The Fluke meters I am familiar with are specified as +/-(given percent of measurement + given number of least significant digits). For the given info, using the 400 ohm range, this yields +/-(0.004 * 0.22 + 2 * 0.1) = +/-0.20088 or approximately +/-0.2 ohms, if the meter is accurately calibrated.
When measuring low resistance values, especially at the sub 1 ohm level, you will need to consider the impedance of your probes and the connections to the meter and the resistor. If you make a resistance measurement with your probes shorted together, you can get an idea of the resistance of you probe connections.
To more accurately measure sub 1 ohm resistors without using a Wheatstone bridge or other indirect measurement methods, I would recommend using a bench DVM with a 4-wire (Kelvin) resistance measurement capability. This will remove the resistance of your probes from the measurement.
- I think the voltage drop in your top example is caused by the voltmeter's input impedance (probably around 10M) that slowly gets into range of the ohm-meter.
- For range 20k and up it is again the voltmeter's input impedance issue. I think the 200Ω range is related to the diode measurement which requires a similar current source at a relatively high voltage. That leaves the 2kΩ range which is probably implemented in a cost effective way based on the current source for the 200Ω range.
Only with the circuit diagram the answer can be 100% sure.
Your multimeter will attempt to measure ohms by sending a known/set current through the attached resistor. This set current varies with the range your meter is in. However your multimeter has no ideal current source on board, but rather attempts to implement a current source from your battery voltage and a couple semiconductors, hence the open clamp voltage will never rise beyond the battery voltage.
Unsure why the voltage drops so much for the higher ranges, this will have to do with the way the current source is built. Notice that the 'high' voltage is not useful (forth column below) when you realize that the product of range times measurement current is much lower than the open clamp voltage (second column).
Also notice that the voltage measured in the lowest resistance range is identical to the voltage used for diode measurements for all three meters. For diode measurement you want a relatively high voltage to test the relatively high voltage drop across a diode. In that case you still use a constant current, but you are no longer interested in the resistance rather than the actual measured voltage. Useless to build two separate current sources for more or less the same current. On the other hand it is easier to build an accurate current source if you allow yourself a higher voltage drop across the current source and you don't need the voltage anyway (forth column).
Below are the results for my meters. For two out of three the input impedance of the voltmeter (10MΩ) was lower than the ohm-meter's range, so I skipped that value. The columns are as follows:
- range
- open clamp voltage
- measurement current
- maximum voltage required for measurement (range × current), notice how that voltage is reasonably constant!
DVM2000 (6V battery)
\begin{array}\\
\text{range} &\Rightarrow& \text{open clamp voltage} &\Rightarrow& \text{constant current} &\Rightarrow& \text{full scale voltage}\\
\hline\\
\text{diode} &\Rightarrow& 3.25\text{V} &\Rightarrow& 785\text{µA}\\
500Ω &\Rightarrow& 3.25\text{V} &\Rightarrow& 785\text{µA} &\Rightarrow& 500Ω × 785\text{µA} = 400\text{mV}\\
5\text{kΩ} &\Rightarrow& 1.19\text{V} &\Rightarrow& 91.5\text{µA} &\Rightarrow& 5\text{kΩ} × 91.5\text{µA} = 460\text{mV}\\
50\text{kΩ} &\Rightarrow& 1.18\text{V} ^{*)} &\Rightarrow& 11.5\text{µA} &\Rightarrow& 50\text{kΩ} × 11.5\text{µA} = 575\text{mV}\\
500\text{kΩ} &\Rightarrow& 1.09\text{V} ^{*)} &\Rightarrow& 1.1\text{µA} &\Rightarrow& 500\text{kΩ} × 1.1\text{µA} = 550\text{mV}\\
5\text{MΩ} &\Rightarrow& 614\text{mV} ^{*)} &\Rightarrow& 0.1\text{µA} \text{(last digit)}\\
50\text{MΩ} &\Rightarrow& ? ^{*)} &\Rightarrow& ?\\
\end{array}
*) The open clamp voltage for ranges > 5kΩ will probably be influenced by the 10MΩ input impedance of the voltmeter. They should probably all read 1.20V.
SBC811 (3V battery)
\begin{array}\\
\text{range} &\Rightarrow& \text{open clamp voltage} &\Rightarrow& \text{constant current} &\Rightarrow& \text{full scale voltage}\\
\hline\\
\text{diode} &\Rightarrow& 1.36\text{V} &\Rightarrow& 517\text{µA}\\
200Ω &\Rightarrow& 1.36\text{V} &\Rightarrow& 517\text{µA} &\Rightarrow& 200Ω × 517\text{µA} = 103\text{mV}\\
2\text{kΩ} &\Rightarrow& 645\text{mV} &\Rightarrow& 85.4\text{µA} &\Rightarrow& 2\text{kΩ} × 85.4\text{µA} = 171\text{mV}\\
20\text{kΩ} &\Rightarrow& 645\text{mV} &\Rightarrow& 21.7\text{µA} &\Rightarrow& 20\text{kΩ} × 21.7\text{µA} = 434\text{mV}\\
200\text{kΩ} &\Rightarrow& 637\text{mV} ^{*)} &\Rightarrow& 3.71\text{µA} &\Rightarrow& 200\text{kΩ} × 3.71\text{µA} = 742\text{mV}\\
2\text{MΩ} &\Rightarrow& 563\text{mV} ^{*)}&\Rightarrow& 0.44\text{µA} &\Rightarrow& 2\text{MΩ} × 0.44\text{µA} = 880\text{mV}\\
20\text{MΩ} &\Rightarrow& ? ^{*)} &\Rightarrow& 0.09\text{µA} \text{(last digit)}\\
\end{array}
*) The open clamp voltage for ranges > 2kΩ will probably be influenced by the 10MΩ input impedance of the voltmeter. They should probably all read 645mV.
DT-830B (9V battery)
\begin{array}\\
\text{range} &\Rightarrow& \text{open clamp voltage} &\Rightarrow& \text{constant current} &\Rightarrow& \text{full scale voltage}\\
\hline\\
\text{diode} &\Rightarrow& 2.63\text{V} &\Rightarrow& 1123\text{µA} \\
200Ω &\Rightarrow& 2.63\text{V} &\Rightarrow& 1123\text{µA} &\Rightarrow& 200Ω × 1123\text{µA} = 224\text{mV}\\
2\text{kΩ} &\Rightarrow& 299\text{mV} &\Rightarrow& 70\text{µA} &\Rightarrow& 2\text{kΩ} × 70\text{µA} = 140\text{mV}\\
20\text{kΩ} &\Rightarrow& 299\text{mV} &\Rightarrow& 23.0\text{µA} &\Rightarrow& 20\text{kΩ} × 23.0\text{µA} = 460\text{mV}\\
200\text{kΩ} &\Rightarrow& 297\text{mV} ^{*)} &\Rightarrow& 2.95\text{µA} &\Rightarrow& 200\text{kΩ} × 2.95\text{µA} = 590\text{mV}\\
2\text{MΩ} &\Rightarrow& 275\text{mV} ^{*)} &\Rightarrow& 0.35\text{µA} \text{(near scale low end)} &\Rightarrow& 2\text{MΩ} × 0.35\text{µA} = 700\text{mV}\\
\end{array}
*) The open clamp voltage for ranges > 20kΩ will probably be influenced by the 10MΩ input impedance of the voltmeter. They should probably all read 300mV.
Best Answer
An Ohmmeter measures resistance by applying a current to its probes, and measuring the resulting voltage. As a result, in any circuit which generates its own voltage, you will have the potential for errors.
In the case of 2 back to back batteries, that's a good try. However, any slight difference between the voltage of the batteries (and there will be, battery voltage changes with discharge state, discharge history and temperature) will give a non-zero voltage.
When measured one way, this voltage will add to the resistance*current voltage, and so artificially increase the apparent resistance reading. When measured the other way, it will subtract, and may even give a negative voltage. A meter will not have been designed to interpret a negative reading, and so will probably read zero.
The only reliable way to measure the internal resistance of a battery is to measure changes in terminal voltage when you change the terminal current, either by changing the load, or using an AC excitation signal.
When using a meter to measure resistance, a different reading for either polarity is a good indication that 'something is going on', and neither reading should be trusted. Badly behaving circuits could include (not an exhaustive list) a battery, a diode or other semiconductor, a big capacitor with some residual charge on it, or junctions with thermo-electric voltages being developed.
With experience, you can use the fact that a DMM on ohms is a current source with a voltage measurement, and use it to investigate components other than resistors. The rate at which the reading changes when measuring the 'resistance' of a large capacitor can give you some indication of its value. Be aware that different ranges will use different currents, and an auto-ranging meter will quite happily switch ranges when you don't expect it.