Veneer Moisture Measurement
A radio frequency moisture meter is a device which gives the measurement of the radio frequency dielectric properties of a condenser or electrodes directly on a meter scale. The plates of the condenser are the electrode plates and the dielectric of the condenser is the sample material or Veneer pressed against the electrode. The veneer under test is penetrated by an electrostatic field which is radiated by the electrode. The dielectric loss of the sample is reflected through coupling coils into the circuit of a self-excited oscillator. This changes the conditions of oscillation so as to cause a shift in the grid circuit of the oscillator tube. A sensitive microammeter in the grid circuit is used to correlate these meter readings caused by varying amounts of moisture in the test sample.
In simple lay language; the radio frequency instrument tends to average out the moisture content in the sample directly beneath the electrode down to the penetration depth of the instrument which is between 1 and 3mm. This is achieved by carefully choosing the size of the electrode coverage, thus the penetration depth is varied. The RF instrument does not attempt to indicate the exact moisture content at any particular depth but tends to give an average of the entire area covered by the electrode. By taking several quick spot checks across a sheet of veneer, a good average moisture indication of the piece can be obtained. As we have discovered, if one measures a veneer sample by placing a specialized moisture meter like the HM8-WS1(HD) over the sample, one must be careful as to what is under the veneer sheet. It would be wise to measure the veneer over Styrofoam or foam, i.e. a material that contains no moisture and that is not a metal.
There has been some misunderstanding to the effect that the radio frequency meter is only a "surface tester." Since the radio frequency type electrode (like the HM8-WS1(HD)) does not actually probe the sample beneath the surface of the sample, it is easy to see how such a misunderstanding can occur. But this is not correct. The reading obtained on the microammeter is affected by the average of the moisture contained in the entire area covered by the electrode down to the penetration depth. As a matter of interest, if you try taking a reading with a radio frequency instrument on the surface of a telephone book, then place a wet blotter between the pages of the book and then take another measure, you will notice the measurement increases from the original reading.
General characteristics of radio frequency type moisture meters (Pinless):
1. Portability. Can be carried from station to station easily.
2. Rugged construction. Designed for field use as well as laboratory.
3. Fast operation. Can make average tests within a few seconds.
4. Easy and convenient operation. Non-technical personnel can use instruments with confidence.
5. Does not mark the sample under test. This is not so important when testing rough lumber but becomes very important when testing finished surfaced boards, veneer, etc.
6. Calibrated for common species. Calibrations typically start at 4-6% moisture content and are carried up to the saturation point in most cases.
RF meters are affected mostly by the density of the wood or species under test and, to a minor degree, by the wood temperature. It is for this reason that separate calibration charts are required for each species. This prevents a direct reading scale on the microammeter for all woods. However, our calibration charts are all based upon check points made from many samplings which are compared to careful oven determinations. Therefore, in most cases, we feel that a reading taken with a radio frequency type meter which is based on an average of many check points which is more accurate than a single oven determination.
Radio frequency Moisture Meters are now being widely used in the manufacture of veneer. Tests are made on single sheets of veneer before passing into the driers as well as after the drying operation. The instrument is also used on thin stacks of veneer in storage before shipping.
Radio frequency type moisture meters are also used on stacks of veneer in the plywood industry. The instrument will not give the moisture content of the individual sheet due to the increased penetration depth. However, it will give a good average indication of the moisture content of the veneer stack. In addition, the Merlin ECO-10 or Wagner MMC 220 moisture meters can be used on veneer stacks, plywood panels over 1/8" thick, as well as on dressed lumber. Therefore, they are more versatile and can be used in more spots. Samples of approximately 3/4" can be tested perfectly.