What does the horizontal bar indicate and how to use it in my measurement?
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From the Fluke 87V datasheet page 11 point 18 :
The number of segments is relative to the full-scale value of the selected range. In normal operation 0 (zero) is on the left. The polarity indicator at the left of the graph indicates the polarity of the input. The graph does not operate with the capacitance, frequency counter functions, temperature, or peak min max. For more information, see “Bar Graph”. The bar graph also has a zoom function, as described under "Zoom Mode".
The bar is a fast-responding 'analog meter', that can be useful for following variations in the input voltage, for instance when you're adjusting a circuit with a potentiometer in real time.
In the Fluke I had, the digital display updated once per second to 3200 count resolution, while the 32 step bar updated at 25Hz.
This was introduced shortly after DMMs were to counter the criticism that analogue meters were better when you had a varying input and could watch the needle swing in real time. Waiting a second for the digits, and then needing to think what they meant, makes tuning circuits unnecessarily awkward.
I know this question has been answered, but I don't agree with the answer.
The horizontal bar shows you how much of your current range you have used up, ie. how much the voltage/current/whatever can change before the meter has to switch to a higher input range
Anyone who has spend hours upon hours using a multimeter will understand why this is usefull; sometimes when you are measuring for instance a voltage with a multimeter what you are measuring might be very close to the upper bound of one range, and so it can happen, at least with some older multimeters, that the range selection will start toggling between two ranges, which can be super annoying. If you are measuring something which you know has spikes that are larger than what you are trying to measure then the horizontal bar will give you an indication of whether the spikes that can occur will be likely to cause the multimeter to change range while you are measuring, letting you change to a higher range if what you are measuring is close to saturating the value of the horizontal bar..
Without the horizontal bar you can have no way of knowing, by just looking at the reading from the multimeter, how much of the current range you are using.
The bar is also useful in the case that you are using manual range, because it may tell you that what you are measuring is only using up a small part of the current range, ie. you are not getting the full resolution of the instrument, knowing this you can turn down the range to use up more of the range of the instrument for you measurement and hence get better resolution.
Several of the answers here claim that it is for DMMs to "compete" with analog meters. I very much disagree with that, DMMs are inherently faster than analog meters, although you might not be able to rapidly read the changing numbers on the display, there is a physical limit to how fast the nedle on an analog meter can move as well, and it is driven by a coil which futher limits its bandwidth. Besides, looking at the bar moving back and forth might let you know the voltage is changing, but not by how much, as you have no way of knowing how large a value a certain size of bar corresponds to. And besides; You're an idiot if your trying to measure a changing voltage (other than pure AC) using a multimeter rather than an oscilloscope.