Manuel, Michael Briand D.BSEE V -3 Prof. Engr. Daniel P. DuriasInstrumentation and Control (ELEN 3503)

1) Why we convert signal to 4-20mA? 4-20mA is the most common current signal standard that use in instrumentation. Constant current is easily to maintain rather than voltage, therefore if transmitter device it construct that in current dependable device therefore we can create a system that depend on current signal which it is easy to manipulate than to voltage signal.

Notice now how the indicator is an ammeter rather than a voltmeter (the scale calibrated in inches, feet, or meters of water in the tank, as always). Because the circuit is a series configuration (accounting for the cable resistances), current will beprecisely equalthrough all components. With or without cable resistance, the current at the indicator is exactly the same as the current at the transmitter, and therefore there is no error incurred as there might be with a voltage signal system. This assurance of zero signal degradation is a decided advantage of current signal systems over voltage signal systems.

2) Why we use digital signal in process of instrumentation? Digital signal is discrete to time. Actually digitally signal is sampled from analog signal.In the conventional method they use two discreet levels for transmitting the signal from instrument to controller. Logic high And Logic low. Relays are used for implement the digital signal transmission. Recently, different types of digital signals are introduced for communication between instrument and controller. Some signals are Profibus, HART, FieldBus.