As a test to determine the effective wall roughness of an existing pipe installation, water at 10°C is pumped through it at the rate of 225 L/min. The pipe is standard steel tubing, 40 mm OD × 2.0 mm wall. Pressure gages located at 30 m apart in a horizontal run of the pipe read 1035 kPa and 669 kPa. Determine the effective pipe wall roughness.