These PTFE thermocouple adapters (from Chemglass) are designed to allow a thermocouple probe to fit snugly into a 24/40 joint.

You’ll notice the screwable cap, which lets you tighten or loosen the adapter as needed.

Inside the cap is a Viton O-ring, which expands when pressure is applied to it. This, in turn, seals the thermocouple probe inside the adapter.

This seal provides an invaluable airtight connection, letting you read temperatures without sacrificing vacuum levels.