And I've seen it before
And I'll see it again
Yes I've seen it before
Just little bits of history repeating.

("History Repeating," by A. Gifford performed by the Propellerheads featuring Shirley Bassey, 1997)

PTOA SEGMENT #114: RTD – TWO

A nuclear reactor core was chosen as the lead picture for this second focus on RTD technology because ...

as PTOA Readers and Students learned ...

one crucial application of RTD technology is in the electrical power production industry ...

where RTDs are used to obtain repeatable, accurate measurements of the nuclear reactor's coolant temperature.

Yeah! We all want THAT to be able to happen!

 

PTOA Readers and Students learned that the supreme accuracy of platinum RTDs is also used to calibrate other temperature measuring devices that measure in the -259 to 631 °C temperature range (aka -434 °F to 1167.8 °F).

 

PTOA Readers and Students learned that the circuitry which translates the electrical resistance output of a platinum RTD into a standard signal of voltage is called a "Wheatstone Bridge."

 

 

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order could then easily understand why the supreme accuracy of RTD technology depends upon not just one ... but TWO ... linear relationships:

• The linear relationship between the detected temperature and the ohm output of the platinum RTD wire (illustrated in the graph that is above and to the right).
• The linear relationship between the measured temperature and the millivolt output (aka "potential") of the Wheatstone Bridge (shown below).

 

Next, PTOA Segment #114 listed the physical similarities between RTDs and thermocouples:

 

• Both are electrical temperature detecting/measuring devices that can be installed to generate a standard signal output which will be transmitted to "a remote location" ... meaning farther away ... like a control room.
• The measurement accuracy of both technologies is dependent upon the linear relationship between the temperature being detected/measured and both instruments'  respective electrical signal output.
• Looking at an RTD or thermocouple from the outside, their housings appear similar.

Then PTOA Segment #114 listed differences between RTD and thermocouple technologies:

 

• The RTD does not have a hot junction made by joining two dissimilar metals; the RTD is a single, helically wound wire.
• The RTD's fine, pure platinum wire is never exposed to the media having its temperature measured; RTDs are always enclosed in a protective sheath and thermowell.
• All RTDs are fabricated from pure metals, not metal alloys as some thermocouples are.
• To generate an accurate temperature measurement, thermocouples must incorporate CJC and dedicated Extension Wires whereas platinum wire RTDs must have a Wheatstone Bridge for signal conversion and a small DC excitation circuit.
• Although the temperature measurement of an RTD is more accurate, stable, and repeatable than that for a thermocouple, the measurement response time for a thermocouple is faster.

 

PTOA SEGMENT #115: INSTRUMENT TECH MUST-KNOWS: RTD BRIDGE CIRCUITS

"Instrument Techie" PTOA Readers and Students must possess a fundamental understanding of Wheatstone Bridge circuit function because it is used in both temperature and pressure detecting/measuring instruments.

In PTOA Segment #115, "Instrument Techie" PTOA Readers and Students learned:

• Wheatstone Bridge architecture features four "legs of the bridge" that make a diamond shape.
• Three of the "legs" have "fixed resistors" with known resistance magnitude and the fourth leg has a resistor with unknown and variable resistance.
• The "balanced state" of the Wheatstone Bridge refers to the state wherein the resistance of the unknown leg matches the magnitude of the three fixed resistors.
• The RTD wires ... actually, the RTD's lead wires ... tie into the bridge architecture as the unknown, and variable resistor of the RTD.
• The greater the detected temperature increases above the reference temperature defined by the "balanced state," the greater the potential difference (aka voltage) across the Wheatstone Bridge.

"Instrument Techie" PTOA Readers and Students learned that there are two sources of RTD measurement error that will generate electrical resistances that are not related to the detected industrial process temperature change.

• The DC excitation current.
• The copper "Lead-Wires" that connect the platinum wires of the RTD to the Wheatstone Bridge.

"Instrument Techie" PTOA Readers and Students learned that the DC excitation circuit must be limited to the minimum needed and that "Lead-Wire Error" can be significantly reduced by using a multiple lead wire architecture.

PTOA Segment #115 showed how the observed 5.9% measurement error caused by lead-wire error in a 2-lead wire RTD could be reduced to 0.1 °C for a 3-lead wire RTD and to a mere 0.03 °C for a 4-lead wire RTD.

 

PTOA SEGMENT #116: ALIKE … THEN AGAIN … TOTALLY DIFFERENT

This intro to thermistor technology began by comparing and contrasting thermistors to RTDs.

PTOA Readers and Students learned that the list of similarities between the two technologies pretty much stops at both having a predictable relationship between temperature and an output in the form of resistance (aka ohms).

PTOA Readers and Students learned that "NTC thermistors" have an inverse relationship with temperature which means their resistance output decreases as the temperature being detected and measured increases.

 

The NTC thermistor's resistance output-to-temperature relationship is illustrated (in the graph to the above right) as a sharply downward plunging line as the temperature increases.

Astute PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order immediately realized that the steep slope of the NTC thermistor graph meant that ...

in the real world ...

the resistance output signal from a thermistor was much more robust than the resistance output of an RTD.

Those same, smart PTOA Readers and Students probably also realized that the more robust, sensitive output signal would eliminate the need for a Wheatstone Bridge to convert the non-standard signal  output of ohms into a functionally useful output.

PTOA Readers and Students also learned that thermistors can be manufactured to have a direct relationship with temperature (as thermocouples and RTDs do) and these are called "PTC thermistors."

 

 

The differences between thermistor and RTD technologies made quite a longer list in PTOA Segment #116 and included:

• Thermistors are semiconductors which are human-made metal oxide blends; RTDs are made of pure metals mined from the earth.
• Thermistors are dinky and can be molded into a variety of shapes that correspond to their wide variety of temperature monitoring and indicating uses; RTDs have a single consistent architecture that was detailed in PTOA Segment #114 which support their function of detecting and measuring industrial process temperatures.
• Thermistors are used in everyday consumer electronics applications requiring temperature monitoring and indicating but are not  used in analog automatic temperature control loops; RTDs are used for industrial process temperature detecting and measuring and can be used as the temperature sensor in a temperature control loop.

In summary, PTOA Readers and Students learned that they are blissfully unaware of being surrounded by useful thermistor technology used in everyday, common consumer electronic applications.

PTOA Readers and Students learned that there are a few crucial thermistor applications in the process industries which include:

 

 

• PTC thermistor technology can protect motor windings against excessively high temperatures.
• PTC thermistor technology can protect electrical circuits against excessively high currents.
• Preventing food borne illnesses in the Food Manufacturing Industries.
• Thermocouple technology CJC.

©2016 PTOA Segment 00130
PTOA Deja Vu Review 4-10

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