Appendix 1

Integrated Design Example

I often use a particular Siemens ultrasonic level instrument to measure tank fluid levels. The instrument can display just the distance from ultrasonic transducer to liquid surface, or it can be programmed to display fluid level or even fluid volume (even with quite complex tank shapes).

It can output fluid level or volume as a 4–20 mA signal. It has five volt-free contacts which I can program to become active or inactive at various tank levels. It can have two transducers and be programmed to make decisions about which transducer’s signal is the more reliable. It has all kinds of sophisticated signal processing onboard, and can be programmed as to which state to fail to if the signal from transducers is ...

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