Hey!) Over, under, sideways, down,
(Hey!) I bounce a ball that's square and round.
(Hey!) Over, under, sideways, down,
(Hey!) I bounce a ball that's square and round.
When will it end? (When will it end?)
When will it end? (When will it end?)

("Over Under Sideways Down," by the Yardbirds, 1966)

PTOA SEGMENT #112: INSTRUMENT TECH MUST-KNOWS: THERMOCOUPLE FINAL ANALYSIS

By the time smart "Instrument Techie" PTOA Readers and Students who already knew they should read the PTOA Segments in the intended sequential order arrived at PTOA Segment #112, they had a fundamental understanding about:

• Thermocouple calibrations types.
• How a thermocouple converts a detected temperature into a millivolt output.
• The architecture and application of different thermocouple probe styles.
• Why thermocouple technology requires Cold Junction Compensation and dedicated Extension Wire to accurately detect and measure temperatures.
• Why thermocouples have a "range" of measurement accuracy instead of an exactly quantified amount for  "inherent measurement error."

 

After all this hard work and dedication focused on thermocouple technology ...

"Instrument Techie" PTOA Readers and Students were probably blown away to find out that  ...

due to their greater measurement accuracy and superior repeatability ...

 

RTD technology is the preferred temperature detection and measurement technology found in the process industries!

"Instrument Techie" PTOA Readers and Students learned that there are a few "niche" applications of thermocouple technology in the process industries, for example:

• When the temperature that needs to be detected/measured is greater than the 650 °C (1202 °F) high temperature range limit of RTDs because thermocouple technology can detect temperatures up to +2316 °C (4200 °F).
• When "point-source" temperature changes need to be monitored.
• When the temperature detecting/measuring environment has a significant vibration.

A video made by Agent JayZ of Jet City clarified the "point-source" application of thermocouples.

 

 

Agent JayZ showed how thermocouples can be arranged around a gas turbine exhaust for the purpose of detecting and measuring temperatures that reveal whether or not the turbine is operating as expected.

PTOA Segment #112 concluded with a thorough assessment that considered the benefits and limitations of thermocouple technology.

 

The instrument assessment process included a typical decision tree that might be used to determine which of Type E, Type J, or Type K thermocouple calibrations would be best for a temperature detecting/measuring application.

"Instrument Techie" PTOA Readers and Students learned that the decision process includes stepwise consideration of:

• The temperature range that needs to be measured.
• The linearity between millivolt output and correlated temperature within the process temperature range that needs to be measured.
• The working environment.

 

"Instrument Techie" PTOA Readers and Students also learned that thermocouple technology will not work satisfactorily in any application that has less than a 33 °C (59.4 °F) anticipated temperature measurement range.

That's because the output of thermocouple technology ... millivolts ... are too small to adequately measure temperature changes in a smaller temperature range.

 

PTOA SEGMENT #113: RESISTANCE … IS NOT FUTILE!

PTOA Readers and Students had learned in PTOA Segment #112 that  "RTD technology" was superior to thermocouple technology with respect to temperature measuring accuracy and repeatability.

The introduction to RTD technology began with revisiting Ohm's Law which had first been featured in the introduction to electrical temperature detecting/measuring instruments way back in PTOA Segment #106.

 

PTOA Readers and Students learned that electrical components called resistors are inserted into electrical circuits to perform the very useful function of controlling the flow of current (measure in amps) through the circuit,

PTOA Readers and Students were not surprised to learn that the Ohm's Law expression that was relevant to this focus on electrical Resistance was ...logically ...

the version that defined electrical Resistance:

R (in Ohms) = V/I (Volts/Amps)

PTOA Readers and Students learned that they could visualize electrical Resistance as the slope of a graph that plots the electrical current flowing through a circuit (I) on the X axis and the corresponding circuit Voltage (V) on the Y axis.

 

This graph revealed two important phenomena about electrical circuits:

• The straight line of the slope (aka circuit Resistance) was dependably steady ... meaning does not change ... at all values of Current (I).
• As electrical Resistance is increased in a circuit, electrical Current decreases.

The last observation listed above was also predicted by the Ohm's Law expression that defines Resistance and shows the inverse relationship between electrical Resistance (R) and electrical Current (I):

R (in Ohms) = V/I (Volts/Amps)

"How convenient" it was for Mankind to discover that the steady and predictable nature of electrical circuit Resistance could be correlated to temperature changes.

Yes Indeedo!

One more example of a naturally occurring linear relationship that can be incorporated into useful technology.

PTOA Readers and Students learned that a popular process industry instrument that correlates changes in temperature to changes in electrical Resistance is called a Resistance Temperature Detector (aka "RTD").

Extremely alert "Instrument Techie" PTOA Readers and Students were left pondering how electrical Resistance ... measured in the non-standard-signal unit of ohms ... could be converted into a standard signal format.

Righteeo!

Some kind of converting device would have to bridge the gap between the resistor's output of ohms and an easy-to-transmit standard signal type ... like millivolts or milliamps.

 

Extremely alert "Instrument Techie" PTOA Readers and Students had also realized that a power supply must be wired into each RTD so that it could generate an ohm output from a resistor ...

....and yet

the explanation for how this "excitation" power would not interfere with the temperature measurement was left as another cliffhanger.

And because every PTOA Reader and Student knows they must read the PTOA Segments in the intended sequential order to master the content, none of them were surprised to learn that the metals purposefully chosen to fabricate RTDs had an established linear relationship between Resistance Output and Temperature Changes.

 

©2016 PTOA Segment 00129
PTOA Deja Vu Review 4-9

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