Why 4-20 ma is used in Analog Signals?

>Why 4-20 ma is used in Analog Signals?
>How come ISA selected this. Why not
>5-21ma?

During an Internal training at our company this topic came up. One answer which I liked was 4-20 ma gives 16 ma range which can easily be translated to 4 bit and if we take 0.5ma step then giving 32 levels which can be dealt by 5 bits and so on. I don't know if this was the reason but if not its a good coincidence. Easy Analogue to digital conversion.
 
R
There is much miss information here.

Obviously for a two wire loop you can't have a zero based scale otherwise where would you get the power to operate the transmitter, all electronics needs some power, the 4 mA provides that.

When I started in process control 10-50 mA was the standard, this converts into 1-5 Volts with a 100 Ohm resistor.
As better electronics came along the power required to operate transmitters became less and less so the current standard dropped.

I don't pretend to know why 4-20 was chosen but I doubt it had anything to do with an ancient teletype, thats communications, nothing to do with process control.

Replace the 10-50 mA transmitter with a 4-20 one and change out the 100 ohm conditioning resistor for 250 Ohms and you're off to the races, you could have a mix of the 2 as I'm sure many old plants did.

I'm sure there were other standards before 10-50 mA

Roy
 
R
>During an Internal training at our
>company this topic came up. One answer
>which I liked was 4-20 ma gives 16 ma
>range which can easily be translated to
>4 bit and if we take 0.5ma step then
>giving 32 levels which can be dealt by 5
>bits and so on. I don't know if this was
>the reason but if not its a good
>coincidence. Easy Analogue to digital
>conversion.

Nice theory but 4-20 was standard before digital control systems became available, the electronics was still all analog.

Roy
 
In reply to Roy Matson: While I can't state for certain the reasons why 4-20ma was chosen, teletype equipment was used in remote telemetry applications. I don't have any details on this beyond some old Teletype sales brochures. Those however show instruments connected to some sort of signal conditioning, which in turn is connected to a standard Teletype "RT Set" which would automatically send the readings to a remote location. This is a completely automated set-up with no operator. Their other literature also makes numerous oblique references to the use of tele-typewriter equipment in process control industries. These are sales brochures however, so they don't go into any technical details. However, it's still possible to see that there were connections between the communications and process control industries.

This is of course aside from the rather practical consideration that there used to be a lot of off the shelf components such as power supplies, regulators, rectifiers, etc. which were designed for the tele-typewriter industry. It's like asking why do we use D-shell connectors when those are communications equipment and not process control equipment? The hardware is there, it's off the shelf, and it does the job.
 
W
The SP50 committee decided that 0-20 (which was the European choice) didn't provide for a "line fail" signal, while 4-20 did, and since it was also easy to convert between 4-20 and 1-5 vdc, it became the recommended signal.
0-20mADC persisted for many years, but 4-20 really took off when HART was developed.

Walt Boyes
Editor in Chief
Control and ControlGlobal.com
555 W. Pierce Rd Suite 301
Itasca, IL 60143

[email protected]
www.controlglobal.com
 
G
only at 4-20ma output, we are getting linearity. Also, at 4-20ma with +24V power supply, no spark will be generated. for these reasons only 4-20 ma is selected.
 
A

Atul H. Misar

Normally the sensor measure physical quantity & the transmitters transfer the signal at 4-20 ma. because if the circuit get interrupted or short circuit occur then the signal shows 0 ma if the signal rating is 0-20 ma then ckt can not be identified.

Also at 4-20ma output,we get linearity & 4-20ma with +24V power supply, no spark will be generated.
 
C
I'm fairly sure they picked 4-20 ma to keep this ML in business. The question is a keep-alive. :^)

Regards
cww
 
> Why 4-20 ma is used in Analog Signals?
> How come ISA selected this. Why not 5-21ma?

4-20ma and 0-10 volts are the two main control signals we see both in commercial controls and industrial controls. The 4-20 ma will allow you to overdrive a device such as a valve actuator, which you might want to do for something like proof of closure.

A very good video at a site called controltrends.com has several really good videos on how and when you would use different types of control signals.

Here is link to a really good video on that site that goes into this is
http://bit.ly/r8iuJ9
 
M
> Why 4-20 ma is used in Analog Signals? How come ISA selected this. Why not 5-21ma?

Because transmitter needs a minimum operating current of 3.8mA. if go below 4mA resolution will be least.

More over 4 to 20mA across 250Ohms resistor give 1 to 5v.
If we increase beyond 20mA power dissipation will be on higher side.
 
C

Colin Dallimore

Originally transmitters used a technique where a beam within a wound coil generated ma as it moved proportionally in and out of the coil reacting to the measured variable. This was then made 4-20ma for fail safe indication
 
Looking ahead, the future continues to look bright for the 4-20mA signal transmission standard in industrial environments. Its lossless nature, lower-sensitivity to induced noise, its live-zero offset, fail-safe operation, and easy scalability contribute to its longevity. Plus its adaptability to many different wire conductors and connectors, and its relative immunity to poor quality connections, all contribute to its popularity. And because it is so widely supported by thousands of compatible devices, including wireless transducers, it would be difficult to unseat it as the leading analog transmission standard for industrial I/O. Likewise, modern variations of 4-20mA such as HART1 continue to drive support for the standard.
 
J

Jebastin anand

> Why 4-20 ma is used in Analog Signals?
> How come ISA selected this. Why not 5-21ma?

All PLC & DCS controllers having input and output processing signal is 1vdc to 5vdc. so according to the Ohms Law,<pre>
V=IR
V=4mA*250 Ohm
V=0.004*250
V=1V
V=IR
V=20mA*250 Ohm
V=0.02*250
V=5V
here,
R=250ohm is constant instrument cable resistance.</pre>
so only we have to use 4mA to 20mA.
 
>> Why 4-20 ma is used in Analog Signals?
>> How come ISA selected this. Why not 5-21ma?

As far as I know, the early hydraulic gas turbine controls needed a 40-200 psi reference signal to position the hydraulic servos of the fuel valves. The first Speedtronic control system, used a 4-20V signal to emulate that. I supose the 4-20mA standard appeared in order to improve the noise inmunity, keeping the relationship in number modules. In all cases 0 means some problems in reference signal and causes shutting down of fuel valve.
 
Y

yukongrizzly

When I began in the power industry several years back there were two commonly utilized current control signals. They were: 4-20mA and 10-50mA. As with the pneumatic control signals, the most commonly used were 3-15psi and 6-30psi.

With respect to control signals, a live zero (a value greater than zero) is desirable so that it would be easy to determine if there were a power supply/source failure. In your case, 4-20mA, 4mA would represent your zero (lower range value) and 20mA would represent your span (upper range value). If there were ever a time a 0mA signal was observed this would be a telltale of a power supply failure. Another benefit to the 4-20 mA signal is that it can be easily converted to a 1-5 volt signal merely by passing the current through a 250 ohm resistor. This is a handy feature, as many control system components utilize voltage inputs rather than current inputs. As any technology develops there must be conventions or standards agreed upon so that the system components can interact compatibly. Thus the 4-20 mA signal was largely adopted as the "universal" current control signal.

I hope this helps.
yukongrizzly
 
I agree, lower voltages are safer but you can still get a spark from a 24vdc loop. The loop is fused back at the marshalling panel but usually significantly greater than 20mA, so it may let by 100's of mA before the fuse blows. Think of your car battery, its only 12vdc but it can make a pretty big spark, even with a small fuse on the circuit.

My opinion on the 4ma as zero is so the system and operators will know if there is a problem on the loop, like blown fuse, transmitter fault, etc.

> Normally the sensor measure physical quantity & the transmitters transfer the signal at 4-20 ma. because if the
> circuit get interrupted or short circuit occur then the signal shows 0 ma if the signal rating is 0-20 ma then ckt can
> not be identified.

> Also at 4-20ma output,we get linearity & 4-20ma with +24V power supply, no spark will be generated.
 
The point will be argued until 4 - 20 mA is dead and buried. Did anyone mention it was just a natural progression from 10 - 50 mA i.e. the old 10 - 50 mA transmitters are still compatible with a 1 - 5 Volt controller input by changing from 100 to 250 Ohms.

There are also many instruments operating on 0.2 to 1 Volt, here again it's just a matter of swapping out the shunt resistor.

Roy
 
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