Pressure

Traditional Pressure Measurement Methods
The most traditional method of method of measuring and recording pressures is a dial gage and a notebook. Mechanical pen-and-chart recorders are a significant step in improving accuracy and detail of pressures, and have been in use for decades, offering a good solution to many pressure-recording needs. They are relatively inexpensive, easy to use and continuously monitor all pressures. The disadvantages of the mechanical recorders are that they need to be manually reviewed and the charts need to be changed every day or every few days, are not typically designed for negative pressures, and they are not capable of providing detailed information on the duration of a transient since the width of the line drawn by the pen may cover up to 60 seconds of time.

Digital data loggers have become a standard within the water and petroleum pipeline industry. Several portable high-speed electronic data loggers are available. Most do record negative pressures, and if set to record frequently enough, they are capable of recording the details of a specific surge event over a period of seconds to minutes. If you know when the transient will occur and can set the data-recording rate accordingly, and can upload the data from storage immediately after the event, these devices may fulfill the need. Data recording intervals of 0.05 seconds (20 Hz) are available.

Digital data collection with insufficient sample rate

Digital data collection with insufficient sample rate

On the other hand, if there is a need to monitor over longer periods of time, the data interval will have to be set at a longer interval to stay within the memory capacity of the device. An interval of once per second might be selected, which would provide 86,400 pressure readings per day. Surely this would convince a pipeline owner that the pressure is rigorously and carefully monitored. But is this sufficient? Not necessarily, because many of the most damaging transients may last only a fraction of a second, and would not be detected at all. It has been difficult to detect and measure an unexpected transient that may last a fraction of a second, and that may be 100 times the operating pressure or more. Not only are these events difficult to detect, they may be the most damaging of all and may go unnoticed for long periods of time. As is noted in Figure, an insufficient sample rate will lead to inaccurate and misleading portrayal of pressure.

Mathematicians will recognize the inadequate sample in the context of the Nyquist principle: the sample rate must be no less than half the wave length of the shortest transient of interest, in order to accurately depict the event. If an event has a duration of 1 second, the data must be recorded at least every ½ second. Waterhammer events may have durations of 0.05 seconds, requiring a sample rate of at least 0.025 seconds, or 40 Hz.