Lumber moisture content formula

[Zachary Hoyt]

I'm hoping someone will be so kind as to help me with a math/wood moisture question that I seem to be unable to solve. I seem to remember years ago reading about a way to calculate MC from weighing and then oven-drying wood, but I can't find the thing I read before.

I bought some soggy red maple back in the fall and brought it inside, where the RH was ~40% at the time and has been dropping since. On December 30 it seemed to feel fairly dry to my hand, so I took a low-value piece with wane and cut off the last 16" or so. I weighed it at 554 grams right after cutting it off the parent board. Then I put it on top of the wood boiler and weighed it every few days.

1-1 538g

1-3 526g

1-6 524g

1-8 524g

It seems that it has stopped losing weight, so I imagine it may be as dry as it will become. The top of the boiler is very warm and dry and there is good airflow in the boiler room due to the air intake fan. Are there any conclusions I can draw from these numbers about how dry the lumber in the stack may have become as of the 30th? I'll be very grateful for any assistance.

[Bryan Lisowski]

I don’t know of any formula, but once you get several readings that are the same, then your board is at equilibrium. Based on your reported numbers, I would say you are there, but you may want to go 6-7 days and weigh again. If it comes in at 524 you know it’s done.

[Mike Gresham]

https://www.timberaid.com/calculator/fundamental/moisturecontent

[Kevin Jenness]

Your wood may be close to oven dry but probably not quite, which is what you need for an accurate calculation. Here is an "official" method. https://erc.cals.wisc.edu/woodlandinfo/files/2017/09/FEM_089.pdf

[Bill Dufour]

We need numbers. Simply measure length, width, height in cm then multiply to get cubic volume in CC. then 538-524=14grms
So 14/cc. yields grams per cc of water lost.

Sounds like your wood is not rectangular so....
Fill a bucket, big enough for the piece of wood, cut it into pieces if needed to make it fit. Fill bucket near the top and hold all the wood under water. make a mark on the side of the bucket. Remove the wood and slowly add water from a measuring cup to the mark. The water added is the volume of the wood.
Bill D

[Greg Funk]

We need numbers. Simply measure length, width, height in cm then multiply to get cubic volume in CC. then 538-524=14grms
So 14/cc. yields grams per cc of water lost.

Sounds like your wood is not rectangular so....
Fill a bucket, big enough for the piece of wood, cut it into pieces if needed to make it fit. Fill bucket near the top and hold all the wood under water. make a mark on the side of the bucket. Remove the wood and slowly add water from a measuring cup to the mark. The water added is the volume of the wood.
Bill DThe volume of wood isn't necessary for moisture content. Provided the wood was actually oven dried with no moisture the first three numbers are all you need.

[Richard Coers]

This might help. https://www.extension.purdue.edu/extmedia/FNR/FNR-156.pdf

[Kevin Jenness]

The top of the boiler is very warm and dry and there is good airflow in the boiler room due to the air intake fan. Are there any conclusions I can draw from these numbers about how dry the lumber in the stack may have become as of the 30th? I'll be very grateful for any assistance.

If you have an accurate hygrometer you can approximate the moisture content of the sample which has achieved equilibrium with the boiler room atmosphere and extrapolate the mc of the rest of the lumber from that, but it will be more accurate to take the sample to oven dry.https://www.bing.com/images/search?view=detailV2&ccid=xSAXq4QB&id=648D74EE9DE67BFEDAE26B0C614A677A52E71967&thid=OIP.xSAXq4QBNNN2KJdXGKnTTAHaDd&mediaurl=https%3a%2f%2fth.bing.com%2fth%2fid%2fR.c 52017ab840134d37628975718a9d34c%3frik%3dZxnnUnpnSm EMaw%26riu%3dhttp%253a%252f%252fwww.wood-database.com%252fwp-content%252fuploads%252frh-chart.jpg%26ehk%3dKM6rhRXIDlDfPHJrRdaAXo8qxkKbzpaS 7XlYXy0AJj8%253d%26risl%3d%26pid%3dImgRaw%26r%3d0% 26sres%3d1%26sresct%3d1%26srh%3d608%26srw%3d1300&exph=281&expw=600&q=graph+moisture+content+vs+relative+humidity&simid=608048080896462995&FORM=IRPRST&ck=2D8813638295F9F66AE0CDE66DA9940A&selectedIndex=0&idpp=overlayview&ajaxhist=0&ajaxserp=0

[Zachary Hoyt]

Thank you all very much for your help. Not having the right kind of oven for oven drying I won't be able to be very accurate, but the calculations will still give some rough idea, perhaps. I will keep leaving the piece of wood in the boiler room and see what happens in a week, and so on.

[Alan Schwabacher]

Dry weight is where it stopped changing: 524g, approximately what oven dry would be quicker. Your initial sample weighed 538g, so 538-524=14g of water was initially present. If you express this 14g of water as a percentage of the dry weight we get (14/524) * 100 = 2.7% water, which is very low.

If that is correct, your initial moisture content would be 6%, so maybe your stabilized water content at 524g is not fully dry.

[Zachary Hoyt]

The initial number on the 30th was 554g, so the amount of weight lost was 30g. When I used that in the formula I got a result of just under 6% for the initial MC, which is lower than seems plausible to me but I don't know much about it. I also don't know enough to know if eventually the wood will get as dry on top of the boiler as it would if dried in an oven. My ignorance is staggering.

[Tom Bender]

The moisture content of the end of a board will be lower than the center, unless you had the end sealed.

The moisture content of the sample on top of the heater is not relevant. It will regain some moisture as it acclimates to the shop.

[Kevin Jenness]

The "boiler dry" piece is unlikely to be at 0% moisture content. The fact that it stopped losing weight indicates it is at equilibrium with the relative humidity and temperature there. It's reasonable to think in your location in January that it's below 5% but that's just an educated guess.

Suppose that the sample is at 3% mc. The oven dry weight would be 524 x 100/103 = 508.7g, the difference in weight between that and the initial weight is 45.2g and the initial mc would be 45.2/554 = .0817 or 8.2%. To get an accurate reading you should follow the oven dry procedure.

Failing that, you could put the sample in your living space and see how much weight it gains (or loses). If it were to rebound to its initial weight then you might conclude that the stickered lumber is safe to work with.

Keep in mind that your initial sample, cut from the end, was probably drier than the average mc of the board. A piece cut at least 12" from the end would be more representative.

In situations like this a moisture meter is well worth having, even though pin types are not good below 6%.