Abstract: A conduit conducting a dirty gas is connected by one line to an interface device connected by another line to a pressure measuring device having a cavity and a diaphragm defining one wall of the cavity. The interface device has a pair of chambers in fluid communication at their upper ends. The bottom end of one chamber is in fluid communication, via the one line, with the conduit conducting the dirty gas. The bottom of the second chamber is in fluid communication, via the other line, with the cavity in the measuring device. The second chamber, the cavity and the line connecting them are filled with a liquid which keeps dirt in the dirty gas from entering the cavity but transmits the pressure of the dirty gas to the diaphragm in the cavity.