With the introduction of the new instruments, speed is basically taken for granted. It is a very important parameter for reciprocating compressors, however, because speed is one of the factors in generating displaced volume. For the axial and the centrifugal compressor, speed offers a multiple influence. In the fan laws stated in Chapter 5, speed was the common parameter in both capacity and head. In fact, since head is proportional to speed squared, it becomes quite important that the speed be accurate.

The electronic counter circuit contributed to the development of modern tachometers. By using a toothed wheel and a magnetic pickup with the counter, a direct reading digital speed output may be derived. While not too common, the signal can be put through a digitai-to-analog converter and an analog meter reading made available. The digital readout is useful for performance testing because it requires no interpolation. The analog meter reading is good for startups or gross adjustments where the rate of response is part of the information.

Figure 8-31. Multiple diaphragm flexible element coupling with a strain gauge type torque meter. {Courtesy ofZum Industries, Inc., Mechanical Drives Division)
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