Method and error in selecting the resistance range of a multimeter
Each range of the resistance stopper measures resistance values from 0 to ∞. The scale of an ohmmeter has a non-linear, non-uniform inverted scale. It is expressed as a percentage of the arc length of the scale. And the internal resistance of each range is equal to the scale arc length of the central scale multiplied by the multiplier, called "centre resistance". In other words, when the measured resistance is equal to the centre resistance of the selected range, the current flowing in the circuit is half of the full-scale current. The pointer indicates the centre of the scale. The accuracy is expressed by the following formula: R% = (△R/centre resistance) × 100%.
With a piece of Uni-Tech (UNI-T) digital multimeter UT33B measurement of the same resistance, the choice of different ranges of the error generated by the
For example: MF-30 type multimeter, the Rxl0 block of the centre resistance of 250Ω; R × l00 block of the centre resistance of 2.5kΩ. accuracy level of 2.5. A standard resistance of 500 Ω is measured with it, and it is asked which error is greater when measured with the R×l0 block and the R×100 block?
The maximum absolute permissible error of the R×l0 block is △R(10)=centre resistance×R%=250Ω×(±2.5)%=±6.25Ω. If it is used to measure a 500Ω standard resistor, the value of the 500Ω standard resistor is between 493.75Ω and 506.25Ω. The maximum relative error is: ±6.25÷500Ω×100%=±1.25%.
The maximum absolute allowable error of R×l00 block △R(100)=centre resistance×R%2.5kΩ×(±2.5)%=±62.5Ω. Using it to measure the 500Ω standard resistor, the value of the 500Ω standard resistor ranges from 437.5Ω to 562.5Ω. The maximum relative error is: ±6.25÷500Ω×100%=±1.25%. The maximum relative error is: ±62.5 ÷ 500Ω × 100% = ±10.5%.
Comparison of the results from the calculation shows that the choice of different resistance ranges, the measurement produces a large difference in error. Therefore, when selecting the range of stops, try to make the measured resistance value in the centre of the arc length of the range scale. The measurement accuracy will be higher.
