Figure 118

Internal damping chamber

The simple, first-order, filter approach may not always be adequate. In fact, if too large a time constant is chosen, it may interfere with control. A more sophisticated approach is to determine experimentally the noise-frequency spectrum and to design notch filters (i.e., band rejection filters) for the dominant frequencies.

AP Transmitters with High-Viscosity Fill

At least one commercial AP transmitter is available with a high-viscosity liquid fill. Its dynamics approximate those of a first-order lag with t — 0.7 second. The damping affects both input signals and the feedback, and thus minimizes pilot saturation and output errors. This instrument has the desirable feature of nonadjustable damping and so is "fiddleproof." Experience to date (10-15 years and several hundred applications) has been very favorable on flow-control applications, and a bit less favorable on level. Apparendy many level applications, particularly in column bases, need additional damping.

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