Comment summary #11
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Comment 101 ... 110
| date | article | author | comment |
| 07 aug 2018 15:35:30 | Multimeters - measurement deviation | Freddy | Yogi Bear, you are missing the point of this article. The goal was to show what measurement errors can be expected in certain situations besides by the manufacturers specified measurement errors. Why do you think that an oscilloscope is not suitable to measure voltages? It is a measurement instrument! How can the specified measurement error of the instrument be an argument. What is the error border that determines if an instruiment can measure voltages? 0,1%, 1%, 5%, 10% or maybe 50%? Just tell me. Beforehand the oscilloscope was calibrated against the Agilent 6 1/2 digit multimeter. You can observe what the deviation was between these two instruments. Yes, Agilent and Fluke are in error by calling it RMS. They should call it Standard Deviation. RMS is a descriptive abbreviation of Root Mean Square, and nowhere is mentioned that first the AC-part should be taken from the whole signal. And why would you rename standard deviation to AC-RMS? There are already mathamatical correctly defined terms! When I perform a RMS measurement then I do it with a reason. For example to determine the heat devellopment in a wire. I expect that a True RMS multimeter measures the correct value regardless the shape or DC-level regardless if I know the shape of the waveform or not. |
| 04 aug 2018 22:16:57 | Multimeters - measurement deviation | Yogi Bear | Frequency dependency Beautifully recognized - you should only measure signals that are also in the bandwidth of the meter. What does Fourier tell us about the bandwidth of the measured signals? With a square wave signal we need ten times the fundamental frequency as bandwidth. Cheap multimeters quickly find their limits here. Unfortunately, the bandwidth is kept secret by many manufacturers. AC Range true RMS Fake News: "Non-RMS meters in the AC range can not measure anything else than sinusoidal wave shapes correctly." With a known waveform, one can measure other signal forms within the bandwidth of the measuring device. But you have to convert readings by hand (ie use of the own brain). In addition, I recommend basic books on electrical measurement from the vintage area, as non-RMS multimeters were the standard. Btw., even an RMS value does not allow any conclusions about the waveform at the input. Knowledge of the signal form at the input is also required here. Therefore an oscilloscope is quite useful. The input determines what comes out at the output. As long as the tail does not wag its dog. Throughout the text I miss the explanation of the crest factor and its significance for the TRMS measurement. |
| 04 aug 2018 22:00:05 | Multimeters - measurement deviation | Yogi Bear | First, a fact that many ignore: an oscilloscope is not a device used to measure voltages. Why? Look in the user manual and search for the measuring error for voltages. Then it becomes clear why. An oscilloscope makes voltage waveforms visible and allows "precise estimation" of voltages. Consequence: The "reference values" in the table are therefore only to be used a rough guide, but at least have the advantage that they are correctly calculated. And they show, the Scope must have been switched to DC-Coupling when taking the samples. Btw., If one already makes such considerations, then one should also consider the traceability. Where are the amplitude and frequency specifications for the used ref. signals? RMS for multimeters As can be seen in the formulas, the input voltages are used to determine the RMS value. The consequence is that in the real True RMS value also input DC voltages play a role! In this respect, the indication "True" RMS on almost all voltmeters is a hoax, since almost always only the RMS value from the AC component of the signal is determined. Better would be the name "True AC-RMS"!Better would be the name True AC-RMS! There are also multimeters that look at the DC component and you have to know that, otherwise you won't be able to interpret the readings correctly. Pulsating DC voltage / sine rect. / Square asym. As the TRMS voltmeter (Agilent & Fluke) only measure AC-RMS it is clear that they show not the ref. RMS voltage, as this is a true (AC+DC) RMS voltage (oscilloscope is / was in DC!). If you want the oscilloskope to calculate the AC-RMS, you can do that by switching to AC-coupling. Or you have to make a computational effort yourself. Agilent and Fluke aren't in error. So to get the right expected value, determine the DC component of the signals and subtract them from the input signal. Then convert to RMS. Just as is done in the formula for the standard deviation: V (t) - Vmean |
| 17 jul 2018 21:59:41 | Additive synthese waveform generator | Harri | To hear the current setup sound. Adjust the volume from it's default off to some value. You can find the control for this under the right of the graph. |
| 15 jun 2018 15:39:50 | Additive synthese waveform generator | phil | How do I get sound? I can't get anything! |
| 12 jun 2018 01:07:17 | Magnetic hysteresis | Freddy | When do you get a circular shape, and what is CRO? |
| 11 jun 2018 06:15:10 | Magnetic hysteresis | Probodha Sathish | I want to know , why we get a circular shape graph in CRO |
| 26 mei 2018 12:33:14 | Measurement Errors | aaaa | Can you tell if a digital multimeter or a digital panel controller will show negative reading for an AC current especially for Power Factor Correction panel? Does that mean the reverse of CT polarity? |
| 23 mei 2018 22:01:25 | Fast Lux meter | Freddy | The MAX4239 has a lower input offset voltage, so the negative output voltage sould become less by using this opamp. If the bandwidth is less important you could make the feedback capacitor larger. This will reduce the noise level. |
| 23 mei 2018 14:25:01 | Fast Lux meter | Marcos Fraga | Hi Freddy! Thanks for the circuit :) What would you recomend to do to make this circuit sensitive from 0 - 50 lux? With the setup in fig 6. the noise is low but it still makes a problem at lover voltages. I get negative average voltage when going under 5 lux because ov pk to pk noise. Maybe I could offset the whole thing? All comments are appreciated. BR Marcos |