E & H Temperature probe not reading correctly

Hi Guys,

I have two temp probes on one pipe . lets call it probe 1 and probe 2
the distance between probes is approx 3-4 meteres away but the closest temp probe called probe 1closest to the heat exchnager reads less that the the probe 2 down stream by 2 deg, Intially probe 1 was a pt100 and probe 2 is a thermocouple type t, thinking different sensing types might cause this,I changed probe 1 to a thermocouple type t. the scales of both is 0-250. The are from the same brand Endress and Hauser. Probe 2 was also changed and had a 5 point calibration. Theoretically probe 1 should read higher temp than probe 2 but its actually not the case> Any ideas on what this could be?
 
Hi Guys,

I have two temp probes on one pipe . lets call it probe 1 and probe 2
the distance between probes is approx 3-4 meteres away but the closest temp probe called probe 1closest to the heat exchnager reads less that the the probe 2 down stream by 2 deg, Intially probe 1 was a pt100 and probe 2 is a thermocouple type t, thinking different sensing types might cause this,I changed probe 1 to a thermocouple type t. the scales of both is 0-250. The are from the same brand Endress and Hauser. Probe 2 was also changed and had a 5 point calibration. Theoretically probe 1 should read higher temp than probe 2 but its actually not the case> Any ideas on what this could be?
Hi Yashaar,

Hum , I have read this post and my question is following:

-Is there any link with ( regarding ) the cold junction compensation??

-How do you read ( with wich support ) the so called probes temperatures?

Hope this can be an help,

Thank you for let us know your answers,

James
 
Yashaar,
You measurements suggest that you do not have uniform temperature profiles near the HEX with hot and cold regions, the sensor located a few meters away, provides a more representative measurement due to turbulent mixing. You may actually have a problem with the flow/heat distribution within the exchanger.
 
If you have flow and nominal fluid data, I'll run a measurment error calc for the well mixed flow condition to indicate what you're dealing with.
 
Yashaar,
You measurements suggest that you do not have uniform temperature profiles near the HEX with hot and cold regions, the sensor located a few meters away, provides a more representative measurement due to turbulent mixing. You may actually have a problem with the flow/heat distribution within the exchanger.
Could this be due to a block tube inside the tube bundle which would cause the turbulant flow
 
Hi Yashaar,

Hum , I have read this post and my question is following:

-Is there any link with ( regarding ) the cold junction compensation??

-How do you read ( with wich support ) the so called probes temperatures?

Hope this can be an help,

Thank you for let us know your answers,

James
The output signal is 4-20ma
With regards to the flow. In order to start the system. There need to a constant flow of 262 m3/hr. At low temperatures from 38c to 90c . The profile is how it should be but as temp increases to setpoint of 180 the error is +-2

With regards to the cold juntion compenation. Please can yoi elaborate more on the question ?
 
If you have flow and nominal fluid data, I'll run a measurment error calc for the well mixed flow condition to indicate what you're dealing with.
Hi Dave ,
Flow is 262 m3/hr coming into the hex.
Product is oil
Density 825
Specific heat capicity is 2.33
 
The output signal is 4-20ma
With regards to the flow. In order to start the system. There need to a constant flow of 262 m3/hr. At low temperatures from 38c to 90c . The profile is how it should be but as temp increases to setpoint of 180 the error is +-2

With regards to the cold juntion compenation. Please can yoi elaborate more on the question ?
Ok so to answer to your question about cold compensation i suggest you to have read on this article :
https://blog.beamex.com/thermocouple-cold-junction-compensation

Also you didnt say about how the temperatures are measured?

-Dcs?
-other Instrumentation device??

Some questions before going forward:

-When this problem started?

-Was the temperatures reading correct before this problem occur??

-Did you or somebody else changed/adjusted any setting(s) on DCS for the designed probe or circuit ??

I know that cold compensation is automatically calculated by DCS software.
You can have look on that maybe it can help for troubleshooting

Also do you know how RTD is connected to DCS or other device for reading ??
Are you sure about the RTD wiring and parameters all ok for the excitation circuit??

I would advise you to check about RTD configuration and see if you can troublesshoot /test it to get sure that it is properly connected and parameters properly setted in the Controls/Monitoring system.




It may also be a problem with the analog to digital converter ??? ( The device which getting the 4-20 mA to DCS can be source of problem sometimes but very unfrequent)

There could be a problem with the type of probe detected by Dcs ?
Check at DCS software if the assigned input for the RTD corresspond to PT100.

You can check proper fonctionnality of the assigned inputs by swapping with another "correct " reading probe....

Now this what i can say till we get more answer of these questions,

Hope this can be a help,
Do not hesitate to write us back for proper feedback,

James







-
 
Hi Dave ,
Flow is 262 m3/hr coming into the hex.
Product is oil
Density 825
Specific heat capicity is 2.33
With a bit of guess work regarding pipe size etc.
The measurment error is small, the time constant listed is the exponential time constant with a settling time for 2% error being 4X the time constant.

Most heat exchangers are characterized by heavy turbulence to improve heat transfer. With shell in tube exchangers you have baffles etc.
 

Attachments

1. Is this a correct summary?

At the present time the measuement system is:

A. Probe #1 is Type T Thermocouple with a temp transmitter sending a 4-20mA signal scaled 0-160 Deg C to a DCS.

B. Probe #2 is Type T Thermocouple with a temp transmitter sending a 4-20mA signal scaled 0-160 Deg C to a DCS.

The probes are in thermowells in the pipe, the probes are not mounted to the outer pipe surface.

Since the DCS is getting 4-20mA signals scaled 0-160 Deg C, the DCS configuration has not been changed throughout this episode. True?

Are the statements above correct ?

2. My questions

A. Are the probes wired directly into close coupled temperature transmitters so that thermocouple extension wire is not needed? (In all this probe changing the cabling connecting a thermocouple to the transmitter is NOT copper conductor cable, at either point?)

B. Have you personally seen the scaling for both the temperature transmitters AND on the DCS side ? Not just taken someone’s word for it?

I can’t tell you how many times I’ve encountered mismatched scales where a scale value like 0-160 got entered as 0-150 resulting in an interpretation error.

An error that increases with temperature could easily be due to mismatched/incorrect scales on either end.

3. Observation/opinion

I think you need to rule out measurement error before you conclude that it’s a process issue.

A. I would have used Pt100’s for measurements of this nature to avoid the inherent inaccuracy of thermocouples vs RTDs and to avoid cold junction error.

B. DCS AI’s tend to be reliable and stable. So they’re taken for granted. But for measurements of this nature where you’re looking for very close or nearly identical values, then checking the DCS AI with a certified calibrator would be warranted. Check the transmitters, too.

C. It would pay to check that there is no offset/bias in the AI function block on DCS side for either signal.

D. When you’re looking for matching temperatures or very small differences, even minor variations in the resistance of the 250 ohm dropping resistor at the DCS AI can make a difference. Sometimes there’s a mix of 249 ohm or 250 ohm resistors, or 1%, 0.5% or 0.1% tolerances. For your measurement, those 2 inputs need matching resistors.
 
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