How do I add a check such as cooking temperature or rice pH?

Open Settings › Modules › Service temperature: the Custom protocols page lists your checks beyond Hot and Cold.

1. Tap Add a protocol.
2. Pick one of the Ready-made templates (editable): Cooking core for poultry, stuffed foods and leftovers (≥ 165 °F), for ground meat (≥ 155 °F), for whole meats, fish and eggs (≥ 145 °F), Blended / minced preparations, Rice pH (≤ 4.6 pH), Hot and cold transport links, Sous-vide… Or tap Custom to start from a blank card.
3. Check the Check name, the Unit (°F or pH), the Min and Max (at least one of the two bounds) and the Suggested value shown at entry, then Save.

The protocol appears right away in the kitchen, as a pill next to Hot and Cold, with its rule. To change it later, tap its line; to remove it, Delete: readings already logged keep its name and the bounds that applied at the time.