A method for testing a tunnel fire suppression system in the form of a water deluge system having a wet side and a dry side separated by a valve, includes the step of providing a nozzle arrangement comprising a plurality of nozzles, with each nozzle disposed in or coupled to an outlet of the water deluge system. The nozzle arrangement comprises, is coupled to or operatively associated with a sensor arrangement configured to measure the pressure of air at each of the plurality of outlets of the water deluge system and output one or more output signals indicative of the pressure of the air at the one or more outlets. The method includes providing a supply of pressurised air through the water deluge system using a blower coupled to the water deluge system. The method further includes measuring the pressure of the air at the one or more outlets of the water deluge system and outputting an output signal indicative of the pressure of the air at the one or more outlets, and conveying the output signal to a processing system configured to determine from said one or more output signals the flow rate of the air supply at the one or more outlets.