The presence of vacuum in a process is a very important factor when selecting the diaphragm seal fill fluid and the way of mounting of the instrument. The relation between the vacuum value and the process temperature should be checked in the vapor pressure curves of the fill fluid to see whether the fill fluid is suitable. When mounting the instrument for a vacuum application, the instrument should be placed below the (lowest) diaphragm seal to protect the instrument. For absolute pressure measurement the instrument should be mounted below the diaphragm seal in order to protect the instrument at all possible conditions.
As a rule of thumb, we recommend to place the instrument at least 50 cm below the (lowest) diaphragm seal. The 50 cm distance, between nozzle and instrument creates a positive fluid column, which protects the instrument from measuring absolute zero. With this mounting style the instrument has additional protection before it measures absolute zero. This is presented in the picture.