Temperature Transmitter linearity
Temperature Transmitter linearity
(OP)
Hi all,
I have a PLC reading a thermocouple transmitter in the range of 4-20mA for a temperature range of 0°F to 500°F. After I calibrate zero and span to match 0°F = 4mA and 500°F = 20mA, I found that there is no linearity at all on such devices, drifting almost 8°F down on midscale when checking its input using a temperature simulator from Fluke.
I am using a SCL series PLC from AB, then scaling input using SCP where parameters are 0-4095 equals 0°0 to 500°F.
Now my question: how can I linearize the input to show a better reading of the temperature considering the nonlinearity spec of the Omega J type transmitter?
Any technique would help. Thanks
I have a PLC reading a thermocouple transmitter in the range of 4-20mA for a temperature range of 0°F to 500°F. After I calibrate zero and span to match 0°F = 4mA and 500°F = 20mA, I found that there is no linearity at all on such devices, drifting almost 8°F down on midscale when checking its input using a temperature simulator from Fluke.
I am using a SCL series PLC from AB, then scaling input using SCP where parameters are 0-4095 equals 0°0 to 500°F.
Now my question: how can I linearize the input to show a better reading of the temperature considering the nonlinearity spec of the Omega J type transmitter?
Any technique would help. Thanks





RE: Temperature Transmitter linearity
xnuke
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RE: Temperature Transmitter linearity
I opened the table and I am not quite sure how to proceed as far as use an automated formula to covers the entire range. Is it how it works?. I will read about the related topic and this NIST table concept.
RE: Temperature Transmitter linearity
Then, calculate the "Corrected Counts":
Corrected Counts = Actual Counts + [-4.8919E-13*(Actual Counts)4 + 7.078E-9*(Actual Counts)3 - 3.9143E-5*(Actual Counts)2 + 7.5141E-2*(Actual Counts)]
Then, use your Corrected Counts as the Input into the SCP function with the range parameters you gave above for Input Min., Input Max., Scaled Min., and Scaled Max. 0-4095 equals 0°0 to 500°F.
xnuke
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RE: Temperature Transmitter linearity
So it will be a matter to use a math function such as CPT to process the input channel (N17:1) and convert the input raw bcd to a corrected value of it.
Since I am using 0F to 500F, I guess I also need to use the formula from -200C to 0C in order to perform the scale below 0C and use logic to select which one to use depending on input value. Is that correct?
This is the first time I see this concept in place, but for sure wont be the last.
Let me give it a shot today.
RE: Temperature Transmitter linearity
xnuke
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RE: Temperature Transmitter linearity
Not necessarily. That would depend on how accurately you need the measurement and how accurately the probe was calibrated. What is your accuracy requirement? Many algorithms will work at the peripheries of their defined specs, but the error obviously increases in those regions.
TTFN
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RE: Temperature Transmitter linearity
Xnuke
You certainly correct, I am working on the 0-500F, however, the Excel file I download stars on the 32F (= 0C) and that is why I am wondering how to make it run on the range we have set the application for. Would you please check the file and let me know if that is a tipo error or certainly need to be corrected.
IRStuff
My accuracy is 0.1% FS
Thans both
RE: Temperature Transmitter linearity
Where did you get this file and how do you know it applies to your temperature sensor? What accuracy does your temperature measurement system have?
TTFN
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RE: Temperature Transmitter linearity
Where did you get this file and how do you know it applies to your temperature sensor? What accuracy does your temperature measurement system have?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Temperature Transmitter linearity
File was provided kindly by Xnuke based on what I using here. File is located at
http://files.engineering.com/getfile.aspx?folder=9...
Application: this is a forced convection oven previously running with a PartLow +1400 temperature controller. The project is about automate oven process and recipe loading using a PLC. PLC reads temperature and controls power on SCR and customer will, pretty much, validate it using the same resolution (1F) and accuracy they where using (+/- 5F) on the previous controller. The actual range of usable temperature goes from room temperature (about 65F) to 250F, however, and unless otherwise requiered, range will be calibrated to 0-500F.
Thanks for your comments.
RE: Temperature Transmitter linearity
I agree with IRStuff, that is an uncommonly tight uncertainty specification, and not achievable with a type J thermocouple (which has at least an uncertainty of +/- 2 F), let alone the additional uncertainty in the measurement provided by the transmitter, the PLC, etc. I suggest you revisit the following questions: What is the process temperature range that you need to measure, and what accuracy do you need?
IRStuff, I created the Excel file. For fun. (Yeah, I know, I'm strange!)
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Temperature Transmitter linearity
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Temperature Transmitter linearity
I have been testing the new values and so far so good. I will be testing it over more point on the entire range as soon as I get a simulator, because using the real temp from oven does not allow me test the lower and high-uper ones. The simulator will tell me the real truth, however, let me tell you that so far, values on PLC input are super close to the ones shown on table. I.e, for 200F, BCD is 1637-38.
Very impressive job on table concept!
I will keep you posted.
RE: Temperature Transmitter linearity
Looks that I let my soul get happy to fast...
On the corrected counts value, the formula is retorning the correct values, however, the PLC input raw value is a bit high when compared to theoretical values shown on the table. Ie. for 200F, corrected value is 1638, and Actual count, as per table is 1581, however, PLC raw data from that channel is about 1500. Again, this is only hipotetical worries that should be settle down once the simulator is hookup to transducer. One of the elements to lead to be worry, is that I have a reading from a chart recorder thermocouple on the oven which is showing -9F respect to PLCs (200F). The chart recorder is not calibrated and TC not in the same point as the one read by PLC, so that, I have not all the info to conclude what the actual difference is. I will keep you posted.
David
RE: Temperature Transmitter linearity
I have run the entire range using a Fluke 477 simulator.
Attached my finding: we are running about 7 degree above the standard. Without correction, the error is about 7F below standard. Looks like we need something in between.
RE: Temperature Transmitter linearity
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Temperature Transmitter linearity
I will perform a test using a calibrated mA source just to make sure PLC input is working correctly and linear throught the entire range, unfortunately, tomorrow is holiday here and no equipment will be available to me, but I will still be working on this to have it done for official calibration on Thrusday.
The closer I can go, assuming PLC output is in good shape, is to source the Ain with the Aout, just as a quick test.
Ill keep you posted.
Thanks a lot for your support.
David
RE: Temperature Transmitter linearity
I managed to use a the same fluke simulator on DC mA mode to input 4-20 mA to my PLC Analog in module. Results works perfect when scaled to 0-500F, therefore, the module is working just fine. Did you have any chance to check your correction formula yet? Later on, I will chat you on the know how behind the polynomials, and this is the first time I see it and I want to be sure I understand what is involved to be ready for next time...but I hope as well, not use face this issue again by using better xmiters...
Thanks.
David
RE: Temperature Transmitter linearity
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.