Following up on a fair amount of belly chat recently, there is a very basic question which is assumed in all the discussions of board types. That is, we are trying to quantify board EMC at various points in the construction process. I know for myself that I don't have a sufficiently accurate and efficient measurement protocol to really know what EMC I'm at at various point along the process. Many of the pinnless meters that many of us own are inadequate to the task. My meter has 2 issues: 1- its designed to measure to the center of a 3/4" board. THerefore probably reading some mixture of the board and the air behind the board...or something...but not the board accurately. 2-mine only gives reading to 7% 3-the margin of era on these tools is +/- 2%....In this work, 2% is huge and in my opinion unacceptable. There are a couple of meters designed to measure to the center of 5/16 stock, but not down to 4% to my knowledge. To get around this problem, I have in the past, when working with a board in the hot box, gone through an oven drying protocol with the household microwave, but really... that's way too inefficient. How are people who are talking about low EMC's in the 4% range quantifying their as built EMC's? Same question for those of us in the higher EMC Ranges. The RH tables are useful way to gauge EMC, and I have used them. But, how long it takes a sample to reach equilibrium is an open question in all of the hot boxes I've seen, including my own, which do not qualify as scientific devices. I am not always able to prove by oven drying that my board is where the RH table says I should be. That brings another inaccuracy to mind. How do we measure the RH in the box. My first unsuccessful attempts used digital meters. I quickly found that by putting 3 meters in the box I could get 3 different (sometimes significantly) different RH readings (hey that way I choose the one I like best). Further when I pushed the temp up past 110 degrees my meters lost their minds. I finally ended up mounting a good old dual bulb thermometer with a small computer fan mounted in-front of the wet bulb which I turned on to take a reading. But my question here is very basic step 1 material. When people are talking about their board EMC during construction whatever their construction choice, how are the EMC's being quantified...accurately. Jim I
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