Wood Drying - Part Five
How is Wood Moisture Content measured. What do the numbers mean?
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In earlier parts, Wood Moisture Content (WMC) has been widely discussed. However the methods to obtain measurements of WMC weren’t explained. In this part, I shall shed more light about this property of wood.
To begin, let me introduce an instrument designed to measure WMC, i.e. a wood moisture meter (see left picture). Through the elaboration of WMC, I hope to debunk the myths and assumptions pertaining WMC in guitars.
For details of wood moisture meters, go to: http://www.buyelectronicstore.com/images//upload/home/MD818_Digital_Mini_Iintelligent_Wood_Moisture_Meter5.jpg
As you gather from the picture, it is not a hi-tech instrument that requires a rocket scientist to operate. The method of measuring is invasive nevertheless. Press both probes (looks like needles) against any wood surfaces to depth up to 4mm. Simply read off the number displayed on the meter for the corresponding WMC. While this act will leave marks behind but they are not going result in structural failures to guitars. The picture left illustrates the way to use the meter.
Picture left was taken from: http://www.euroheat.co.uk/images/product_images/Log_Store/600/1402548039Moisture-meter-in-use.jpg
It is unlikely such invasive process will find favor among guitar lovers. There are in fact roller types wood moisture meters. The approach in taking measurements is definitely less invasive but they are much more pricier…
Picture left was taken from:
Setting WMC Baselines
Firstly, I went ahead to establish certain WMC or EMC levels of various forms of woods. Green woods, a.k.a. trees were the first targeted material. As living trees are within outdoor conditions, I have assumed that the WMC within trees have reached stability and it should not be fluctuating tremendously unless something dramatic or extreme happened to these trees. Therefore the WMC measurements can be taken as the EMC percentages too. In the cause of the investigation, it was discovered that most trees locally carry a WMC between 25% and 40%. As the meter’s working range is 5% to 40%, I assumed that any readings of 40% could mean more but the actual WMC levels were curtailed by the meter’s limits. Younger trees tend to carry higher WMC than older trees but tree species was a factor that influenced the WMC. It was also interesting to learn that the WMC is consistent along the tree trunks height with certain exceptions. When measurements were taken on the anomalies parts of the tree, e.g. truncated part, which turned in stumps, it exhibits higher WMC. However it was not a significant factor to this investigation.
The next wood form is outdoor fixtures and furniture that are made from woods. With the knowledge that tree species account for discrepancies in WMC, I would expect similar outcomes. After all, processed woods still possess the basic biological made-up and the behavior should not deviate drastically. However, the generally WMC measurements are expected to be lower than green woods. The measured WMC or EMC supports my hypothesis.
Moving along, the focus is on wooden fixtures and furniture found indoor, typically in residences. A few categories were sought for. They are wooden furniture, single piece of solid wood, single piece of plywood, wooden fixtures. Learning from previous experiences, wooden items in indoors were expected to carry less moisture than those within outdoors. The outcomes were congruent with the expectations.
The above readings were obtained by using the wood moisture meter mentioned in earlier sections. They will serve as baselines to the recommended WMC or EMC in guitars locally.
Making reference to description of WMC below, it was clear that most indoor wooden items appeared to carry slightly elevated WMC levels. However, most are not in danger of excessive moisture according.
Below 12% - Readings in this range are common to kiln or oven-dried woods and furniture grades of wood, and represent dry conditions. Most interior wood is in this range.
12% - 16% - Readings in this range are common to lumber during construction, air dried lumber and “healthy” residential substructures (beneath first floor in crawl spaces). These are typical readings for exterior wood.
16% - 20% - Readings in this range indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture. The excess moisture source should be corrected if found.
WMC on Acoustic Guitar
An old guitar was used as a specimen to establish its WMC at various parts. This guitar has been left in a room with average RH level of 77% to 59% for at least 6 months. Given this period, it would be sensible to assume that the moisture content within this guitar has reached stability. Therefore, I would suggest the moisture content to be taken as the EMC of this guitar residing in the mentioned environment. It has no strings on it. Different parts of the guitars are measured. See measurements below.
Under column EMC (A), the old guitar averages at 17.8% of WMC, a.k.a. the average EMC. It indicated slight elevated moisture content. As for the data presented under column EMC (B), it will be elaborated in later sections.
However, the percentage does little to indicate the tonal quality and playability of the guitar. A more definitive indicator is the string height at 12th fret which governs playability of guitars. Detecting changes in tonal quality has been suggested as another indicator but tone is highly subjective. It was also widely accepted among guitar builders that if the playability is adversely affected, chances are the tone will too be diminished.
Using all three indicators, that are WMC, string height at 12th fret and changes in tonal quality, one should be able to make an informed decision regarding to dehumidify or not.
Dehumidify The Old Guitar
To satisfy my curiosity, the next step is to dehumidify this old guitar and measure it exactly the same ways to ascertain the WMC levels. Dehumidification started on the 29 Dec 2010, at 1647hrs and it went on continuously until the 31 December 2010, 1118hrs. The guitar has been dehumidified for 42hrs. The approach and device used to dehumidify this guitar was a portable guitar dehumidifier that I have invented. More on this invention later… lets’ assume that the dehumidification process was administered with no anomaly.
Comparing column EMC (A) and EMC (B) shows the overall reduction in the guitar’s WMC levels for all areas. The average has been decreased to 14.9% from an initiate level of 17.8%. As this guitar has no string fitted, it was not possible to ascertain the playability. Among the measured areas, the bridge (from 20% to 8%) and back-front side (16% to 4.5%) experienced the most reduction in their WMC levels. It was clear enough to say that dehumidification was effective in combating excessive moisture within woods, in this case our guitars. Moreover for acoustic guitars, the critical areas are around and beneath the bridge. The mentioned portable guitar dehumidifier has indeed delivered its intended results, i.e. targeted dehumidification on critical areas. The outcome was indeed promising.
The average of 14.9% indicates the guitar moisture level is within the healthy region of exterior wood. However our guitars are not meant to perform within the outdoor. The desire WMC levels is 8.5% and more can be done to bring the WMC level to the healthy range of interior woods.
In Part 5, we have set WMC baseline of various woods regarding Singapore's humidity characteristics. An old guitar was experimented upon. The WMC on various parts of the old guitar was measured and recorded. Both before and after dehumidification regarding WMC recordings were taken. Compare and contrast was carried. From the outcome, it was clear that most guitars in Singapore were sufferring from slight elevated WMC. Ensuring critical areas were effectively dehumidified was most optimal.
In Part 6, we shall discuss the correlationships between temperature and humidity. Above it, we wish to ascertain the impact of this relationships on our guitars.