By default, the variables are centered and scaled to have mean 0 and standard deviation 1. Without centering, both the mean variable value and the variation around that mean are involved in selecting principal component loadings. Scaling serves to place all process variables on an equal footing relative to their variation in the data. For example, if Time and Temp are two of the process variables, then scaling says that a change of in Time is roughly equivalent to a change of in Temp.