A specialized glass flask with a close-fitting ground glass stopper that has a fine capillary tube through it. By weighing the flask empty, filled with a reference fluid like pure water, and filled with a test liquid, a technician can calculate the test liquid's exact density. The capillary ensures the volume is identical every time by letting excess liquid escape.
Heard on the job
Make sure the pycnometer is completely dry before you take the tare weight.