Fisher 4194B Series Instruction Manual page 10

4194A, B, and C Series
The vent must be protected against the entrance of
any foreign material that could plug it; or, if a remote
vent is not required, the vent opening in the case,
2–4
must be protected against the entrance of any foreign
material that could plug it. Check the vent periodically
to be certain it is not plugged.