It's so hard to believe but
It's all coming back to me
(It's all coming back, it's all coming back to me now)

("It's All Coming Back to Me Now," sappy ballad by J. Steinman & made famous by Celine Dion, 1996)

PTOA SEGMENT #107: NOW I KNOW MY K, J, Ts … NEXT TIME WON’T YOU SING WITH ME?

PTOA Segment #107 began by enumerating the steps that make it possible for the structure of a thermocouple to generate a standard signal in the form of millivoltage.

PTOA Readers and Students learned:

The temperature sensing junction of the thermocouple generates a millivoltage upon sensing heat being transferred into it or out of it ... which is of course caused by the media surrounding the thermocouple that is having its temperature detected and measured.

The reference junction of the thermocouple provides the basis from which the temperature change described above can be measured.

The thermocouple works because the millivoltage generated by the transfer of heat can be correlated to a temperature ... yet one more example of mankind exploiting a one-to-one linear relationship that can occur when certain metals are joined at two ends.

The remainder of PTOA Segment described the different calibrations (aka types) of thermocouples.

PTOA Readers and Students learned that each calibration is designated by an alphabet letter which is short hand code for the two metals and/or metal alloys that the thermocouple wires are fabricated from.

PTOA Readers and Students learned that the Type K thermocouple has been the most popular choice and over the years has replaced the popular Type J that are still in service ...

However ... technological advances are invented every day ...

... the popularity of the Type N thermocouple is increasing because it has less drift and does not react with the environment (aka oxidize) as much as Type K.

PTOA Readers and Students learned that the graph that compares "temperature-versus-mV generated" for all types of calibrations revealed that the Type E thermocouple has the steepest slope of all thermocouples ...

...which indicated the Type E thermocouple could generate the most accurate change in temperature over its more limited temperature range.

Type T thermocouples are used for ultra low temperature detecting and sensing; the fancy name for this service is "cryogenics."

 

PTOA Readers and Students learned that thermocouple technology will lose accuracy and lifespan when subjected to constant extremely high temperatures and for that reason very expensive Types S, R, and B (aka "noble metal thermocouples" ) were fabricated out of precious metals.

 

PTOA SEGMENT #108: SHOW ME HOW THIS THING WORKS

In PTOA Segment #108, PTOA Readers and Students learned exactly how the generated millivoltage output from a thermocouple is correlated to a temperature because they were challenged to perform the function!

With the help of a Reo Temp's Type J Calibration Table, PTOA Readers and Students expertly learned how to find the millivoltage output that would correlate to a given temperature.

PTOA Readers and Students were then reminded that the real world process is exactly opposite; a millivoltage output is generated from which a temperature measurement is inferred!

Using the plausibly real-world example of Type J thermocouples placed at the inlet and outlet of a reactor, PTOA Readers and Students determined that the reactor was experiencing an endothermic reaction.

The exercise of determining the inlet and outlet temperatures of the reactor also showed PTOA Readers and Students why and how the millivoltage output of the reference junction must be taken into account to infer an accurate thermocouple temperature measurement.

PTOA Readers and Students were then shown a photo that depicted how the thermocouple ... protected within a thermowell ... extends out of a reactor with a flanged connection to a device that automatically and continuously converts the millivolt signal output into a standard signal that is transmitted to the control room.

PTOA Segment #108 concluded with a reference to the different types of thermocouple probes since correctly matching the probe style to the temperature-measuring service is also required to achieve an accurate temperature measurement.

 

PTOA SEGMENT #109: INSTRUMENT TECH MUST-KNOWS: THERMOCOUPLE PROBE STYLES

PTOA Segment #109 explained that the wide range of thermocouple uses is due to the various styles of thermocouple probes.

The thermocouple probe was defined as the two wires that are joined together to measure temperature ... which are  inserted into a metal stainless steel or Inconel sheath.

"Instrument Techie" PTOA Readers and Students learned that:

• The two wires of the thermocouple only touch at the measuring junction.
• Mineral insulated cable and magnesium oxide powder keep the two wires in the sheath separated.
• The orientation of the measuring junction to the sheath defines the probe type.

"Instrument Techie" PTOA Readers and Students were probably not surprised to learn that Exposed Junction Probes directly contact the media that is having its temperature sensed ...

Thus without any protection, "Instrument Techie" PTOA Readers and Students easily understood why this probe style would work best determining the temperature of slow-moving, non corrosive gases ... like the temperature of air flowing through an air duct.

"Instrument Techie" PTOA Readers and Students learned that Grounded Junction Probes are physically attached to the sheath at the tip ...

Thus protected, Grounded Junction Probes can detect the temperature of more corrosive and higher pressure environments ... because they are not in direct contact with them!

Although Grounded Junction Probes could be fabricated to measure the temperature of rolling surfaces, "Instrument Techie" PTOA Readers and Students learned that the significant drawback to Grounded Junction Probes was their tendency to create ground fault error.

Although it sounds wrong to the ear, "Instrument Techie" PTOA Readers and Students learned that the Ungrounded Junction Probe is the thermocouple probe style that is not prone to ground fault error and easily interfaces with field instrumentation and microprocessor-based controls in DCS systems (which are electrically grounded).

Using their previously gained knowledge about the mechanics of heat transfer, "Instrument Techie" PTOA Readers and Students could visualize why:

• The Ungrounded Junction Probe had the best measurement accuracy yet had a greater measurement response lag time than that observed for the Grounded Junction Probe.
• The Exposed Junction Probe had the fastest measurement response time.
• Probe size and wire diameter impacted the rate of heat transfer through the probe and thus likewise impacted the measurement response lag of a thermocouple.

©2016 PTOA Segment 00127
PTOA Deja Vu Review 4-7

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