The amount of data generated during an inspection is large. The amount depends on the number of sensors, the number of samples per unit distance and the distance traveled, as given by:

As an example, typically a high resolution MFL pig has at least 2 sensors for each circumferential inch of pipe. For a 40 foot joint of 24 inch diameter pipe, recording data every 0.1 inches (120 samples per foot), the number of data points is:

Repeating this calculation for 100 miles of pipeline

The data rates (data points per time) can also be large. A tool with the above configuration, traveling at 5 miles per hour, would have to record 132,000 data points every second. As an example, a pig must

• find a 2-inch by 2-inch defect (a set of keys)
• in a joint of pipe (40 feet long x 2 feet in diameter = 250 sq. feet, a large kitchen)
• in less than 6 seconds, and
• determine whether they are yours or your spouses.

As a result of the high volume of data, many tool designers use data conditioning systems to compress the data and reduce storage requirements. For example, the amount of data stored can be reduced by recording only channels with changing signals. This type of data compression technique requires a threshold to be defined prior to the inspection. Such data compression systems are largely proprietary and are not discussed in this report.

Data are stored in an analog or digital fashion on either tape or in solid-state memory. Early MFL tool designs used analog recording systems to store the sensor output. These systems were built using modified audio recording technology including multiple track (usually 24 or 28) record/playback heads and reel-to-reel magnetic tape. The tape speed was reduced to increase the inspection run time to up to 100 hours. These systems have proven themselves under the in-line inspection conditions.

Many recently developed MFL tools use digital recording systems. Digital systems require analog-to-digital converters, which add power needs, size, and complexity to the tool. Digital systems provide more safeguards on data integrity (error checking) and allow easier data compression. Digital systems also provide data in a format suitable for use in computerized analysis systems. These systems are built on high-density computer tape storage technology. Recent advances in tape storage have increased data storage capacities enabling the storage of over 25 billion data points on one tape cartridge smaller than a standard VHS video cassette. These data are often transferred to a compact disc for fast access to specific locations when using computer analysis programs.

All tape systems have operating temperature ranges that can restrict their use. As a result, tape systems are not used on all in-line inspection tools. Solid-state memory is used when the operating temperatures are too high for tape units or where a tape unit is physically too large for the inspection tool. Solid-state memory are also used on some small-diameter tools, where the data storage requirements are less than for large-diameter tools.

The data conditioning electronics low pass filter the flux leakage signals so that the bandwidth of signals does not exceed the bandwidth of the recording system. This filtering reduces high frequency noise from other components. Unfortunately, signals from short, deep defects at high inspection velocities can produce signals that are effected by the filtering. This filtering can affect length and depth characterization.