This circuit is designed to provide two measurement ranges. The low range will measure inductors with inductance value between 3uH to 500uH, and the high range will measure inductance values between 100uH and 5mH.
To calibrate this inductance meter adapter, connect a digital voltmeter, switch the voltmeter to 200 mV range, short the test probe and adjust the zero (R1) to give a zero millivolts reading on your digital voltmeter.
To calibrate the low range of this inductance meter adapter, switch the voltmeter to low range position, and select 2 V range for the digital voltmeter.
Test a known inductor that has a value around 400uH; adjust the low calibration pot to give correct reading of 1mV/uH. If you use a 400uH inductor then you must adjust the calibration to give exactly a 400mV reading.
For high range calibration, switch the range selector to high position and use a known inductor around 5 mH, adjust the high calibration pot to give 100mV per mH. A 5 mH inductor should give a 500mV reading on your DVM.