help a beginner measure mc in wood with confidence

[Gene Nelson]

  Gene,
      We bought an expensive dial type hygrometer with a calibration certificate from Germany, we also have less expensive digital ones that we bought from Radio Shack both give RH and Temperature. I have found that they are all within a few percentage points of each other in accuracy, however, I admit I don't know which one is more accurate, the expensive one or the el-cheapos, but their close to each other. 

  <Probably an average of all meters would then be close enough?
    
       If I understand what your saying in your post, your calculating the MC with the RH (via a Hygrometer) and weighted samples. In my opinion you need a moisture meter, We use one that is accurate down to 5% MC, I don't know what MC you want to attain, but 5% is pretty low.  We keep our shop at 35-40% RH and 66-70 degrees year round in an attempt to be as stable as possible. When our wood has acclimated for a while, the actual MC and EMC (via the chart) are close, but never the same. I admit we have never tried the oven-drying method for determining MC buy the weight, but have oven-dried wood for determining shrinkage based on MC.

  <Yes, I use a cheap hygrometer with temperature and refer to the EMC chart and compare this to the weighted samples - so far I am unable to establish uniformity in measuring %mc but there is a corelation.

      For our purposes we don't need the MC as low as the Bellymen requires so we find that a moisture meter with it's range of accuracy is well within acceptable limits.
      Hoadley is great and easy reading, if you want a supplement to that look at the website for the U.S Department of Agriculture, Forest Service they have excellent books on this type of stuff that you can download as a PDF for free. 

  < Thanks, I do visit the U.S. DOA  site frequently.

      I have to ask where are you that you need a humidifier to get the RH up to 40%? We need DEhumidifiers to get the RH to 40% and you can tolerate a temperature of 95 degrees F?

  <Northern California foothills - east of Sacramento. Hot and dry - outside can get up to 105 F -sometimes higher - with rh in the lo-twenties to teens.
  My shop is something that my wife calls her garage. I have a swamp box that helps keep the temperature down a bit and the rh up so things don't dry out too much.
  When I need to address things like setting bearing or measuring crown is when I need control of the mc in the wood accurately. Do not have a hot box set up yet.
  When I dry my panels down I do it with the panel on sawhorses, covered with blankets and an oil filled space heater underneath. The weighted samples are in there with the panel. I only dry down to 6%-6.5% mc. However, if the shop swamp box is left off, the wood will get below that without any help so dry down strategy changes depending on the time of year and what else is in the shop - winters are relatively mild around 30-40f and 45-70%rh.

  Gene

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    ----- Original Message ----- 
    From: Gene Nelson 
    To: Pianotech List 
    Sent: Saturday, August 16, 2008 11:53 AM
    Subject: help a beginner measure mc in wood with confidence


    I have been using the technique Bruce Hoadley talks about - cut several samples of spruce (similar dimensions) to the same weight - 100g is easy to deal with - oven dry one of them to get a dry weight to compare the others with. Sounds straight forward enough.
    Then I compare calculated mc using weighed samples with my hygrometer and the EMC charts and the disagreement begins.
    If conditions are changing, certainly there is a lag time for equalization but the disagreement continues. Also the samples must be placed in the shop so they are representative - still the disagreement continues.
    Probably it would pay to get a much more accurate hygrometer? 
    Is the weighing technique really accurate? How would you determine if it was?
    Would it be helpful to add a moisture meter to the lot?
    I am curious how shop folks gain confidence in their technique to measure mc so that if the measurements are telling you the mc is 7% it really is 7% +/- an irrelevant amount?


    I can give one example: I use my humidifier to get the rh up to 40% at 95 deg f - this should get my weighed samples up to about 7%rh but they insist on weighing in at 6.1%. Is this a typical discrepancy between cheap hygrometer and weighed samples?

    Thanks for any comments. 
    Gene Nelson
    Thanks,
    Gene Nelson
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