advertisement
from the department...
why 4-20mA? Why not from 0?
Engineering and workplace issues. topic
Posted by Mikey on 30 August, 2002 - 2:42 pm
Is there a technical problem to produce output start from 0 V or 0 A in any type of trasformer and transducer? Just want to know because the common products are usually with output starting from non-zero, like 4.


Posted by don gray on 1 September, 2002 - 11:21 am
> Is there a technical problem to produce output start from 0 V or 0 A in any type of trasformer and transducer? Just want to know because the common products are usually with output starting from non-zero, like 4.<

The reason for a "live" zero value such as 4 mA or 1 volt is it allows one to see the difference between a 0 value, and a broken connection. This means if the loop is 4 - 20 mA, and the reading is 0 mA, there is something wrong with the instrument loop. If the reading is 4 mA, the loop is ine.


Posted by Vikas Rawat on 1 September, 2002 - 11:23 am
This is a design feature and not a technical problem. These devices are designed to start output from 4 mA or 1 Volt so that one can differentiate between an open circuit and a live circuit. For example, you are getting 0 mA from a transmitter how sure you are about weather the 0 is coming because of a process condition or an open circuit or a device failure? On the other hand when you get 4mA from a transmitter you are sure that the control circuit is ok but the process condition is such that the transmitter is giving the lowest output.

With best regards,

Vikas


Posted by James Ingraham on 1 September, 2002 - 11:24 am
If it starts at 0, you can't tell if there is a wire breakage or missed connection. With 4 being the lowest, you automatically know when the device has been disconnected, as opposed to getting minimum value.

-James Ingraham
Sage Automation, Inc.


Posted by Bob Peterson on 5 September, 2002 - 5:46 pm
The zero elevation also gives you the ability to use the voltage drop this has to create to power the transmitter, thus allwoing for 2 wire transmitters.


Posted by Fernando Villamarim on 1 September, 2002 - 11:33 am
It is very possible to use 0-20mA instead of the popular 4-20mA current loop. In fact, some process transducers do offer the option of working with 0-20mA. However, there are two distinct advantages of using 4-20mA: First, as the output current generated by the transducer is allways flowing, one can easily setup an alarm system by monitoring the current. If there's no current flowing in the loop, then the loop is open somewhere or the sensor is bad. The second advantage concerns with the signal-to-noise ratio (S/N)of your measurement: Suppose you have, say, 100mV of 60Hz noise pickup, or high frequency pickup, in your loop. If the loop is 0-20mA (or, say, 0-5V in case the transducer generates an output voltage instead of current), them at 0mA the interference level could be quite significant for your particular application, and sometimes is hard to eliminate. Now, if the loop starts with 4mA, the S/N would be much better and noise filtering, if required, much simplified. Finally, computer-based data acquisition systems can be programmed to take into consideration the current off-set (4mA) coming out from your process transducers and adjust your measurement scale accordingly.


Posted by Daniel Chartier on 1 September, 2002 - 11:33 am
Hello;
If you use an offset in transmission levels (4-20 mA, 1-5 Volts) you have an inbuilt wire break detection. Any 0 value signal indicates incorrect or defective wiring. Which can be useful, no? There is nothing stopping you from using 020 mA or 0-5 V if you wish.
Hope this helps,
Daniel Chartier


Posted by Anonymous on 1 September, 2002 - 11:33 am
There is no technical problem to produce output start from 0 V or 0 A & there are many of them but if you will use 4-20Ma you can measure the sensor itself and also you can measure that the sensor cable is broken or no connected !


Posted by John Beck on 6 September, 2002 - 12:05 pm
There is no technical reason why 0 ma cannot be used IF you are using a field powered device (usually called 4 wire transmitters). If however
you are using loop powered devices (usually called 2 wire transmitters), the device itself needs to obtain power to operate, even with a zero indication. The power available in that 4 ma or 1 V offset is sufficient to provide this function, and still retain the open loop detection as well.

John Beck
Control System Specialist, Retired


Posted by Andrew Ward on 1 September, 2002 - 11:36 am
0-20 mA is an option in most PLC IO cards, but the biggest reason for starting at a non-zero value is for instrument failure detection. If the value is less than 4 mA, the instrument could be out of calibration or have bad electronics. At zero, the instrument could be powered down/dead or the loop could be open. A lot of instruments also fail to 25 mA to indicate a problem.

Andrew Ward
andyward2@comcast.net


Posted by Felix Lee on 1 September, 2002 - 2:30 pm
Hi,
The reason why 4 to 20 mA is because in some applications, a wire break in the signal is very critical. If you start at zero, there is no way the controller can distinguish between a true zero signal and a wire break condition !!!


Posted by Anonymous on 1 September, 2002 - 2:32 pm
excuse for my english, it is not my first language; it is for security
the devices with 4-20ma is connected e.g to a plc , the variable is temperature, in the wire there are 4 m.a to the "min value" when the wire is broken this measure will be lower than 4 m.a.
the plc detect this condition.


Posted by Biekasan on 1 September, 2002 - 2:33 pm
Hello mikey
I know 1 reason for this:
Cheking signal line is important in process control.We must check line for open circute conditin.If you have minimum current in your line
(like 4ma),you shure that your line is live.If use
0ma set,is there any diffrent between zero condition and open circute?

Biekasan


Posted by JCBlanch on 1 September, 2002 - 2:33 pm
This subject was brought up a few month ago on this forum. Search the PLC Archive for 4-20 mA.

Good Luck
Jcblanch


Posted by Ian on 1 September, 2002 - 2:34 pm
The 4mA gives a positive indication for Zero.
0mA therefore equates to lost signal.
0-4mA allows for underrange indication in some equipment.
system is therefore easier to fault find.


Posted by Roland Lilja on 1 September, 2002 - 2:35 pm
Hi,
No there is no technical problem! But the main reason for using 4-20 mA instead of 0-20 mA is that you can detect a broken circuit by reading a value less than 4 mA. Usually PLCs are generating an alarm if the messuerd curent is lover than 4 mA
The same goes for 0-10 V.

Regards
Roland Lilja


Posted by Sanjay Sheth on 1 September, 2002 - 2:35 pm
This is likely to prevent your analog inputs from seeing random noise in the system of a few millivolts/milliamps etc.


Posted by Marc Le Douarain on 1 September, 2002 - 2:36 pm
I think that the reason for starting from 4 instead of 0, is that a 0 mA measure means an error : link/captor broken... But perhaps there is another reason?


Posted by Jesper on 1 September, 2002 - 4:35 pm
4..20 mA is the accepted standard for two-wire transmitters. One wire provides the power, the other sends back the signal.
The 4mA is what the electronics in the transmitter "lives" of. It also provides you with a simple way of testing if the transmitter is alive (signal less than 3mA = bad).

If the transmitter is a 3 or 4-wire, it can have any desirable range (0..20mA, -20..+20mA, 0..10V, -10..+10V etc.).
But as the 4..20mA range is so common, even most 3 and 4-wire transmitters conform to that range.

One good advice: If you read 4..20mA in a PLC analog input, allways choose 0..20mA range for the input. Do not use a 4..20mA input range, or the PLC automatically generates "underrange" alarms (most PLC's do that) even if the signal barely touches 3.99mA.


Posted by jyothi on 20 May, 2011 - 3:23 am
hello guys,

can you please give me an unique reason for why we are using 4 to 20mA instead of 0 to 20mA

jyothi


Posted by Zacharia, Tomy on 21 May, 2011 - 5:02 am
Please search archives. Answered many times.

Ok, refer http://www.control.com/thread/1026235722#1304111163

Regards,
Tomy Zacharia


Posted by Bill Clemons on 1 September, 2002 - 4:35 pm
Zero output can be interpreted as an open loop condition. So, 4-20 mA signaling can use zero output to determine an error condition.

If the system can tolerate zero signal (and many do), the open loop error (nuisance) can be programmed/configured around.


Posted by Frank Guardabascio on 1 September, 2002 - 4:37 pm
The correct definition for the 4-20 signal is "elevated zero". With an elevated zero, the signal link can distinguish between a correct connection and an open line. If voltage is used, it is not possible to distinguish betwnn zero volts and an open loop


Posted by Joe Jansen/ENGR/HQ/KEMET/US on 5 September, 2002 - 4:56 pm
Check the archives, as this is another question that comes up about every 3 to 6 months....

The technical reason is that by starting at 4mA, you can:

1. Power the device from the 4 mA present, if it is small enough.
2. More importantly, if you get a 0mA reading, you know that there is a fault somewhere in the line, as opposed to simply having a 0 level.

--Joe Jansen


Posted by Arnold Dillon on 5 September, 2002 - 5:47 pm
The signals could just as easily start at 0 milliamps. One of the main reasons for choosing a non-zero value was that it gives a means of error checking called a "Live Zero." If the loop reads 4 milliamps then a zero measured value is
detected, but a valid current loop exists. If the same loop reads 0 milliamps then an open loops exists, and something is wrong.


Posted by Walt Boyes on 6 September, 2002 - 4:29 pm
The reason is that if there is a line break, the current drops to zero in a 4-20 mADC system, while in a 0-20 mADC system this alarm feature does not exist.

Walt Boyes

---------SPITZER AND BOYES, LLC-------------
"Consulting from the engineer
to the distribution channel"
www.spitzerandboyes.com
walt@waltboyes.com
21118 SE 278th Place
Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office
fax:801-749-7142
--------------------------------------------


Posted by Jonas Berge on 6 September, 2002 - 5:11 pm
Apart from the wire break detection, another reason for 4-20 mA is that two-wire devices that power themselves from the 4-20 mA need the guaranteed 4 mA to operate. I.e. the internal workings of this type of device is that about 4 mA is used to power the circuitry, and about 0-16 mA is shunted past, thus adding up to the 4-20 mA signal.

==================
jberge@smar.com.sg
www.smar.com


Posted by Roy Matson on 22 May, 2011 - 1:40 am
I can't believe how many of you think the 4 mA is just for break detection. If that was the reason why not 1 mA?

It's to power the transmitter.

Some of the old guys will remember when it was 10 -50 mA, as electronics improved we no longer needed 10 mA to power the electronics.

Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2014 Nerds in Control, LLC. All rights reserved.

Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy.


Fortune
Look out! Behind you!
Advertise here
Advertisement
our advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive