Why are many industries using 4-20 mA signals when there are other types of signals that can be used? What makes 4-20 mA current signal above the other signals?

 

The analog signal, 4-20mA, is used worldwide because
- it is simple and rugged
- in its 2 wire, loop powered form it gets its power over the same wires as the signal, which reduces the cost of wiring extra wires over long distances.
- it has an inherent 'live zero' where zero engineering units are 4.0mA not 0mA. 0mA indicates an open circuit failure or fault mode, that is, a dead transmitter, not a valid zero engineering units reading. (current below ~3.6mA is used to drive the electronics of loop powered devices).
- in its 2 wire form, it can be designed for Intrinsically Safe approval for use in hazardous areas, which allows troubleshooting the circuit while it's 'live'
- in its 2 wire form, the 4-20ma circuit on commercial instruments is isolated and floating so ground loops are relatively rare.
- has no signal loss in its loop circuit; Kirchoff's Law says that the loop current has to be same in all parts of the circuit. Voltage signals can incur voltage drop.
- its response is adequate for the vast majority of process sensing needs
- values slightly above and below the nominal 4-20mA can be used to signal a major fault (3.6ma or 21.5mA)
- relatively good at working in electrically noisy environments compared to voltage signals.
- electricians find it easy to wire and troubleshoot, easily understood as simple DC electronics
- it is generally tolerant of copper cable length and type
- it works at relatively long distances (>1km at nominal 24Vdc). Boosting the power supply voltage will drive the signal even further.
- When used with a nominal 24Vdc power supply, no one who touches the wires even feels it, nobody gets hurt.
- current is less suseptible to noise pickup than voltage, so 4-20mA is relatively free from interference and cross-talk from other signals
- it has an installed working base of millions of points
- a cheap, $10 digital volt/milliamp meter can be used to troubleshoot the circuit
- it 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 ratio as the 3-15 psi pneumatic signal it has largely replaced.
- after the usual political bickering and dithering, 4-20mA was declared and still remains an international standard, the ISA SP50 (1966). Of course, the market had already settled on 4-20 by the time the standard was approved, but hey, it is a standard.
- wide choice of conversion factors when converting to a IR voltage drop by picking a suitable resistor. Every resistor value from 1 ohm to 500 ohms is used, with 250 ohms being the most common.

 

Taking your question a bit further, if 4-20 mA has so many advantages for analog signals, why is voltage (0-10, etc) still a common practice?

We just put up an article about that on the editorial side of the Control house, might want to check that one out. I've been doing a lot of work with IO-Link recently, and it has some really handy features that, in some ways, are superior to a single digital or analog channel, but the extra hardware and software configuration can be a bit much, especially for a small system. Plus, you can't grab a multimeter and test an IO-Link, or Modbus, or whatever signal.

https://control.com/technical-articles/a-tale-of-two-standards-comparing-analog-voltage-and-current/

For questions of 'why is it still used?', I hear that same question debating the popularity of PLCs vs. IPCs, ladder logic vs. text, and protocols like Modbus vs. Ethernet. Sometimes the simplest answer is... It's nearly impossible to get rid of in one mass sweep, so it has to stay supported. Therefore it will keep being used. And the cycle starts again.

 

Ethernet-APL has a real chance of supplanting 4-20mA. Ethernet-APL is 2-wire, loop powered Ethernet communications that can be intrinsically safe. The 2-wire (power to run the field device over the same wires as the output signal) part is a big deal for long cable runs encountered in the process world. The Ethernet part is exciting for getting more data than just the process variable. It's coming. Will be interesting to see how quickly it's adopted.

 

Thanks man, it helps a lot

 

In my opinion 0-10Vdc signals are -not- common practice. 4-20mA is mostly used and can easily be converted to voltage at the point where the signals need to be measured with a simple resistor.
Voltage signals are way to susceptible to noise in industrial environments.

 

@patrickduis, this is something many self-proclaimed technicians don’t realize—most control systems/monitors don’t measure current they only measure voltage so a dropping resistor is necessary to develop the voltage for the control system/monitor to read. AND they think that because the current in a series loop is the same at any point in the loop they can just add measuring points without limit. They don’t know about the typical and usual 500 ohm maximum in the loop, OR they don’t add the dropping resistor which causes a break in the loop which means nothing in the loop works.

David_2 did a good thing when he mentioned that.

Many of these same technicians don’t know about current loop isolators and “duplicators” which can be used to get the signal to other systems/monitors.

OJT (On the Job Training) without a curriculum or testing/certification isn’t very good most of the time. And there are apprenticeship programs anywhere any longer. And with the retirement of baby boomers there often isn’t anyone to do the OJT for the new hires, and many companies don’t want to hire replacements until a week or two before the retiree departs.

It’s a problem—just about everywhere, except in some countries in Europe. Where unions are strong.

 

@WTF: very much so. Just had one on the phone that wasn't able to start his 4-20mA controlled burner...............I asked did you install the 500ohm resistor, answer: yesyesyes. Send me a picture and: tadaa. In series with the 4-20mA.....but it has to be between the + and - of the connection point of the 0-10Vdc burner modulation input.

Always the same.
And this was in the Netherlands by the way.

Most young folks today don't want all this fuss with studying many years hard for a technical job with low pay. They prefer to be influencer/youtuber so they can sleep out every day and live in Dubai.

O, good thing in the Netherlands, government is planning to re-initiate military service for young people. Now -that- will also help a lot, that they learn to listen to older people and do what they say.

 

Voltage Outputs

For some reason, analyzers and scientific instruments that find their way into the process world are the suspects for voltage output signals. And probably not isolated which will require an isolator module by the time the signal gets to wherever it goes.

There are some "low power" field transmitters (pressure and thermocouple transmitters) that use a voltage output for use in remote location, battery powered systems where every uA or mA isn't used extends the battery life.

 

That is true, I used Teledyne oxygen analysers and they had a 0-1V ouptput, this was always a pain in the #ss in noisy industrial environments.....