# 4-20 mA calibrator



## Mbit (Feb 28, 2020)

Was browsing Tequipment.com looking at the Fluke 773. Came across this little blurb about accounting for resistance in a 4-20 circuit. Came across this:

_Milliamp Calibrators (Loop Calibrators) are important for compensating for wiring runs

At the other end of the wiring run from the transmitter is an instrument such as a PLC (programmable logic controller), DCS (distributed control system), data logger, or controller. These devices actually will be measuring voltage so an external precision 250 ohm shunt resistor is installed across the terminals or built into the instrument. The instrument will be reading a voltage signal from 1 to 5 VDC. How? Look at ohms law, V = I x R, at 20 mA ==> 0.02 A x 250 ohms = 5.0 Volts.

Great, but what happens when you are using 200 feet of 20 gauge wire. Quick internet search and you will find a resistance of roughly 1 ohm per 100 feet, meaning the instrument will see 0.02 x 252 = 5.04, an increase of 0.8%. So now your brand new, NIST calibrated device is +0.8% in error before even installing it. Depending on your application, this could be significant. And it gets worse with longer runs and higher gauges. For 22 gauge, it is approximately 1.6 ohms/100 ft and 24 gauge is 2.6 ohms/100 ft. 20-24 gauge are typical wire gauges for process instrumentation.

Rather than taking estimates of the length of wire and loss in resistance, use a calibrator to quickly correct the error. These adjustments are often called 4-20 mA trim or current loop trim. Connect the milliamp calibrator in the source mode in place of the transmitter and source 20 mA and 4 mA to the instrument and setup the instrument to read properly 100% and 0%. Make sure to wait sufficient time to let the reading stabilize and/or set any instrument damping (sometimes called filter) to zero._ 


I've never done what they're talking about in here. Normally when I "calibrate" a "normal" pressure transducer that only has zero and span(with a known pressure source) I set the zero and span until they're both as close as I can get them and then check for linearity at 4, 8, 12, 16, and 20. If it's all good I consider that calibrated.

https://www.tequipment.net/fluke/process-calibration/signal-calibrators/loop-calibrators/

I've never worried about the resistance in the wire because that's the whole point of a 4-20mA loop. The transducer acts as the transmitter and controls the loop. I'm not talking about a loop that might have a transmitter and 2 or 3 I/P valves or somthing like that. Just a single pressure transducer on a 4-20mA circuit.

I've never had to do this, maybe I should be lol? Fire away gents!


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## splatz (May 23, 2015)

Mbit said:


> I've never worried about the resistance in the wire because that's the whole point of a 4-20mA loop. The transducer acts as the transmitter and controls the loop.


I agree, you don't really care about the resistance in the circuit because the transducer is going to adjust the current irrespective of the resistance and you're going to measure the current. Kirchoff's current law tells you it's the same through the whole loop. 

Was it maybe something about calibration where you had to account for resistance in the wire?


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## gpop (May 14, 2018)

test the instrument then test the analog input with a calibrated meter. Adjust either end or both so they match.

You have no idea if a offset was programmed years ago in the card or plc (then a new sensor was installed) or if the hardware has drifted so you still have to test the input which is the whole point of calibrating the input and the transducer. We also do this to spot scaling errors that seem to happen way to often.


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## emtnut (Mar 1, 2015)

Mbit said:


> I've never worried about the resistance in the wire because that's the whole point of a 4-20mA loop. The transducer acts as the transmitter and controls the loop. I'm not talking about a loop that might have a transmitter and 2 or 3 I/P valves or somthing like that. Just a single pressure transducer on a 4-20mA circuit.
> 
> I've never had to do this, maybe I should be lol? Fire away gents!



You're correct the way you are doing it.
Author is an idiot, current is constant.


Maybe the guy should google 'constant current source' instead of Ohms law:biggrin:

Or maybe, my LED strings are dimmer at the far side of the house :vs_laugh:


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## Jlarson (Jun 28, 2009)

Lol that was one of that whole points of current over voltage instruments. 





gpop said:


> test the instrument then test the analog input with a calibrated meter. Adjust either end or both so they match.
> 
> You have no idea if a offset was programmed years ago in the card or plc (then a new sensor was installed) or if the hardware has drifted so you still have to test the input which is the whole point of calibrating the input and the transducer. We also do this to spot scaling errors that seem to happen way to often.



What he said too. I always check the input cause just tossing xmitters will bite you eventually if you don't.


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## micromind (Aug 11, 2007)

The article quoted in the OP was written by some sort of an educated idiot engineer who has more papers pinned to his office wall that he has brains. 

I wonder if he has ever heard the terms 'current is the same in all parts of a series circuit'. Apparently not. 

Maybe he needed to make a payment on his mcmansion to avoid foreclosure so he wrote a sensational article that would transform the entire industry!!And make him rich.......

The actual reality is that the size and length of cable matters only if it causes enough impedance to reduce the voltage across the loop powered instrument to a level that the instrument cannot function. 

I do the same thing as a few of the others do; test the analog input then hook up the instrument. But now that I've read the article, my next one is sure to fail.......


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## paulengr (Oct 8, 2017)

I check 0 (4 mA, 50% (12 mA), and 100% (20 mA). If I’m ambitious I might check over and under range too. As stated that guy is an idiot. If I get one of those I will ask what happens if the air temperature goes up, changing the resistance of the wire dramatically.


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## paulengr (Oct 8, 2017)

Also if you are recalibrating everything how do you know why it’s losing calibration? If it’s drifting something is going bad. Better to just leave it alone and track the error.


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## Mbit (Feb 28, 2020)

paulengr said:


> Also if you are recalibrating everything how do you know why it’s losing calibration? If it’s drifting something is going bad. Better to just leave it alone and track the error.


I don't really do calibrations unless it's a new pressure xducer or pressure switch, don't have time. I have a lot of ametek 88s with zero, span, and a turndown so if I want to change the range I can. Pressure switches go bad all the time due to vibration here. Generators and reduction gears have low lube oil shutdowns and they get killed by vibration. 

Plus sometimes checking the calibration on a pressure switch is the easiest way to get the mechanics to do their job.


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## MotoGP1199 (Aug 11, 2014)

I don't "really" calibrate anything with 4-20ma. I do however have to adjust my range on some longer runs as I find the bottom of my scale at 0 output might actually be 4.21ma or 3.90ma and full output might be a little different. Some meters will read negative numbers if the ma reading is under 4.00ma and the customers do not like that. So I am sort of calibrating the sytem.


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## gpop (May 14, 2018)

When your working an a 20-000 gallon tank and the product is $40-50 a gallon they tend to get a little upset if your calibration is a slightly out on the level sensor. Its a boring job but some people like that type of work and the pay is good.


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## Mbit (Feb 28, 2020)

The "cal guys".

We had 2 cal guys come on board one time for a week or so. The day before we were gonna leave port we did some engine checks and the loop for the pitch on one of the propellers was dead. We had a fun time even figuring out where the circuit ran, where the xmitter was or even exactly where it got it's speed reference. There were 8 or 10 pitch indicators in the loop.

I asked the cal guys "did you guys do anything with the pitch indicator loop, it's not working"

"we never touched any of that"

Ok, hmm.

after about 4 hours I went outside where one of the meters was. It had a big red "REJECTED" sticker on it. Popped it open and jumped it with my meter and the loop came up normal.

That was a sunday and my entire department was ashore hosting a BBQ on the beach drinkin beer...with the "cal guys" in attendance.


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## gpop (May 14, 2018)

Mbit said:


> The "cal guys".
> 
> We had 2 cal guys come on board one time for a week or so. The day before we were gonna leave port we did some engine checks and the loop for the pitch on one of the propellers was dead. We had a fun time even figuring out where the circuit ran, where the xmitter was or even exactly where it got it's speed reference. There were 8 or 10 pitch indicators in the loop.
> 
> ...


Why would the cal guy know what hes working on. I been to lots of places where i have no idea what the sensor does. I know how to test the sensor and check the cal but where it fits in the scope of production i have no idea. 
This is why we generally have a helper/runner assigned by the company who puts us on the equipment and ensures it safe for us to work on it.


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## Mbit (Feb 28, 2020)

gpop said:


> Why would the cal guy know what hes working on. I been to lots of places where i have no idea what the sensor does. I know how to test the sensor and check the cal but where it fits in the scope of production i have no idea.
> This is why we generally have a helper/runner assigned by the company who puts us on the equipment and ensures it safe for us to work on it.


Because he took a working meter and during the course of testing made it not work. 

If I'm working and I blow something up or trip something (I've been in the same exact situation so I understand), and someone asks me "hey did you possibly do anything to make XXX not work?" I'm gonna say "well I'm not sure but I did just blow up a meter and reject it, could that have something to do with it?"


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