How do I measure the offset current of my picoammeter or electrometer?

The offset current of a picoammeter or electrometer is the current measured with no circuit connected to the input. To measure the offset current, turn the power on and allow the instrument to warm up for the time specified in the manual. Place a shield cap over the input connector. Connect the LO input terminal to ground (if your instrument has these terminals). Set the meter to the most sensitive current range and disable the zero check switch. After a few seconds, the meter readings should settle to within a few digits. This settled reading is the input offset current. This reading should be compared to the specification in the instrument's manual. If the offset current is too high, the instrument probably needs recalibration.