Smaller probe diameters need a gland sized for the correct mechanical fit. PG16.12 is a metal gland for probes with Ø 12 mm diameter and is suitable for HD9008.31.12.
The gland has a G 1/2" L=8 mm thread, giving installers a defined accessory for fixing Ø 12 mm probes in compatible HD9008.31.12 mounting arrangements.