Adjustment, the Timeless Adjustment
Mike Wyatt has shared a simple yet effective method for calibrating Digital Multimeters (DMMs) using a precision resistor and a thermistor. This approach, while not perfect for metrology experts, can still be useful for general calibration purposes.
The Calibration Process
The calibration process includes the use of a precision resistor and a 1% thermistor, along with a standard dual banana plug. The assembly, housed in a 3D-printed casing, is designed for precision measurements with two wires.
- Calibration using a Precision Resistor:
Connect the precision resistor of a known accurate resistance value to the DMM, set to measure resistance. Measure the resistor’s value with the DMM. Compare the reading to the resistor’s certified value. If the reading deviates beyond acceptable limits, adjust the DMM’s calibration settings accordingly. This confirms the DMM’s resistance measurement accuracy.
- Calibration Using a Thermistor:
Place the thermistor in a controlled temperature environment. Measure the thermistor resistance with the DMM. Convert the measured resistance to temperature using the thermistor’s calibration data. Verify the DMM’s accuracy for low resistance and temperature-related resistance measurements. Adjust the DMM if necessary.
Detailed approaches include starting by zeroing or offsetting the meter with open or short circuit as required. Measuring precision resistors over a range of values covering the DMM’s scale is also crucial. For thermistors, stabilizing temperature before measurement is essential due to their sensitivity. Documenting each measurement and adjustment helps maintain calibration traceability.
Practical Considerations
For practical calibration, consider using a laboratory setup with a voltage/current source (if voltage or current scales need calibration), precision resistance elements, and temperature-controlled environments for thermistor-based tests. After measurement, calibration adjustments are made via internal DMM trim pots, software calibration modes, or external calibration tools depending on the DMM model.
Benefits of the Method
The precision resistor provides a stable reference to check and adjust the DMM’s resistance measurement circuit. The thermistor adds a temperature-dependent resistance value useful for verifying temperature measurement or resistance that changes with temperature, testing the response linearity and sensitivity of the DMM’s resistance measurement function.
The integrated thermistor helps differentiate between the ambient temperature and the meter's temperature. Bench meters tend to get warmer than hand-held units during operation, which can affect measurements.
In conclusion, calibrating a DMM with a precision resistor and thermistor involves applying known stable resistance values, taking careful measurements, comparing against standards, and adjusting the DMM settings to reduce measurement errors. While the process does not necessarily require extensive debate, it provides a practical and effective solution for maintaining the accuracy of digital multimeters.
[1] Signal Conditioner Calibration using Precision Components and Stability Controls. (n.d.). Retrieved from [https://www.tek.com/content/dam/tek/docs/application-notes/signal-conditioning/tektronix-signal-conditioner-calibration-using-precision-components-and-stability-controls.pdf]
[2] Scale Calibration. (n.d.). Retrieved from [https://www.tek.com/content/dam/tek/docs/application-notes/oscilloscope/tektronix-oscilloscope-scale-calibration.pdf]
The utilization of a precision resistor and a thermistor in the calibration process of Digital Multimeters (DMMs) allows for precise measurements for both resistance and temperature-related parameters. This method, while not as complex as metrology-grade calibration, is a practical and effective solution for maintaining the accuracy of digital multimeters, relying on stable reference values, careful measurements, and calibration adjustments for reducing measurement errors. Electronics and technology are vital components, with the precision resistor providing a stable reference for resistance measurement, and the thermistor adding a temperature-dependent resistance value for verifying temperature measurements.
