Magnetic pickup, output

The output from the magnetic pickup and the output from the pressure transducer are connected to an oscilloscope. After a computer conversion of the data, the resulting pressure/volume plot is printed out. This plot is called an indicator card. The indicator card shown in Fig. 29.3 was generated from a 4000-hp natural-gas compressor in Hebron-ville, Texas. 

These usually consist of a non-magnetic casing, a rotor, and an electromagnetic pickup. The rotor is either a propeller or turbine freely suspended on ball bearings in the path of the flowing fluid with the axis of rotation in line with the flow. The rotor turns in the fluid flow stream at a rate proportional to the flow rate. As the rotor turns it cuts through the lines of force of an electric field produced by an adjacent induction coil. The electrical pulse output from the induction coil pickup is amplified and fed to readout instruments or recorders to give either total flow or flow rate [Holland and Chapman (1966)]. 

Another type of electrical pressure transducer is a variable-reluctance device. The deflections of a diaphragm of magnetic stainless steel are sensed by two inductance pickup coils. With ac-bridge circuitry, this transducer delivers a full-scale output of 40 mV per volt for a variety of ranges from 0 4 Torr (0- 0.08 PSIA) to 0- 3200 PSIA. 

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