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1. What means HART communicator / where it is used /
how to use it?
2. Types of Thermocouples and its temperature ranges /
output of thermocouple?
3. Types of RTDs and its temperature ranges / output
of RTD?
4. Idea about Temperature Transmitters?
5. Difference between Analog and Digital signals?
6. Difference between “2 wire” and “4 wire” system /
instruments?
7. Difference between “Potential” and “Potential free”
contact?
8. Types of flow meters / transmitters?
9. Types of level transmitters?
10. Difference between “Power/Control” cable
and “Instrument” cable?
11. What means “shield” and “screen” in cables / how it
is useful?
12. Draw a schematic of closed loop circuit of flow /
level / pressure / temperature?
13. Idea about calibration of Control valve /
positioned / I/P?
14. How to bypass a digital/analog signal in PLC/DCS by
hardwire and software?
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Answer Posted / manish kumar singh
Ans.1-
HART
The HART system (and its associated protocol) was originally developed by
Rosemount and is regarded as an open standard, available to all manufacturers. Its
main advantage is that it enables the retention of the existing 4-20mA
instrumentation cabling whilst using, simultaneously, the same wires to carry
digital information superimposed on the analog signal.
HART is a hybrid analog and digital system, as opposed to most field bus systems,
that are purely digital. It uses a Frequency Shift Keying (FSK) technique based on
the Bell 202 standard. Two individual frequencies of 1200 and 2200 Hz,
representing digits ‘1’ and ‘0’ respectively, are used. The average value of the
1200/2400Hz sine wave superimposed on
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