The combination volt-ohm-milliammeter is a basic tool in any electronic laboratory. The proper use of this instrument increases its accuracy and life. The following precautions should be observed:
1. Start with the highest range of the instrument and move down the range successively, to prevent meter overloading and possible damage when checking voltage or current.
2. For higher accuracy, the range selected should be such that the deflection falls in the upper half on the meter scale.
3. Choose a voltmeter range such that the total voltmeter resistance (ohms per volt x full scale voltage) is at least 100 times the resistance of the circuit under test - For higher accuracy
4. Make all resistance readings in the uncrowded portion on the meter scale, whenever possible.
5. Take extra precautions when checking high voltages and checking current in high voltage circuits.
6. Verify the circuit polarity before making a test, particularly when measuring dc current or voltages.
7. The power to the circuit should be turned off while checking resistance in circuits. Otherwise the voltage across the resistance may damage the meter.
8. Recalibrate the instrument at frequent intervals.
9. Renew ohmmeter batteries frequently to insure accuracy of the resistance scale.
10. Protect the instrument from dust, moisture, fumes and heat.
Working of Dead Weight Tester