How Is Air Flow Measured?
By far, the most accurate way to measure air flow is to count individual molecules. To make this measurement more manageable, the molecules are measured by the box full. The box size is a standard cubic foot (12" x 12" x 12"), measured at sea level (a point of standard atmospheric pressure). The amount of standard cubic feet consumed is measured either per hour or per day. This measurement lets you measure high consumption rates (air dryer output) or relatively low consumption (air flow into a cable) in a meaningful way, using Standard Cubic Feet per Day (SCFD) or Standard Cubic Feet per Hour (SCFH). These measurements help in monitoring for system leaks and in cable maintenance.
Because of maintenance activity, cable splices, and the use of pneumatic fittings along the cable route, all cables tend to leak to some extent. For this reason, an allowable air flow rate, called Optimum Air Usage (OAU) has been established. OAU is the calculated air consumption rate that an air pressure system should use under normal operating conditions. It is based on a consumption rate of 1.25 SCFH per sheath mile of cable.
Like the minimum air pressure standards, OAU is important in dispatching maintenance technicians. A flow increase at an air source is not valuable information unless you also know the OAU, or what the air source should be flowing. OAU is also important in the design of an air pressure system and for evaluating system quality.
To make sure that adequate delivery pressure is supplied to the cables in the field, most telephone companies set a minimum pipe endpoint pressure standard of 7.5 PSI. If the pipe pressure falls below this standard, cable protection is jeopardized. For example, it's impossible to maintain 5 PSI in an underground cable when the delivery pressure from the air pipe is only 4 PSI. For this reason air pipe delivery pressure must be carefully monitored. Think of it as a main artery in the system. If pressure in the air pipe is low, the whole system is low.
What About Air Pressure?
As mentioned previously, most of the air distributed into a cable system is provided by a mechanical air compressor/dryer. This central office equipment compresses or squeezes air molecules together within a given area to create greater air pressure. It's kind of like a garbage compactor in some ways, but with a more useful end result. The amount of compression is measured in Pounds per Square Inch or PSI. For years, PSI readings were the only measurements being used in air pressure maintenance. They're still very important, but nowadays people have come to rely on both pressure and air flow measurements.
And What Is Pneumatic Resistance?
Another important concept in cable maintenance is pneumatic resistance. This is the amount of resistance that air flow meets as it moves along inside the cable sheath between conductors. What determines the amount of pneumatic resistance in a particular section of cable are its length, the gauge of the conductors, the type of insulation (PIC or pulp), and the number of pairs in the cable.
Pneumatic resistance must be taken into account when using pressure and flow measurements when leak locating along a route. Together these three components comprise the important information needed to perform many of the successful leak locating formulas being used today.