Last month, I presented a webinar for our customers. At the end of Control 101, when we opened the session up for questions, a customer asked about making the choice between 4-20 mA and digital signal outputs. It got me thinking about this list I put together a while back — one that I pull out whenever customers ask me “Is 4-20 mA still valid?”
The answer: Yes, it is. And here are the 18 reasons I came up with… so far.
- 4-20mA has an inherent ‘live zero’. Zero engineering units is 4.0mA, not 0.0mA. 0.0mA indicates an open circuit, a failure or a fault mode like cut wires or a dead transmitter. 0.0mA is not a valid reading of zero engineering units and a wake-up call that there’s something seriously wrong in the loop. (electrical current level below about 3.6mA is absolutely necessary because it is used to drive the electronics of loop powered devices).
- Any electrical circuit needs two conductors. In 4-20 mA’s ‘two-wire, loop powered’ form, a field transmitter gets its power over the same two wires as the signal, without a third or fourth wire for power. This is a major plus when large numbers of field instruments or long wire runs are involved.
- 4-20 mA can be designed for Intrinsically Safe approval for use in hazardous areas, which allows troubleshooting the circuit while it’s ‘live’ and powered.
- 4-20 mA can carry HART® digital data superimposed on its primary DC signal, without sacrificing any of the properties above and without interfering with non-HART enabled analog inputs.
- The 4-to-20 span and offset (with live zero at 20% of the span from zero) has the same 1:5 ratio as the widely used 3-15 psi pneumatic control signal.
- Values slightly above and below the nominal 4-20mA can be and are used to signal a major fault. (See the NAMUR NE43 standard.)
- A 4-20 mA current signal is less susceptible to noise pickup than a voltage circuit, so 4-20 mA is relatively free from interference and cross-talk from other signals.
- 4-20 mA has little, if any, signal loss in its circuit; current regulation compensates for minor resistances in terminal boxes, whereas voltage drop across resistances for voltage signals creates an error.
- 4-20 mA works at relatively long distances (>1km at a nominal 24 VDC). Boosting the power supply voltage will drive the signal even farther.
- Two wire 4-20mA commercial instruments are isolated and floating so ground loops and their associated errors and faults are relatively rare.
- 4-20 mA is generally tolerant of copper cable length and type.
- The response of 4-20mA is adequate for the vast majority of process sensing needs.
- The original published standard for 4-20 mA, ISA SP50, was published in 1966, almost 50 years ago, generating widespread acceptance and creating an installed base of millions of points.
- Electricians find it easy to wire and troubleshoot 4-20 mA. Its working principles are easily understood DC electronics.
- When used with a nominal 24 VDC loop power supply, no one who touches the wire terminals gets shocked or even feels a tingle.
- A cheap, $10 digital multimeter meter can be used to troubleshoot a 4-20 mA loop circuit.
- 4-20 mA can easily be measured in multiple ways:
- With a milliameter inserted into the circuit or across a diode in the circuit
- With a voltmeter as a voltage drop across a dropping resistor
- With a Fluke 77x clamp meter
- 4-20 mA offers a wide choice of resistor values when converting to a IR voltage drop across the receiver input. Lots of resistor values, from 1 ohm to 500 ohms, have been used (250 ohms is the most common).
In summary, 4-20mA is simple and rugged. It works well. As a contributor to a controls web forum once stated, “beat that”.
Did I miss any?
Why do you think 4-20 mA has survived so long?