The Air Sampling Unit (ASU) is designed to be used with Crowcon’s Xgard detectors to monitor potentially hazardous gases within ventilation ducts.

The ASU uses the Venturi principle, which states that the airflow rate within the duct must be between 5 and 20 metres per second.

Installation

The Xgard gas detector must be fitted to the ASU before being assembled onto the duct. The ASU is not compatible with the Xgard Type 4 high-temperature gas detector.

• To avoid the adverse effects of turbulence, the ASU should be installed in the centre of a straight section of the duct, which has a length of at least six times its width.
• Check the airflow direction in the air duct system.
• Apply the drilling template onto the centre line of the air duct parallel to the airflow. Check that the directional arrow on the drilling template is the same as the direction of the airflow in the duct.
• Punch the two centres indicated for the drilling template using a centre punch.
• Drill out to 25mm (1”) diameter. Remove burrs and sharp edges around the two holes.
• Remove the ASU cover.
• Fit the sample tube elbows with rubber washers inside and outside the sampling chamber. Fit the securing nuts and tighten. Ensure that the black sample tube holes face 180º from the sample chamber and the white sample tube chamfer faces away from the sample chamber.
• Check the thickness of the air duct metalwork and fit the black rubber grommets into the two duct holes drilled for point 5 of these instructions.
• Fit the ASU probes through the two grommets, check that the inlet and outlet probes are correctly installed, and push until the unit has been sealed.
• Mark the six ASU plate fixing holes and drill 5mm (1/4”) holes for securing the plate. Secure the plate to the duct using suitable bolts.
• Connect the detector cable as instructed in the Xgard manual.
• Check that there is no air leakage into the sampling chamber from outside the duct through the probe joints, grommets or sampling unit cover gasket.

Testing and Commissioning

Calibrate the detector according to its instructions. Test the operation of the ASU by introducing gas into the duct system.

Warning: it is essential to ensure that procedures are in place to prevent people and equipment from being affected by any test gas introduced into the ventilation system.

Operation and maintenance

The ASU needs no specific maintenance; however, the Xgard gas detector must be maintained according to its instructions.