http://breezewiki.catsarchywsyuss6jdxlypsw5dc7owd5u5tr6bujxb7o6xw2hipqehyd.onion/minecraft/wiki/Cauldron
With a cauldron behind it (either directly, or separated by an unpowered solid block), a comparator outputs a signal strength proportional to how full the cauldron is: 0 for empty, 1 for one-third full, 2 for two-thirds full, and 3 for completely full or filled with lava. However, if there is a block between the cauldron and the comparator, the comparator does not immediately update.