Cold loop checking is about to identify and ensure the not powered / not connected loop. Hot loop checking is done once the instruments is connected with.
It fascinates me that the accepted answers (listed here) to this question of “Why 4-20 mA?” reflects ignorance as to what the real primary reason was in the development and acceptance of a live zero electronic replacement for 3-15 psi (20-100 kPa) pneumatic process signals.
The core reason for the development of 4-20 mA was for 2 wire, loop powered instrumentation, where 3.5-3.6 mA is used for powering the instrument. Without a minimum 3.6 mA field instruments would not have any energy or power to run themselves! They’d be 3 or 4 wire devices with separate wiring for DC power.
You can’t have 2 wire, loop powered 0-20 mA or zero to anything mA because there is no energy to power the transmitter at zero mA. There is energy to power the transmitter with an ‘elevated zero’ at 4 mA.
3.8 mA has become a range for low failsafe; 3.9 mA is a range for underflow leaving 4.0 mA for a true engineering ‘elevated’ zero.
There are multiple advantages of 2 wire loop power 4-20 mA, as the accepted conventional wisdom lists:
- live zero is handy to indicate a broken circuit or a dead transmitter
- the loop can be made intrinsically safe
- it has the advantages of less susceptibility to noise that is inherent in current vs voltage signals
- a current loop adjusts for minor resistance changes in loop wiring (rusty terminals)
- it does create 1-5 volts when shunted through a 250 ohm resistor, but there neither was (historically) nor is anything particularly superior about 1-5Vdc. The 1:5 ratio of 4-20mA is the same as its predecessor, 3-15 psi (20-100kPa) pneumatics, which is nice, but again, there’s nothing inherently superior about one ratio vs another.
But those are advantages only, none are the real primary reason for an elevated live zero: 2 wire loop powering of the instrument.
Don’t fail your interview because the interviewer doesn’t know either, but if you’re a professional in the field, you should know the real reason for the 4-20mA standard.
i got 14.46 ma for the current?
Thanks Sir for your information sharing Kindly explain how does it make equalization and what the benefits from that equal.? and thanks again…
Please send me how get the answer ? if any equation…
Proportional-Integral-Derivative (PID) control is the most common control algorithm used in industry and has been universally accepted in industrial control. The popularity of PID controllers can be attributed partly to their robust performance in a wide range of operating conditions and partly to their functional simplicity, which allows engineers to operate them in a simple, straightforward manner.As the name suggests, PID algorithm consists of three basic coefficients; proportional, integral and derivative which are varied to get optimal response. Closed loop systems, the theory of classical PID and the effects of tuning a closed loop control system are discussed in this paper.
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The HART standard specifies “master” devices in a HART network transmit AC voltage signals, while “slave” devices transmit AC current signals.
I got 419.1deg and 14.64 mA
The measuring device has 250 ohm resistor. As for current signals,they are converted to voltage signals like this
4 ma * 250 ohm = 1 v / 20ms * 250 ohm = 5v