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RE: Inexpensive LCR-Multimeter



Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

With respect to the measurement frequency issue that there is a bit of a 
concern...

If the "box" measures the component at 60 Hz or 1 kHz, there is a good 
chance that parasitic C or L won't be much of a factor.  For the typical 
telsa coil running at a few hundred kHz, the parasitic C (particularly of 
the secondary, where the C is part of the resonant circuit), an "apparent 
inductance" measurement at 100-200 kHz might be very very different from 
that measured at 60 or 1000 Hz.  Of course, one could argue that the 60 Hz 
measurement of the L is actually more accurate, because the error in 
measurement due to C is much, much less.

It kind of depends on what you are trying to measure, I suppose....

I would think that as long as the device you are measuring is within the 
measurement range of the instrument, you're probably ok with the cheap 
device.  I wouldn't try measuring 0.05 picofarad caps with a $60 
instrument, both because the reactance is so high at the likely measurement 
frequency that it is hard to make the measurement, and because the "test 
fixture" is going to contribute a lot of error.  Likewise, measuring the 
inductance of a 12" piece of wire (about 0.3 uH) is going to be iffy.  But, 
in both cases, it's because the instrument isn't suited to the test 
article, not because the $60 widget isn't a decent meter.

I suspect that environmental effects and test fixtures will dominate any 
but the most careful of component measurements in tesla coiling, and a 
meter that claims, for instance, 5% accuracy, would be more than good 
enough. The 27XT claims basic accuracy of 0.5%, capacitance range from 
0.1nF to 2000 uF, and inductance from 0.1mH to 20H.  Since the latter has 5 
ranges, I'm going to guess that the ranges top out at 20H,2H,200 mH, 20mH, 
and 2mH.  The 0.1 mH is 5% of full scale on the 2mH and since it is a 1999 
count device, that reading is 100 counts, so the 0.5% accuracy is 
consistent (the meter only has a display precision of 1% for that 0.1mH 
inductor).

http://www.metermantesttools-dot-com/downloads/DataSheets/DMM/XTSeries.pdf

Looking at the spec sheet (above) in more detail, I see that the 
capacitance accuracy is 5% +10 (counts?) on the 27XT (interestingly, the 
25XT is actually a bit better..)
For inducatance, the accuracy is +/-5% + 30/1uH and they use 1000 Hz as a 
test freq on the two low ranges (2 and 20 mH).. At 1000 Hz, the reactance 
of 2 mH is about 12.5 ohms, so things like lead resistance shouldn't throw 
the reading off that much.
(I wonder how the meter measures inductance?  If it measures voltage and 
current, does it also measure the DC resistance, and take that into account?)


At 04:57 PM 1/8/2003 -0700, you wrote:
>Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><Gary.Lau-at-hp-dot-com>
>
>Which things are useless, LCR multimeters?  I can't speak for the $60 unit 
>offered by Ocean State Electronics, but the experience of myself and other 
>on this List with the Wavetek 27XT has been that it's very accurate for 
>both primary and secondary inductance measurements, as well as 
>capacitance, and frequency.
>
>Why is it necessary to measure inductance at different frequencies?  My 
>characterizations of different primary coils with a _very_ expensive 
>HP4194 impedance/gain-phase analyzer showed negligible variation of 
>inductance with frequency.
>
>40 years ago, who would have believed that restaurants would give away 
>digital wristwatches with a hamburger?  Technology advances and what used 
>to be costly becomes cheap.
>
>Gary Lau
>MA, USA
>
>
>Original poster: "Mccauley, Daniel H by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
>
>In my experience, these things are basically useless for inductance
>measurements.
>
>You should really try to get a dedicated LCR meter which can measure
>inductances at a variety of
>different frequencies.  There is a place that sells very high quality LCR
>meter kits for about $50.00
>High quality in that the readings are extremely accurate.
>
>I'll try to dig up that source.
>
>Remember the old addage.  You get what you pay for.  Multimeters that have
>every single function known to man
>especially for those prices are usually garbage.
>
>Dan