Measurement Resistance Quiz

Messwiderstand

• Messwiderstand, or measurement resistance, refers to the electrical resistance encountered when measuring electrical quantities.
• It's often a parasitic effect, meaning it's not a desired component of the circuit being measured.
• High measurement resistance can distort the measurement process, introducing errors.
• Sources of measurement resistance can include:
  • The resistance of the measuring instruments themselves (e.g., ammeters, voltmeters).
  • The resistance of connecting wires.
  • The resistance of the probes or contacts used to connect to the circuit.
  • Internal resistance of the device under test.

Factors Affecting Messwiderstand

• Instrument sensitivity: More sensitive instruments generally have lower internal resistance, minimizing measurement errors.
• Circuit configuration: The arrangement of components in the circuit can significantly affect the overall resistance encountered during measurement.
• Wire gauge: Thinner wires have higher resistance, which can contribute to measurement errors, particularly in high-current applications.
• Contact resistance: Poor connections or contacts between wires and components create additional resistance, affecting the overall measurement accuracy.
• Temperature: Resistance changes with temperature, thus temperature variations can influence the accuracy of measurements.

Types of Messwiderstand

• Input resistance: The resistance presented by the input terminals of a measuring instrument.
• Output resistance: The resistance presented by the output terminals of a measuring instrument.
• Contact resistance: Resistance between components in electrical contact.
• Internal resistance: Resistance within a component under test, often overlooked in simplified circuit models but crucial for accurate readings.

Minimizing Messwiderstand

• Using high-quality, low-resistance wires.
• Utilizing measuring instruments with low internal resistance.
• Proper contact pressure in connections (e.g. tightening connections securely).
• Using appropriate meters for the current/voltage range being measured to avoid overloading the meter.
• Utilizing shielded cables. Shielding reduces inductive and capacitive coupling, lessening interference.
• Utilizing instruments with high input impedance or employing specialized techniques for measurements where high input impedance is crucial.
• Compensating for known sources of error (e.g., using correction factors for temperature variations or contact resistance).

Impact on Measurements

• Measurement resistance can lead to inaccurate readings. For example:
  • Overestimation of small voltages. (Voltmeter affects circuit).
  • Underestimation of high resistance values. (Inaccurate reading influenced by instruments' high impedance).
• The magnitude of error depends on the magnitude of the measured parameter and the resistance value. A large measured voltage is less affected than a smaller one.
• In high-impedance circuits, the resistance of the measuring instruments becomes critical to the overall measurements.
• Using an ammeter to measure current introduces some series resistance which changes the current the circuit is trying to measure.

Calibration

• Calibration procedures are crucial to ensure accuracy in measurements by controlling for the effect of measurement resistance.
• Instruments require periodic calibration to confirm that their readings are accurate and reliable.
• Calibration results often provide a correction factor to account for the instrument's measurement resistance. Correcting for the internal resistance ensures more accurate measurements.