicator?

Cross cut the chosen plank into strips of the right width (mine's 50mm) 
to fit in the channels in  the frame pieces, leaving a couple 
millimeters free top and bottom so the strip won't bind. It's just 
supposed to slide easily, but not sloppy, in the tracks. Cut enough 
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 
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 
comparison averaging. Record the MC from temperature and RH%. Dry the 
pieces in the hot box for a couple of days until the test piece quits 
changing dimension (and/or weight) at around 6%MC or so. Record temp 
and RH% with the same instruments you used for the high MC measurement, 
and figure the new MC. You now have a dimension @ a specific MC for two 
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 
hygrometer will likely be in the same direction and close to the same 
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 
facilities to do this with a sling psychrometer, that would probably be 
better. Now the scaling.

 From the expansion figures I got, I figured I had room in my frame for 
a strip of the proper length to change 0.010" per 1%MC. That seemed 
sensitive enough to me to be useful, and simple enough to figure out on 
the gage without a conversion table. I can read it directly off the 
dial. From measurements taken on the test piece, find the expansion 
rate per inch per 1%MC increase, and glue enough pieces in line to 
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 
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 
MC at. I mounted mine by driving a small wedge with a bit of glue 
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 
without getting in the way of anything, and My frame can be a bit 
longer than actually needed in case I want to replace the panel piece 
some day. Now, start testing it against as accurate a temperature and 
RH% means as you have available at different humidity levels to certify 
it's accuracy.

The sucker seems to be pretty close, and since it's unconstrained, it 
won't suffer compression set either over time, or from extremes of 
humidity like most of the designs I've seen will. As long as I don't 
overheat it or contaminate it with something that affects it's moisture 
capacity, it should last a very long time. So far, I like it. Here's 
the photo.

Photo:

https://www.moypiano.com/ptg/files/attachments/28/96/b7/66/Dsc00001.jpg

Alternate URL:

http://tinyurl.com/u4ah