Technical problems of measuring the hottest fluke

2022-08-07
  • Detail

Fluke multimeter measurement technical problems

what is the wave crest factor of the oscilloscope

the crest factor of the oscillograph is often asked. If you look at the specifications, the only explanation you can find is "the reading does not depend on any signal, that is, the crest factor". Here we will explain what the wave crest factor is and what the value can be used for. For a multimeter, the crest factor should be taken into account when measuring non sinusoidal AC voltages. The crest factor is defined as the ratio of the peak voltage to the RMS voltage

typical crest factor is:

sine wave: 1.414

square wave: 1

25% duty cycle pulse: 2

for a multimeter, (it can handle very high crest factor), but because you can't really handle the shape of the waveform you measure, your reading may be wrong because your signal peak is too high. The peak value can overload the circuit, and the reading may be too low and not displayed on the measurement problem. A multimeter with a true RMS reading can specify the value of the maximum crest factor that it can truly and accurately process. Due to the saturation characteristic of the true RMS conversion chip, this value is usually near the lowest point of the full scale reading. There will be better accuracy near the half scale. Therefore, it is a good habit to check the reading of your Multimeter in the sub high range and verify that the reading is of the same order of magnitude. The peak factor of fluke 87 can reach 3. The crest factor of Agilent 33401a and 33970a is "there is a maximum crest factor of 5 near the full scale" (the scale has an additional error of 0.40%)

for Scopemeter products, the situation is different. First, there is always a waveform on the screen to indicate whether the signal is overloaded. In addition, Scopemeter calculates the true RMS measurement value from the sampled waveform data. As long as the sampled value is outside the range of the a/d converter, the reading will no longer be displayed, and ol will be indicated. For example, fluke 123 will display ol in a 5V range when the peak value is greater than 8.5V. This will result in a fully scaled crest factor of 8.5/5=1.7. Although this value is not very significant, it is not important for Scopemeter because the device does not display incorrect readings. In addition, when using the automatic mode, it will automatically change to the next higher range. (Scopemeter can measure peak and RMS simultaneously)

scopemeter can correctly measure the true RMS value of the signal without relying on the crest factor. The limitation is that a higher range is required to correctly capture the peak signal. Because of this higher range, you can only accurately measure the limit value of the lower range for the RMS value. If the RMS value of the signal is too small, you can stop using the lower range value with less accuracy. For the oscillograph, the useful range is the range with the maximum crest factor of 10. For example, for a 100Hz 5V pulse with 1% duty cycle, measuring the same signal with fluke 87 (I) will give a reading of 174.1mv. When manually switching to the next higher range, it will give a better measurement effect: 0.483mv

make sure your measuring equipment can process the shape of the signal you are measuring. If you are unsure, double check in a higher range, or select equipment that never gives false readings

● why is the number of reading services obtained on fluke 123 in 45 countries around the world different from that displayed on fluke 190 series or other oscilloscopes

as you know, we have very good specifications for the accuracy of scopemeters. But sometimes you will find that the readings are different between fluke 123 Scopemeter and fluke 190 series Scopemeter (for example, for high frequency amplitude measurement). This difference is too great to be explained by the error of the equipment. Recently, we found that sometimes the reading of fluke 123 has disappeared, while fluke 190 has a good reading. The reason is that different loading circuits

fluke 123 (STL 120) test probe is a 1:1 probe, which tests the circuit with megohm//225pf. Fluke 190 series probe (vps200) is a 10:1 probe, which tests the circuit with the impedance of 10 megohm F. Different from fluke 190 series probes using vps200, fluke 12 provides better market service. 3 Scopemeter uses STL 120 shielded test head, which will load the circuit at about 20 times the frequency (e.g.>1mhz). When the internal resistance of the circuit is small, it will not affect the reading, but when the resistance is large, it should be noted. The test will lead to the reduction of the voltage on the circuit under test, resulting in unrealistic test results

use the 100:1 probe at high frequency

at high frequency (>1mhz), the resistance of the capacitive parts of the probe is very large. A standard 10:1 probe, such as VPS 200, has a capacitive resistance of 14pf. The 100:1 probe, such as the new VPS 201, has a capacitance resistance of 6.5pf. Using a 100:1 probe will greatly reduce the load on the test circuit

(fluke 190 series Scopemeter with 100:1 probe has a maximum sensitivity of 500mv/div)

● fluke190 stopped working after 47 hours. Why

when fluke 190 series works with fluke 123 or Scopemeter B series power adapter instead of bc190, each function will work normally, but after some time, fluke 190 series will automatically shut down due to battery depletion. When you use a recording function of fluke 190 series, other power adapters will not be able to capture the important information you are trying to capture

pm8907/80x cannot provide sufficient current to fluke 190 series, and fluke 190 series battery may be exhausted when using pm8907/80x instead of bc190/80x. This is mainly due to the limited output power of the pm8907/80x. Pm8907/80x provides 300mA output current, while bc190/80x provides 840ma output current

operation:

use only those power adapters provided by the device. Because the device has been tested and certified by the adapter

when sending the demo device to the customer, make sure that the power adapter is included so that you can get a good first impression

suggestion:

do not use fluke 160 series pm9651/00x adapter

this adapter is not suitable for measuring higher than 30vrms

● how to display Lissajous figure

to quickly display Lissajous figure (or X-Y display), use the following method

1 connect probe a to signal a and input B to signal B

2 connect all ground probes to the ground

3 convert all inputs to AC coupling to subtract the effect of DC offset

4 select "a vs B" in the calculation menu

5 lisayu figure will be displayed on the screen

note: use the mobile keys a and B to change the position, or you can use the cursor to locate

● optimize your Lissajous graphic display

for your optimized Lissajous graphic mode waveform display, you'd better have at least one but not less than 3 cycles of signals in the record. Using the analog range, the time base is not used, and the electron beam is directly controlled by inputting X and y of a and B. In a digital oscilloscope, such as Scopemeter, the setting of time base can affect Lissajou's graphic display. In the digital range, waveforms are first obtained, and then microprocessors are used to produce Lissajous graphics

changing the time base setting can improve the signal display

this figure shows the display when the cycle is too small, and only some signals are displayed

if the time base is set too slowly, too many signal cycles will be obtained, and not all signals will be evenly distributed

for mechanical or low frequency applications, it is recommended to use a bandwidth limiter to reduce unwanted noise. Use under normal conditions when the afterglow function is turned off

● how to record three-phase voltage with a dual channel fluke-123 oscilloscope

according to Murphy's law, when you measure phases 1 and 2, the third phase of the three-phase system will be deposited. A simple method enables you to observe three phases simultaneously on the trend drawing screen of two channels

setting:

connect channel 1 to phase a, and the ground wire of channel 1 to phase C. The channel 2 you are connected to is connected to phase B, and its ground wire is connected to phase C. (Note: as you know, COM A and com B are internally connected in fluke 123!). Now select the menu "VAC" (or "vac+dc"), and you will see two sine waves from the input. Ensure that the two channels have the same input sensitivity. Now select the "trend plot" located in channel a, and you will see two graphs with almost the same voltage!

interrupt:

if the voltage in phase a decreases, you will see this change in the channel (trend graph), (above figure) , if the voltage in phase B decreases, you will see this change in 2 (trend chart), (figure below). But when the C phase changes, you will see that the two figures are changing in the same direction

please remember that the ground wire of channel 1 of Scopemeter 123 is automatically the same as that of channel 2, because they are connected by hardware! If your application requires different ground wires, use fluke 190 series Scopemeter or dpl20 differential probe

(Note: you can use this method on fluke43b power quality analyzer, but input 2 will read amps instead of volts. If you set the probe of input 2 to read 1a/v, it is very easy to read the correct voltage on input 2.)

● why do I find graphic breaks on my trend plot

it's like my fluke 190 series Scopemeter's paperless recorder has finished using its ink. The first thing I can think of is "I lost the signal". In fact, when your signal is lost, trendplot will draw a graph with a value of zero. The situation shown in the figure only occurs when Scopemeter cannot perform the measurement function. For voltage or current readings, this occurs only when there is no trigger condition. To prevent this kind of event, you can set your trigger condition to "automatic" or select the "free running" state when using boundary trigger

when you make time-related measurements, such as pulse, load, frequency or phase, at least one cycle of signal should be measured. Otherwise, the measurement will not be carried out and no signal will be displayed. Scopemeter 190 series also detects time and power related quantity measurement. If no relevant quantity is detected, no reading will be displayed

when using FlukeView to output the waveform with interruption to the spreadsheet, you will find that these units are filled with nan, which indicates that there is no data

● how to remove dual tracks in FlukeView

this double track is also called "most

Copyright © 2011 JIN SHI