Inserting probe into pressurized water pipe
The usual way is to use a T with the middle outlet being vertical:

With a proper fitting, you screw this hollow adapter in. Ecnerwal says in a comment it is called a &quot;thermowell&quot;.

Fill this with mineral oil and dip your temperature probe in.
The oil transmits temperature, but the probe is not submerged in water, nor under pressure, so it can be inspected and changed.