Connection, I just can't make no connection.
But all I want to do is to get back to you.

("Connection," by M.Jagger & K. Richards of the Rolling Stones, 1967)

PTOA SEGMENT #110: INSTRUMENT TECH MUST-KNOWS: COLD JUNCTION COMPENSATION & EXTENSION WIRES

"Instrument Techie" PTOA Readers and Students that are reading the PTOA Segments in the intended sequential order could easily visualize the great advantage of electrical temperature-measuring technology ... like thermocouples ... which can transmit a temperature measurement in the form of a standard electrical signal from the pumps and pipes in the processing plant area into a control room.

And yet ... like everything in life ... the advantages of electrical measuring devices are partially offset by the potential sources of electrical interference and distortion that must be identified and minimized to yield an accurate process variable measurement.

PTOA Segment #110 focused on how the installation of thermocouple technology must include the following to minimize erroneous emfs:

• Cold Junction Compensation circuitry.
• Dedicated Extension Wires that are specifically matched to each type of thermocouple calibration.

In PTOA Segment #108, PTOA Readers and Students used a calibration table and "did the work" that Type J thermocouples would do while detecting and measuring reactor temperatures.

During that exercise, PTOA Readers and Students learned how the reference junction temperature needed to be taken into account to generate an accurate thermocouple millivoltage that could be correlated to a temperature that human beings understand.

In this PTOA Segment #110, "Instrument Techie" PTOA Readers and Students were reminded that "there ain't no-way, no-how" to prevent the ambient air conditions that surround the reference junction from impacting the two wires that are joined at the junction ...

hence, the need for Cold Junction Compensation.

The exact circuitry that is used to perform the function of Cold Junction Compensation was beyond the scope of the PTOA to delve into ...

But guess what .... don't stress about it!

Most real Instrument Technicians don't actually have to understand the CJC circuitry either ...

Ergo ...

"Instrument Techie" PTOA Readers and Students will be sufficiently impressive with just knowing that CJC is required to generate a thermocouple millivolt output that is not severely impacted by undesirable emfs caused by variances in the ambient conditions that surround the instrument.

Likewise in this PTOA Segment #110 ...

"Instrument Techie" PTOA Readers and Students applied what they already knew about heat transfer to predict that an undesirable emf would also be generated at the interface of thermocouple wires and their corresponding Extension Wires that are connected to the instrument's reference junction ...

Because by now all "Instrument Techie" PTOA Readers and Students can write a term paper explaining why an emf is generated ANY TIME the junction of two dissimilar metals ... like the interface of thermocouple wires and Extension wires ... experiences a temperature change.

"Instrument Techie" PTOA Readers and Students would conclude in their research papers that dedicated, specific Extension Wires for each type of thermocouple calibration must be used to minimize the emf that would otherwise be generated if just any ole wire lying around were used instead.

Lastly, "Instrument Techie" PTOA Readers and Students were warned that as yet no international committee has convened to decree that just one color coded chart of extension wires shall be universally relevant.

The lack of standardization for extension wires in this day and age of international business and cooperation means that all "Instrument Techie" PTOA Readers and Students must take heed and verify that the thermocouple calibration being installed has been correctly matched to the extension wire used in the instrument.

 

PTOA SEGMENT #111: INSTRUMENT TECH MUST-KNOWS: STUFF THAT IMPACTS THERMOCOUPLE MEASUREMENT ACCURACY

 

In this PTOA Segment #111, "Instrument Techie" PTOA Readers and Students focused on the decision tree used for comparing thermocouple technology to the two other technologies that electrically detect and measure temperature ... the RTD and thermistor.

"Instrument Techie" PTOA Readers and Students were probably not surprised to learn that the sources of "inherent" instrument measurement error were a significant factor in the comparison process.

What was probably surprising to "Instrument Techie" PTOA Readers and Students was learning that thermocouples have a range of "inherent" instrument measurement error that depends upon:

• The calibration ... or type ... of the thermocouple.
• Whether or not the thermocouple is made from standard grade metals or specialty-grade metals which are more expensive but significantly reduce the range of measurement error.
• The diameter of the thermocouple wire because the ability to detect the highest range of temperatures is sacrificed as the diameter of the thermocouple wire decreases.

Otherwise stated ...

"Instrument Techie" PTOA Readers and Students learned that ...

unlike any temperature-measuring technology presented up to this point ...

... the "instrument-related" measurement error of thermocouple technology is stated as a range of scale ... not a specific amount of scale ...and thus thermocouple technology is not as accurate as the RTD or thermistor temperature-measuring technologies.

"Instrument Techie" PTOA Readers probably wondered why the uncertainty related to instrument measurement error wasn't eliminated by simply manufacturing all thermocouple wire to a standard diameter.

But then "Instrument Techie" PTOA Readers and Students learned that the thinner the diameter of the thermocouple wire, the faster the measurement response time (aka lower measurement response lag).

Furthermore ...

"Instrument Techie" PTOA Readers and Students observed the data table from an experiment that clearly showed the wide variance in measurement response time that occurred as the diameter of the thermocouple wire was enlarged.

"Instrument Techie" PTOA Readers and Students observed test results that showed ...

A 0.250 diameter thermocouple wire can take 22+ more seconds to detect and measure a temperature change compared to a much thinner thermocouple wire that has a 0.040 diameter.

In a dynamic temperature-measuring application where up-to-date temperature-measurement accuracy is required 22+ seconds may just be too long of a time interval to be helpful and not misleading.

In summary, "Instrument Techie" PTOA Readers and Students learned that selecting thermocouple technology to detect and measure temperature requires optimizing the temperature range that needs to be measured with the accuracy and response time needed for the measurement.

"Instrument Techie" PTOA Readers and Students were also reminded that the different probe styles of thermocouples add to the measurement response lag of the instrument.

Additional factors "Instrument Techie" PTOA Readers and Students must consider when deciding between thermocouple, RTD, and thermistor technology include:

• The instrument's reliability and stability.
• The instrument's ruggedness ... like the ability to withstand vibration
• Whether or not the technology will chemically interact with the media having its temperature measured (aka "be oxidized" or "be corroded").
• Whether or not the environment surrounding the temperature-measuring device is moist.
• Whether or not the temperature-measuring instrument is a new installation or otherwise must be retrofit into an existing housing or other environment.

 

©2016 PTOA Segment 00128
PTOA Deja Vu Review 4-8

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