What tool is primarily used to determine vat levels?

The correct answer is the oil projection chart, as it is specifically designed to provide a visual representation of oil levels and projections in various contexts. This tool allows managers and operators to quickly ascertain the volume of oil in storage tanks or vats and helps in making informed decisions regarding oil usage and management.

In the context of managing resources like oil, having a chart that projects levels can aid significantly in ensuring that operations run smoothly without interruptions due to unforeseen shortages. The projections consider factors such as consumption rates and expected deliveries, making it a strategic tool for operational efficiency, especially in industries like food service and hospitality.

The other options, while related to measurement or assessment in some capacity, do not serve this specific function as effectively as the oil projection chart does. A thermometer is typically used for measuring temperature, and an oil pressure gauge is used for checking the pressure instead of the oil level. A level measuring stick, while it can be used to measure levels directly, does not provide the same level of insight and forecasting that an oil projection chart offers.