Multimeter Range and Resolution

Question

I'm confused as to how my multimeter takes readings, especially since its not an auto-range multimeter. I have to switch to the desired range to carry out measurements. Unfortunately, the resolution part is what confuses me. How do I know my exact reading?

Answer 1

Your meter uses a DVM, digital voltmeter, module with a range of -199.9 mV to +199.9 mV (or ±0.2 V in round numbers). These are known as 3½-digit displays, the leading '1' being the ½ digit.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Simplified switching diagram for current measurement ranges. Only one switch closed at a time but make-before-break contacts are likely to be used to prevent high voltages being applied to the DVM during range switching. Fuses and input protection not shown.

The current measurement ranges connect the digital display inputs across a current shunt resistor whose value will generate a 200 mV drop at the rated current. So, most sensitive range on your meter, 2 mA full-scale, will need a resistor of \$ R = \frac V I = \frac {0.2}{0.002} = 100\ \Omega\$ for the current shunt. The resolution of the display is the smallest increment it can display on that range.

Range:  2.000 mA    20.00 mA    200.0 mA     2.000 A
            |           |           |            |
Resolution: 1 μA        10 μA       100 μA       1 mA

As Andy has noted the accuracy depends on the stack-up of all the errors.

Answer 2

Unfortunately, the resolution part is what confuses me.

Resolution is the smallest increment that your display can indicate. In other words, if you are on the 2 mA range then the display will be like 1.000 mA when 1 mA is being measured. The least significant digit is 1 μA.

But, the accuracy (1.2%) tells you that if your display reads 1.000 mA then the meter might be actually be measuring a current between 0.988 mA and 1.012 mA. But, it's a bit worse than that because the text says something like this: 1.2% + 2.

The + 2 bit means that the least significant digit could be in error by up to 2 in either direction. So now, if 1 mA was fed to the meter, it might read anything from 0.986 mA to 1.014 mA.

Clearly, if you are only measuring 100 μA, the + 2 bit becomes ten times more significant as an error. For instance, if the current through your meter was exactly 0 mA then the display might indicate a value of -0.002 mA to +0.002 mA.

So how do i know my exact reading?

You don't and, neither does anyone else. There are always measurement and rounding errors.