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Pipe measurements

The VPFlowScope M, Probe and DP are insertion probe type flow meters. They all measure velocity. To provide output directly in volume flow like m3n/hr or SCFM, you need to configure the inner pipe diameter into the VPFlowScope. Thereby it is critical to program the exact inner pipe diameter, as a small deviation can easily provide percentages of deviation on the measurement results, as our example shows. As you can see in the table below, the deviation impact is worse for smaller diameters:

Measuring the outside diameter

Is the nominal pipe diameter unknown, it is possible to measure it. With a flexible measuring tape, it easy to read out the outside diameter by wrapping the measuring tape around the pipe. Is there no flexible measuring tape within reach? It is also possible to use a string instead. Wrap the string around the pipe, mark the point where the string touches each other, and measure the length from the mark you made till the tip of the string. With the measured outside diameter, it is possible to find in the tables the Nominal pipe diameter when needed.

Internal diameter

What to do, if the internal diameter is unknown and it is not given in any table? No problem, it is still possible to calculate it. To get the internal diameter (ID), you need to know the outside diameter (OD) and from this, subtract the wall thickness twice:

ID = OD – (2 x the wall thickness)

Be aware that the wall thickness can deviate. The tolerance on a typical wall thickness is 12,5%, so the actual internal pipe diameter can be slightly different. When you do not know the wall thickness, but you know the schedule of the pipe, you can simply look for the wall thickness in standard tables. The table below gives the internal diameter in inches and millimeters belonging to those schedules.

If only the outside diameter is known and there is no way to get other measurements of the pipe, there is another way to obtain the internal diameter. It is possible to measure the wall thickness with an ultrasonic wall thickness gauge. Ultrasonic thickness measurement (UTM) works with a sound pulse, produced by an ultrasonic transducer. It measures exactly how long it takes for the sound pulse to travel through the pipe and back. From this measurement the device can calculate the wall thickness of the pipe. Once knowing the wall thickness and the outside diameter, the inside diameter can be calculated as described above.

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