The use of the Pitot Gauge when taking a flow reading is not difficult, but it is essential that certain rules be followed if accurate readings are to be obtained.
The tube and gauge are connected to the handle. The handle serves as an air chamber to help keep the gauge indicator steady.
To hold the Pitot Gauge, grasp the handle with one hand and the tube with the other using the first two fingers and thumb. The little finger of the tube hand rests upon the hydrant outlet to steady the instrument. Unless some effort is made to steady the Pitot Tube Gauge, while taking a reading, the movement of the water will make it difficult to obtain an accurate reading.
The point or small opening in the Pitot Tube should be centered in the stream. The tube should be held away from the opening art approximately one-half the diameter opening. For a 2-1/2" opening, the distance would be 1-1/4". The Pitot Tube should be held at a right angle to the stream of water.
After each test, the petcock at the end of the handle or air chamber should be opened and all water drained from the assembly. After each test and draining, close the petcock.
During the test, the air chamber should be kept above the horizontal plane through the center of the stream for the purpose of increasing the efficiency of the air chamber. Remember, the above example is applicable only for 3" or smaller outlets.
Testing the hydrant
When fire fighters are testing the available water supply, two or more hydrants should be opened for the flow. The number of hydrants opened will depend upon an estimate of the flow that may be available in an area.
The amount of water that will flow from the hydrants and the amount of water that is available in the system at 20 PSI residual pressure are two different things. If direct readings of the available water at 20 PSI were made, it would be necessary to open sufficient hydrants to drop the residual pressure to exactly 20 PSI. If, however, the flow is taken at any residual pressure, the available water at 20 PSI can be calculated by formula. The second method will be the most often used by the Crossgates Fire Protection District.
Five arrangements by which fire hydrants in an area can be selected for a flow test are shown in the figures below. The illustration is self-explanatory as is the purpose of the test and the activities at each point of the operation.
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