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Off the Wall measurements

By Chuck Newcombe

When I saw this phrase in the biographical information attached to my inaugural column here, I decided I'd better try to explain the reference.

Perhaps it is most appropriately applied to my activity repairing TV sets and similar electronic devices in the late1950s. People brought all kinds of electrical devices into our shop for repair. I remember my boss calling out, "Anyone every seen one of these?" as he held up a box with a meter in it and a pin-cushion like gadget on a cord. None of the three of us present had, but I asked to take a closer look.

It was called a Plaster Wetness Tester, which upon further investigation I found to be a simple ohmmeter scaled to read "% Wetness." In the days before drywall construction, it turns out this was a pretty useful device - for truly "off the wall" measurements. When I was able to fix it, and a couple of equally odd devices, my employer seemed to find an endless supply of similar puzzles for me to solve. My career at Fluke was simply an extension of the puzzle solving I initiated back then, as I investigated one emerging measurement problem after another.

Fast forward to my last year at Fluke.

Shortly before I retired I was asked to assist a local non-profit organization with some interactive science displays to be installed in a new children's museum. I did as requested, and now that I'm retired I operate one of those exhibits for two or three days a week as a volunteer.

The exhibit is a Van de Graaff generator capable of producing more than 200,000 volts of static electricity, to stand the hair of the visiting children on end and to demonstrate some other mysterious electrical properties. The picture accompanying this column shows Yours Truly testing the installation.

One of the original concerns of the museum's director was that this high voltage device might affect other electronic devices in the museum, since similar installations across the country had suffered such problems. So before we built the exhibit room and installed the machine, I did some experiments of my own - in Fluke conference rooms and in my home solarium. Unsuspecting Fluke peers, family members, and visiting friends were subjected to my testing. We designed the exhibit room using the results.

So how does one evaluate the performance of such a gadget? With his trusty Fluke meter, of course! Using a Fluke 87-V digital multimeter (DMM) with the common lead connected to a ground reference, I have measured dc voltages up to 90 volts or so by simply holding the test probe in the air about three feet from the globe atop the generator. Recalling the meter's 10 Megohm input resistance and performing a calculation involving Ohm's Law tells me that I'm sucking a pretty steady 9+ microamps of current from the charged air around the ball. Not bad for a four foot rubber belt running around in an insulated vertical column.

One night at the museum, I demonstrated a toy called a "Plasma Ball" which I purchased for about $40.00. I was asked by a visitor whether this was similar in nature to the Van de Graaff generator in our exhibit. My answer was a clear, "Yes, No, Kinda…" as I attempted to explain the differences between the plasma made by high frequency ac in a Tesla Coil and the dc particle charging of the Van de Graaff machine.

It turns out the big concern of the questioner was about the amount of current that flowed into one's hand when it was placed on the ball. It seems he was able to detect a warming feeling in the palm of his hand. I tried it and found that indeed, you can feel warmth from the capacitively-coupled ac current.

The manufacturer says the ionizing potential is about 2000 Volts but that would be difficult to measure because of the construction of the device. I could, however, measure the current. Well, you guessed it. That led to a new series of experiments, using my DMM in the ac voltage mode, to measure the current coming through the surface of the ball, and its frequency. With my test probe connected to a piece of foil on the ball to simulate the palm of a hand, I measured 291 Volts at a frequency of about 23 kHz.

So, if the DMM input was just the 10 Megohms mentioned before, this would translate to 29.1 microamps - not enough to cause the heating we experienced. But the meter has a parallel capacitance across the 10 Megohm input. Fluke says it's "less than 100 pF." This leads to a much lower effective impedance at 23 kHz, so using the appropriate math I was able to estimate that the available current to a large area like the palm of a hand could easily exceed a few mA. Now, 1 mA at 2000 V is 2 Watts, which one could easily feel.

It's this kind of experimentation that has occasionally led to the introduction of new and useful features to many of Fluke's meters in the past. And it's why the people at Fluke really want to know about the new and clever ways you use the tools you buy from them.

Got an idea or question for Chuck? Send us an email.