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specific gravity/ wagner mmc220

12/13/18       
Nathan Member

I am wondering if anyone can shed some light on specific gravity settings on Wagner mmc220 pinless meter? The listed SG for american elm in wagner book is .50. I have found the Wood Database online to be an excellent resource. He is listing the SG of American Elm to be .56 at 12%. The question arises because I am attempting to kiln dry some 2" thick American Elm. It was consistently losing 1% to 2% per day until it reached around 10%. It has been stuck there for a couple days now. I am having some issues with my Nyle L53 at temps above 140 so I am set at 136 dry bulb 100 wet bulb. That is as hot and dry as I can achieve right now. I don't want to deceive myself by setting meter to higher SG but if set to .56 SG it reads 1% to 2% lower MC. The other question that I wrestle with is is it practical or even necessary to hold out for 6% to 8% MC on thick material that is intended use is table tops and bar tops? If it gets finished and put into service at 10% MC in Minnesota will I experience some undesirable warping or failure? Thank you for any wisdom and experience you might be able to share!

12/14/18       #3: specific gravity/ wagner mmc220 ...
Bryan "doc" Henderson  Member

Hi Nathan. I have the same meter that basically measures density. it could just give a reading like an ohm meter that you could look up on a chart, but they allow you to put in a generic estimate of that woods density. The water adds to the density and as the wood dries and becomes lighter, it becomes less dense. If you change the setting to a higher wood specific gravity, It will show less moisture content as it attributes more of the density to the wood and not the moisture. The sell a calibration block if you think your meter is the problem. I am not a kiln person, but I wonder if with that thickness it will take longer to get to your goal MC. How long has this been drying? As you know knots will read higher, sap wood less. Also thin boards like 1/4 inch will read lower. Might try the oven drying method to see where the wood is really at.

12/15/18       #4: specific gravity/ wagner mmc220 ...
Gene Wengert-WoodDoc

The density that the Wagner meter uses is the green density. You do not adjust this density as the wood dries. So, use the values in their book, unless the wood you are testing is abnormally high or low in density. In fact, it is the natural variability of density of the same species that causes some confusion with this pinless meter.

You must indeed get the wood to a low MC, thick or thin, for use in MN or just about any environment that is heated. The environment is around 30% RH which means 6.0% EMC. So, any lumber above 7.0% MC will dry a little bit more and with drying comes shrinking and possibly warp.

If nice and dry, then when summer comes, the wood will gain several percent lister, but because there is approximately a 1% delay before swelling begins when moisture is added, and because swelling tends to be less of an issue than shrinking, we are not too concerned. In my book, the ideal is 6.5 to 6.8% MC.

Note that a 36 F depression at 100 F is under 3% EMC, so your wood should be drying toward that value. If it is not drying, then either you MC is wrong or the DB and WB are wrong...stick a small, thin piece of wood in the kiln and then use the oven test to check its MC which will also be the EMC. The pinless meter will not work on a small piece. Also, the pinless meter tends to give a higher reading for thicker stock.

12/16/18       #6: specific gravity/ wagner mmc220 ...
Bryan "doc" Henderson  Member

Very true and well stated Gene. so gases and liquids tend to be more consistant, but solids like wood has a lot of variation. The meter is set to an overall estimate of specific gavity for that type of wood so it can estimate the MC from the density measured above what is expected. The SG of water is 1, and therefore wood with a SG of .55 is less dense than water and will float. That wood is thick and wondering if it needs more time or heat. Gene is clearly the expert and would let him comment on that.

12/16/18       #7: specific gravity/ wagner mmc220 ...
Gene Wengert-WoodDoc

A small correction...136 DB and 100 WB is under 5% EMC.

Another comment...American elm is quite rare as almost all the trees were killed by a fungus infection decades ago. So, you might have a different elm species and therefore a different SG correction.

Overall, I think your moisture values are wrong. Do you know anyone nearby with a pin meter you can borrow?

12/16/18       #8: specific gravity/ wagner mmc220 ...
Nathan Member

Website: http://crosscutllc.com

Thanks for the input. I have been trying to post a response, but after pressing post message it vanishes and I have to start over? Maybe they will all show up someday?

I checked with pin meter and come up with mostly 0%... a couple 5%. I don't think it works below 3 or 4%?

I am as sure as I can be it is American Elm.

Pinless meter still reads as low as 7% and as high as 16%. Mostly around 10 to 12%

I need to know what you all with more experience and wisdom would do as a kiln operator in this situation. I also need to know I am producing a quality product my customers will be pleased with for years to come

Thank you!

Nathan

12/16/18       #9: specific gravity/ wagner mmc220 ...
Nathan Member

Website: http://crosscutllc.com

Gene, I think you were right to question whether I have properly identified the species of elm. Both trees were standing dead when I cut them. One matches perfectly all the american elm pics I can find. The other is a bit darker in color. Does not really match any other elm pics I can find. I am beginning to think it is rock elm, which would explain a lot. Switch wagner meter to SG of .63 for rock elm and readings are in the 7 to 8% range.

Between that revelation and the pin meter readings I am thinking I can pull the load and move on. Would you all agree?

Nathan

12/16/18       #10: specific gravity/ wagner mmc220 ...
Gene Wengert-WoodDoc

Yes.

Pin meters cannot read accurately under 6.5% MC.

12/16/18       #11: specific gravity/ wagner mmc220 ...
Gene Wengert-WoodDoc

I believe that when you look at the end grain of American elm with 10x magnification, you will see only a single row of large open vessels at the beginning of each growth ring. The other elms have several rows. If you google "American elm Hoadley", I believe you will see a picture from his book. Or try link below

Am. Elm magnified

12/17/18       #12: specific gravity/ wagner mmc220 ...
Nathan Member

Website: http://crosscutllc.com

The mystery continues in my mind...I attempted to examine the end grain. The slabs I have came from two different trees. As I said both standing dead when I cut them. End grain appears to be the same in regards to the single row of vessels in earlywood. I did notice an obvious difference in the density of the rings. The larger tree (30" + diameter) has very wide rings. The other that is such a mystery in regards to MC read on pinless meter has rings so tight in areas you can hardly tell them apart. I have always understood each light part of the ring(early wood?) followed by the darker ring (late wood) combined indicates one year of growth in most species? I was conservative when counting some of these very tight rings and came up with some where around 225 rings on a 16" diameter tree! Is that even possible? I will attempt to attach pics. First two are the dense one still getting readings on pinless meter as high as 14% even set at .63 for rock elm. The other is the wider ringed larger tree.


View higher quality, full size image (3264 X 1836)


View higher quality, full size image (3264 X 1836)


View higher quality, full size image (1836 X 3264)

12/17/18       #13: specific gravity/ wagner mmc220 ...
Gene Wengert-WoodDoc

I cannot say for sure that either is American elm. What we need is a surface cut with an extreme sharp razor blade or similar knife.

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