All of the following statements about Automatic Tank Gauging (ATG) systems are true except:

Automatic Tank Gauging (ATG) systems are essential tools in the management and monitoring of Underground Storage Tank (UST) systems. They serve multiple functions, including inventory management and leak detection, but they are not mandated for every UST system. The requirement for installing ATG systems varies based on regulatory frameworks and specific circumstances related to the facility and the types of products stored.

The other statements accurately reflect the functionalities and components of ATG systems. For instance, they indeed have probes installed within tanks to monitor fluid levels and temperatures. Additionally, ATG systems can calculate product volume changes, which is crucial for identifying leaks, helping to maintain compliance with safety and environmental regulations. Furthermore, these systems are equipped with a control panel that presents vital information such as product levels and temperatures, enabling operators to easily access real-time data.

Thus, while ATG systems are highly beneficial and often recommended, they are not universally required, making the statement about their mandatory installation in all UST systems the one that does not hold true.