To zero the balance when the pan is not empty. This is a standard weighing method
whereby the weight of the vessel holding a sample is automatically subtracted from the total
weight of vessel and sample together, by first zeroing the balance while the empty vessel sits
on the pan. After such an operation, a balance may display a negative number when the pan is
empty of both vessel and sample. When the sample is subsequently weighed in the vessel on
the pan, the balance will display the weight of the sample only.