You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

Reply
 
Thread Tools Display Modes
Old June 18th, 2021, 01:59 AM   #1
skyfox
Lifetime Supporting Member
United States

skyfox is offline
 
skyfox's Avatar
 
Join Date: Nov 2006
Location: CA
Posts: 255
Help scale an RTD with a 4-20mA SIG Conditioner

Good day all,

I am having bit of an issue getting my head around this RTD scaling issue. Be forewarned, my head has been known to be "THE" issue for quite sometime.

PHYSICAL DEVICES:

RTD - OMEGA 4 Wire -30 to 260 C PT100 RTD

Signal conditioner - OMEGA PRCTL series 2, 3, or 4 wire RTD to 4-20mA output. The signal fed to the PLC is scaled according to the signal conditioner settings (blue marking) in the chart below.

Issue:


I am about 2 degrees off with these settings. This is the closest I can get to the actual temp. Any higher or lower ranges in the signal conditioner settings skews the actual temp by more than +/- 5 Deg C.



REFERENCE:


In the same loop is a Brooks Automation precision Mass Flow controller with a built in RTD. It consistently reports around 31.5 Deg C which in fact is the correct temperature in the process loop.


Help Needed:

How do I correctly scale this RTD with this Particular signal condition to the range I need so that I am within 0.5 DEG C of the reading from the MASS FLOW CONTROLLER? I do not care about the lower range in this instance. Just the ambient inside the process loop which can never be above 32 C or below 28 C.


SIDE NOTE FYI.

Mass Flow Controller - $5,000 (Used buy one of worlds top 3 semiconductor equipment manufactures and dictated by them that we use it on this test jig.

RTD - OMEGA RTD ~$170
SIG Conditioner - OMEGA Signal Conditioner ~$230

We use OMEGA's -200 to +850C RTD's with this same signal conditioner for this very same customer and they work flawlessly,for both HI-TEMP and Cryogenic applications.

Any thought or suggestions on how to go about properly scaling this RTD with this specific signal conditioner in this particular scenario?

Many Thanks.

Cheers!
Attached Images
File Type: png RTD SIG COND.png (231.5 KB, 131 views)
  Reply With Quote
Old June 18th, 2021, 03:36 AM   #2
cjd1965
Lifetime Supporting Member
United Kingdom

cjd1965 is offline
 
Join Date: Apr 2007
Location: UK
Posts: 1,653
hi, go back to basics.

Measure the resistance of the RTD, is it what your expected for the given temperature?.... Is it connected 2 wire, 3 wire? 3 wire accommodates the resistance for the length of the cables

If the RTD is 'good' measure the mA . Is the mA as expected?

If the mA is 'good' check the PLC is configured for the correct mA (0-20/4-20)

Check the PLC card at 4mA and 20mA.. is it giving you the expected raw counts? Does this agree with your scaled min/max?

This check list will lead you to the problem.
  Reply With Quote
Old June 18th, 2021, 03:38 AM   #3
cjd1965
Lifetime Supporting Member
United Kingdom

cjd1965 is offline
 
Join Date: Apr 2007
Location: UK
Posts: 1,653
Edit, i see the conditioner hasnt a setting for -30 and you have set to -25 so that is an error on your 4-20mA range also
  Reply With Quote
Old June 18th, 2021, 05:45 AM   #4
saultgeorge
Lifetime Supporting Member
United States

saultgeorge is offline
 
saultgeorge's Avatar
 
Join Date: Jul 2015
Location: Detroit
Posts: 415
Quote:
Originally Posted by cjd1965 View Post
Edit, i see the conditioner hasnt a setting for -30 and you have set to -25 so that is an error on your 4-20mA range also
It appears it also doesn't have a setting for 260 deg F either.
__________________
"Instant success teaches one nothing. Failure teaches one everything one needs to succeed."
  Reply With Quote
Old June 18th, 2021, 08:38 AM   #5
cjd1965
Lifetime Supporting Member
United Kingdom

cjd1965 is offline
 
Join Date: Apr 2007
Location: UK
Posts: 1,653
to conclude, your conditioner isnt set up to match the rtd (and cannot be) so you need to get an rtd to match the conditioners available scaling options
  Reply With Quote
Old June 18th, 2021, 09:48 AM   #6
danw
Member
United States

danw is offline
 
danw's Avatar
 
Join Date: Oct 2004
Location: midwest, USA
Posts: 3,203
One is expected to configure/scale the analog input to match whatever the 4-20mA signal represents.

There is never an assumed standard range for 4-20mA signal. 4-20mA is understood to represent a scale or range of an analog signal , hence PLC 4-20mA inputs are ALWAYS scaled or ranged to match the range of the transmitter.

The solution in this case is not changing ranges on the transmitter or changing RTD elements. The solution is to scale the analog input channei so that it matches the range of transmitter output.
  Reply With Quote
Old June 18th, 2021, 11:07 AM   #7
Nova5
Member
United States

Nova5 is offline
 
Join Date: May 2013
Location: Northern Snowball
Posts: 332
Have you verified which device is correct with a 3rd temperature taking device? It doesn't matter how expensive a device is, it can still have a defective part. Or are you assuming because the heating/cooling part of the process is commanded to get it to the target temp and the Brooks is reading that temp.. The brooks could be lying to you. Verify it.

IS your scale consistently off by the same amount from top to bottom, or does it skew? if totally consistent then just correct it in the PLC. You can make it work but someone can **** it right up at least with a consistent offset its easy to leave behind a note, a skew would require figuring out the ramp rate it changes by.


As said above by another.. you're going outside device compatibility. Fix that.

If the omega signal cond lets you play a little. set the input range to match the RTD capability, then set the output to whatever range you desire. I use a Define Instruments Javelin to do similar with the thermocouples we use. the thermocouple comes in the input side, I configured the Javelin to read the proper thermocouple type. then on its output I scale it to a sensible
range.. the process can't go below ambient Nor can it ever hit the top end of the thermocouple range... so I set it as 32-450 for the 4-20ma output.
__________________
Problems that go away on their own mystically are not solved. They will return.
  Reply With Quote
Old June 18th, 2021, 07:35 PM   #8
AustralIan
Member
Germany

AustralIan is offline
 
Join Date: Jan 2013
Location: Leipzig, Germany
Posts: 1,255
You did not mention which 4-20mA analog Input Card you are using.

A class A pt100 is supposed to be +-0.21C at 30C.
A class b would be +-0.45C. so double check your purchase order.

I couldn't find an Omega PRCTL signal conditioner. Is that the part number of the probe itself? Similarly priced conditioners from omega are advertised as "+-0.2%" accuracy, so if you are using the range -30 to 260, that's +- 0.6C.

I have seen plc 4-20mA cards advertised as +-2% accuracy, so this would be +- 6C. (e.g. 2080-IF2)

I guess you would sum the errors to get +- 7C.

So step 1 is getting all your data sheets together and working out your best case scenario.

The transmitter should be set up with as small a range as possible. Even though your rtd can sense down to -30 and up to 260, you should be able to set it to say 20C to 50C.

Without any calibration you should set your PLC scaling Min and max to match the setting on the transmitter, not the rating of the rtd.

But it makes sense to calibrate it. Stick the rtd probe in a temperature calibrator set at the desired temperature, and then make this mA value equal to the desired temperature.

Additionally, I would not take the temperature sensor built into a mass flow meter as gospel.
  Reply With Quote
Old June 18th, 2021, 10:44 PM   #9
willxfmr
Lifetime Supporting Member
United States

willxfmr is offline
 
willxfmr's Avatar
 
Join Date: Nov 2013
Location: Wisconsin
Posts: 440
Keep in mind I'm just a 3rd shift knuckle dragger.
It looks to me like you need dip switches 5,6, and 8 on. In your analog card set the scaling to 4ma/-200C 20ma/850C. I gather this from the box on the lower left that gives the ranges of PT100 RTD.

Bubba.
__________________
Now will come the nay-sayers with all the reasons why you shouldn't do this. Some of them are teachers, and some of us are doers. Besides, you have got to let the smoke out of this equipment every now and then, or it starts to clog up the chimney.
~Lancie1
  Reply With Quote
Old June 19th, 2021, 10:20 AM   #10
danw
Member
United States

danw is offline
 
danw's Avatar
 
Join Date: Oct 2004
Location: midwest, USA
Posts: 3,203
Setting the Range of an RTD transmitter

1. RTD element range

>We use OMEGA's -200 to +850C RTD's

I can't find an RTD with a 850C top end, the usual high end for wire wound platinum elements is 600C. This looks more like a spec for a thermocouple.

But assuming that the device in use is actually an RTD and not a thermocouple, the range of an RTD element describes the temperature range at which the element will provide a signal within the accuracy spec of the resistive element.

2. RTD/transmitter Circuit

The RTD is connected to the input of an RTD transmitter.

The RTD transmitter's output is 4-20mA. The transmitter's output range is scaled over some temperature range according to what makes sense for the application.

Too wide a range and the practical resolution available from the transmitter. If the temperature will be seen at outside of a certain range, why cover that range?

Too small a range will truncate the needed data.

Most transmitters will have some minimum range between the zero at 4.00mA and the span at 20.0mA.

3. Ranging the transmitter's 4-20mA output

There is no adjustment for the transmitter's input range (which is typical). The transmitter's output range is adjusted by some means, in this case, DIP switches.

4. Ranging the receiver's analog input

The receiver can be an analog input on a PLC, PAC, digital indicator, controller, recorder, DAQ.

The 4-20mA signal represents a specific temperature range. The receiver's input range MUST match (be identical to) the transmitters's output range.

A PLC's analog input range is scaled in the PLC's development software and downloaded to the PLC's CPU.

5. What output range makes sense in this application?

The perceived problem is that the transmitter's output range DIP switches does not have a -30 to 260C range. Some other range has to be selected, and the PLC's analog input range changed to match the selected transmitter output range.


In post #1, the process temperature range is described as
>the process loop which can never be above 32 C or below 28 C.

Also, in post #1, it states
>RTD - OMEGA 4 Wire -30 to 260 C PT100 RTD

The -30 to 260C range suggests some recommended range.

What is the source of the -30 to 260C range?
Is -30 to 260C the receiver's analog input range?
Is -30 to 260C a customer specification?

It seems to me -30 to 260C to be an overly wide range for a process that has to maintain a temperature of 28 to 32C.

If I had to pick, I'd use 0-50C because the process temperature range of 28 to 30C falls near the middle of a 0-50 range, but I don't know all the process details.

The suggestion that the RTD's sensor range of -200 to +850C is not appropriate because of the stated 28 to 32C process range; a wide range such as -200 to +850C would result in low resolution at the process operating range.
  Reply With Quote
Old June 21st, 2021, 02:57 PM   #11
skyfox
Lifetime Supporting Member
United States

skyfox is offline
 
skyfox's Avatar
 
Join Date: Nov 2006
Location: CA
Posts: 255
Quote:
Originally Posted by cjd1965 View Post
to conclude, your conditioner isnt set up to match the rtd (and cannot be) so you need to get an rtd to match the conditioners available scaling options

Now this is where I am a bit confused. Is the above statement really a true statement? First of all, no disrespect. I asked this purely based on my understanding that a RTDs resistance should directly correlates to a given ABSOLUTE temperature.

For an example:

RTD from manufacture A = 100 Ohms at 0C
RTD from manufacture B, also = 100 Ohms at 0C. (Ignoring class A/B precision for the sake of this discussion.)

Therefore, this Ohms/Temperature relationship should hold true for all RTDs. Grant it, some RTDs may be specd for certain ranges. But they should all report the same temperature at the same resistance reading, provided their specifications happen to occupy or falls within a given temperature range, regardless of what each ones min max range specifications are.

Again, for an example,

An RTD rated for -200C to +850C should have a resistance of 100 Ohms when temperature is 0 Deg. C as an RTD rated for -30 to +260 C. Likewise, they both should have the same ohms reading at 260 C. Then as temperature increases past 260 C the RTD rated for 260 C is no longer reliable and the other rated for +850 should continue up to 850C. Am I wrong or am I wrong on this summarization?

SPEC for the RTD is -30 C to 260 C

Since there is no exact 1:1 configuration option available with the signal conditioner, I have selected a range that is well within the specification limits of the RTD (-25 to 250) and scale the 4-20 mA signal from the conditioner to match the selected range in the PLC. Temperature reading is still off by more than 5 to 8 Deg. C. However, If I switch the RTD with a RTD that is specd from -200 C to 850 C and configure signal conditioner to the same range as well as the scaling in the PLC, then I am within 0.2 Deg C of the mass flow controllers. I have tried several signal conditioners as well as multiple RTDs.
  Reply With Quote
Old June 21st, 2021, 03:06 PM   #12
skyfox
Lifetime Supporting Member
United States

skyfox is offline
 
skyfox's Avatar
 
Join Date: Nov 2006
Location: CA
Posts: 255
Quote:
Originally Posted by Nova5 View Post
Have you verified which device is correct with a 3rd temperature taking device? It doesn't matter how expensive a device is, it can still have a defective part. Or are you assuming because the heating/cooling part of the process is commanded to get it to the target temp and the Brooks is reading that temp.. The brooks could be lying to you. Verify it.
There are actually three Brooks massflow controllers and the RTD is located down stream of the output path of MFC's within two inches. All MFC's report temperature within 0.1 C of each other. Brooks MFC are calibrated every six months at Brooks Automation.


Quote:
As said above by another.. you're going outside device compatibility. Fix that.
Actually my range selection is well within the rated range for the RTD. Yet it is reporting an incorrect temperature.
  Reply With Quote
Old June 21st, 2021, 03:14 PM   #13
cjd1965
Lifetime Supporting Member
United Kingdom

cjd1965 is offline
 
Join Date: Apr 2007
Location: UK
Posts: 1,653
Hi your correct that an RTD will give 100 ohms at 0C and they are pretty much universal

Your transmitter is set to give 4mA at -25C and 20mA at 250C therefore it wont transmit the correct 4-20mA for your desired range of -30C to 260C

Are you actually measuring from -30C to +260?
  Reply With Quote
Old June 21st, 2021, 03:25 PM   #14
skyfox
Lifetime Supporting Member
United States

skyfox is offline
 
skyfox's Avatar
 
Join Date: Nov 2006
Location: CA
Posts: 255
Quote:
Originally Posted by AustralIan View Post
You did not mention which 4-20mA analog Input Card you are using.
It is a CompactLogix 1769-IF8

RTD is Class 1. OMEGA PN# RTD-3-F3105-36-T

Quote:
I couldn't find an Omega PRCTL signal conditioner. Is that the part number of the probe itself?
You are correct. PRCTL was from a part number for a -200C to +850 C RTD which happens to report the temperature correctly with the range set to match and scaled accordingly in the PLC. However, this RTD cannot be used with this particular system.

SIGNAL CONDITIONER PN# OMEGA DRSL-RTD-LP


Quote:
The transmitter should be set up with as small a range as possible. Even though your rtd can sense down to -30 and up to 260, you should be able to set it to say 20C to 50C.
I have set it to a lower range than the min max ratings of the RTD but the signal conditioner is not reporting correctly. Puzzled as to why. I have tried three different signal conditions (same make/model) with the same results.


Quote:
Additionally, I would not take the temperature sensor built into a mass flow meter as gospel.
Actually there are three MFC's in the test set-up. They are calibrated every six months. They report the temperature within 0.1 Deg. C of one another.
  Reply With Quote
Old June 21st, 2021, 03:33 PM   #15
skyfox
Lifetime Supporting Member
United States

skyfox is offline
 
skyfox's Avatar
 
Join Date: Nov 2006
Location: CA
Posts: 255
Quote:
Originally Posted by cjd1965 View Post
Hi your correct that an RTD will give 100 ohms at 0C and they are pretty much universal

Your transmitter is set to give 4mA at -25C and 20mA at 250C therefore it wont transmit the correct 4-20mA for your desired range of -30C to 260C

Are you actually measuring from -30C to +260?
No I am not using the entire range. This RTD was specified by the customer as they run higher temperature gases thru this sub assembly. We manufacture and test this sub assembly for them but using N2 at ambient temps.

I have set the signal conditioner to -25 to +255. and the 4-20 mA signal from the conditioner is scaled in the PLC for this range and not the actual min max spec of the RTD. With this setting RTD should be reporting the correct temperature. I will try narrowing down the range say 10C to 50C and see if that would yield more desirable results.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
100 Ohm Platnum RTD at sub zero Adriel LIVE PLC Questions And Answers 4 January 26th, 2019 08:49 AM
Load cell scale using leverage grnick50 LIVE PLC Questions And Answers 9 January 23rd, 2015 07:38 AM
GSE 465 Scale issues muusic_man LIVE PLC Questions And Answers 4 October 22nd, 2013 11:13 AM
how to scale the 4 to 20mA signal jcp LIVE PLC Questions And Answers 18 May 16th, 2011 09:41 AM
315-2 DP to Scale on Profibus runx999 LIVE PLC Questions And Answers 4 January 5th, 2008 06:21 AM


All times are GMT -4. The time now is 11:00 AM.


.