Calculating moisture content

      Using a hygrometer and calculator to determine the MC of kiln samples. September 2, 2002

Q.
If I understand correctly, wood will attempt to reach equilibrium with the air. If I place my kiln samples in a plastic bag with a hygrometer/digital sling psychometer, I should be able to calculate the moisture content of the wood given the temperature and RH. I found a calculator at woodbin.com. Has anyone attempted this? How long before the balance is met? A hygrometer at Radio Shack is $29.99. The least expensive pin meter is $89.00.

Forum Responses
Indeed, what you describe is an ASTM test. However, the pieces of wood used are fairly small. They are put into a mason jar, which is capped. Then the equilibrium RH is determined to get the EMC and this is equal to the MC. A kiln is too big to run this test and the kiln is not sealed.

Gene Wengert, forum technical advisor



I'm not sure how you would calculate RH using a sling psychrometer or a hygrometer in a plastic bag, as they both require air movement over the wet wick.

There is a standard easy-to-read chart that shows EMC and RH at a wide range of dry bulb and wet bulb values that appears in most drying publications, including the Kiln Operator's Handbook.



From contributor J:
Putting a sample in a plastic bag with a hygrometer will not give you the true MC all the time. The values on the chart and tables in the various publications are only averages of many samples of many species. At each temperature and humidity, every species will give a fairly wide range of actual MC's. Species with high extractive contents will give a lower EMC than species with lower extractive content.

An extreme example is a company I am working with now that is drying estuary salvaged timbers. The salt gained over the years of soaking in the brackish water initially made the use of electrical moisture meters useless, as the salt influenced the electrical resistance. But more to the current subject, the salt and resin content drastically increased the EMC. The MC which they have to dry to is greater than the normal 6-8%. The salt holds onto more water at the same RH and temperature than normal wood.

On the other hand, composite materials - MDF, plywood, OSB - are exposed to high temperatures in production. Their EMCs are lower than normal wood at the same RH and temperatures.

All this comes into play when a project includes different species and, say, MDF. In order for the project to be in correct moisture content balance, the MDF needs to be at 5% and the normal wood needs be at 8%.

When drying wood, the EMCs on the charts are more guidelines and targets than exact values.



What contributor J says is partly true, but the variation of EMC between species is quite small (of interest mostly to university labs, etc.), so the standard EMC values are used on all North American species without an adjustment and have been for nearly 100 years.

Incidentally, the table values used in the US are based on spruce (Sitka, I believe) and not many species. They used shavings from a hand plane and exposed them to oscillating RH conditions during the first time these pieces were dried (oscillating desorption).

Putting a sample in a confined chamber is an ASTM standard test and I am sure that the ASTM was careful when they adopted the tests that they published, as they are used around the world and can be used legally, etc. So, be fairly confident in the test and the values.

As far as an MC in MDF of 5% when the table values indicate 8% EMC, this seems extreme. Remember that at 0% MC, all agree. The effect of temperature increases with increasing moisture content, but it is hard to believe that it is off by 3% MC already by 8% EMC. This means that at 28% MC (saturation), the difference would be over 10% MC. If this is true, I have not seen it or encountered such differences, except with wood that has been impregnated (e.g., salt, fire retardants, etc.).

Of course, salt that is impregnated into wood is a hygroscopic chemical itself. Most naturally occurring extractives are not highly hygroscopic, so they do not have the same effect as salt. As far as salt goes, there was a process called Morton Cure that was patented and provided a way to dry wood by salt impregnation. Unfortunately, the salt does rapidly corrode fasteners and at high RHs, the wood gets wet.

Gene Wengert, forum technical advisor



From contributor J:
The initial test may have been performed on Sitka spruce, but the current values are a tabulation of many species since the first tests were performed.

The ASTM tests are only concerned with the test procedures. The society establishes the test to be sure the same sample gives the same result under the same conditions and that all labs use the same procedures. No test procedures can dictate how different materials will perform during the tests.

The Morton Cure process resulted in higher EMCs because of the salt along with the problem of corrosion. This was the same result as the timbers soaked in brackish estuary waters.

While a difference of 3% for different material exposed to the same RH and temperatures may seem drastic, it does occur and causes problems in production. Therefore, putting samples in a plastic bag with a hygrometer will not necessarily give you the correct MC of the sample.



From the original questioner:
Gene, I have not been able to locate the ASTM spec. Do you have the number? Would drill shavings work better as a sample?

Describe the Morton Cure method of drying wood. Is there another method involving limestone caves?

Have you seen the calculator on www.woodbin.com? Does it give reliable results?



Contributor J, the original data was for Sitka spruce and the numbers have not been changed since the original, except for a small change at high RH and elevated temperatures. No other species have been used in the U.S. data.

To the original questioner:
The standard is ASTM D-2016.

I do not think that drill shavings would work, due to heating.

For salt curing, the wood was soaked in a water/salt solution, usually heated, for a given length of time that allowed the salt to penetrate the wood thoroughly. Obviously, some species that are not permeable enough will not respond to this treatment.

Yes, there is a patent on curing wood in limestone caves, but the secret is the cool and damp conditions and not the vapors from the limestone. The limestone itself is not in contact with the wood.

The WoodBin calculator for EMC (and also shrinkage) is just fine, but they do state that wood has an EMC, which is not correct. Air has an EMC and wood has a MC.

Gene Wengert, forum technical advisor



From the original questioner:
It appears that D2016 has been discontinued. Can you describe the test method in more detail? (Sample size, dwell time, etc.) I just want to avoid problems attributed to improper drying.


From contributor H:
Be sure that you are looking in the index volume. If you have the ASTM volume for steel products, for instance, it will not show the specs for wood. The last full set of ASTM specs I saw was 4 or 5 feet long on the shelf.

Whatever you dry to, if you don't use the wood right away, it will soon be what the ambient humidity is in your area, anyway. For what it's worth, that means 12% in southeast Michigan.



From the original questioner:
The current ASTM specs are searchable on ASTM.ORG, which indicates D2016 has been discontinued with no replacement.


Try these numbers - often they will have a two digit number after them indicating the year it has last been changed:

ASTM D4442
Direct Moisture Content Measurement of Wood and Wood-Base Materials

ASTM D1037 (Part A, Sec. 126-127)
Evaluating the Properties of Wood-Base Fiber and Particle Panel Materials, Part A, Sec. 126-127: Moisture Content and Specific Gravity

Gene Wengert, forum technical advisor



From contributor H:
I don't believe that anyone has taken serious exception to the last paragraph in my post that normal storage means ambient MC in a month or so, no matter how dry the wood comes out of the kiln. Therefore, if you want to work with 6% wood, you not only have to figure out how to get it to 6%, you have to figure how to keep it there.

One other point in kiln design and drying practice is that you want to be able to get the wood to 135 degrees+ for at least a few days to kill the bugs.



Lumber storage is covered in 'Drying Hardwood Lumber'. If your storage closed-in shed is about 25 F above the morning low temperature, you will keep wood at about 7% MC for many months without any problems. There are other more complicated techniques too.

Lumber off of stickers will have the exposed lumber and the ends change MC within a month, easy, in an ambient shed. If this is a defect, then do not do it!

Gene Wengert, forum technical advisor



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