On-press stability and consistency

A printing press is a device for laying down a film of ink onto a substrate. Each component on each press unit, from ink ductor rollers to ink train oscillation moves or rotates with a repetitive frequency and this shows up as solid ink density variation. When the natural solid ink density variations through the press run are graphed, the result is sometimes referred to as the "heartbeat" of the press because of its resemblance to the rhythm of a human heartbeat.
The heartbeat of a press - the solid ink density variations measured over through the press run. Each of the 3 sections contains 100 press sheets from the beginning, middle, and end of the press run. Each "heartbeat" in each section is the solid ink density variation between 10 sheets. So, each of the three sections contains the SID measurements of 100 sheets.
The human heartbeat.
Every press design has its own "heartbeat" profile that represents its normal condition. What one looks for are abnormalities in the heartbeat profile itself as well as in comparison between different press units. Plotting the heartbeat of the press, just as it does with humans, can help diagnose problems in the system that may need correcting.

Solid ink densities varying naturally through the press run cause dot gains - tone reproduction - to also vary which in turn causes color shifts in the presswork. Below, courtesy of data provided by Alwan Print Standardizer, is a movie showing dot gain variation through the press run.
Click on the expand icon (the four arrows) to enlarge the video for greater clarity. Then click on the play arrow.
It's fair to say that a printing press in proper working condition is stable - but not consistent. And that fact applies equally to the presswork itself. Therefore, since variation is an integral characteristic of the printing process, the important thing is to establish, and communicate between buyer and print provider, what the target for presswork color should be, how it will be measured, and what range of variation is acceptable based on the needs of the specific job at hand.