<html> <head> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; = charset=us-ascii"> <meta name=Generator content="Microsoft Word 10 (filtered)"> <style>  </style> </head> <body lang=EN-US link=blue vlink=purple> <div class=Section1> Well I'll be damned.  Her it is (with a link to a picture = too).  Credit to Ron Nossaman   David Love davidlovepianos@comcast.net   -----Original Message----- From: Pianotech Sent: None To: files@ptg.org Subject: MC gage experiment     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:   <a href="https://www.moypiano.com/ptg/files/attachments/28/96/b7/66/Dsc000= 01.jpg">https://www.moypiano.com/ptg/files/attachments/28/96/b7/66/Dsc000= 01.jpg</a>     Alternate URL:   <a = href="http://tinyurl.com/u4ah">http://tinyurl.com/u4ah</a></font= >     _______________________________________________= pianotech list info: = https://www.moypiano.com/resources/#archives </div> </body> </html>