Flow Measurement Magnetic-inductive Flow Measurement –Choosing The Right Instrumentation Enhances Installation, Production and Maintenance
Magnetic-inductive flow measurement has a long proven track record. Nevertheless, there are a number of instrumentation-related issues that you need to consider to ensure optimal functionality during installation and ongoing operation.
Various types of process noise degrade signals from magnetic-inductive flowmeters in many applications. Solids or chemical reactions in the fluid stream can generate noise. In the past, users have resorted to high signal damping as the only way of getting a handle on the problem.
There are three typical process noise profiles: 1/F noise, peak noise and white noise.
- The amplitude of 1/F noise decreases with increasing frequency. The noise is caused by particulate matter, gas or chemical additives in the process medium.
- Peak noise is detected as higher amplitudes resulting from particulate impact against the electrode. Peaks occur at all frequencies in process media that contain significant amounts of particulate matter.
- High-amplitude white noise remains constant over the entire frequency range up to several hundred hertz. It is caused by pumps and valves located near the flowmeter sensor.
The Effects of Noise
Process noise and valve activity are directly related. Noise forces the control circuitry to continually activate the valves, which in turn induces even more noise in the pipes. Signal damping is often used to compensate for noise.
However, damping on the input signal to the control loop creates a dead zone in the loop. The flow signal is stabilized, but the control loop (and valve) is no longer able to react quickly to real changes in the input signal, resulting in process variation and lower product quality.
The Rosemount 8700 Series uses a more advanced method than signal damping. The flowmeters can operate at two different working frequencies and are factory-calibrated at both frequencies. Built-in diagnostic circuitry constantly monitors the noise level at both frequencies. If process conditions cause degradation of the signal-to-noise ratio (<25), this condition is flagged and the user can switch to the alternative frequency. The electronics subsystem displays the signal-to-noise ratio at both frequencies, making it easy for the user to see which frequency produces the most stable results.
Correct grounding is particularly important on magnetic-inductive instrumentation, because it isolates any magnetic interference fields from the electrical equipment near the point of measurement. Mistakes can easily be made during sensor installation. The Rosemount 8732E transmitter monitors noise in the 50–60 Hz frequency range and can detect whether or not the grounding is correct. This can be a big help to the user during installation and initial startup.