I built a microbalance based on a design by Paul Grohe:    • Weigh an Eyelash--Build a Microgram S...  

It is has a precision of about 5 micrograms, and I measured a single eyelash at about 35 micrograms. The balance is built from an analog panel meter that is controlled by a servo loop which optically monitors the meter's position. Adding mass to the meter's needle requires that the servo loop add more current to maintain the needle's position. This additional current is read, and converted to a mass value.