It's the final countdown
The final countdown
The final countdown

("The Final Countdown," by J. Tempest of Europe, 1986)

 

THERMOCOUPLES MEASURE HOT HOT HOT TEMPERATURES

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have been constantly reminded of the versatility of thermocouple uses in the processing industries.

Therefore it is probably surprising to learn that ... whenever and wherever possible ... RTD technology will be used instead of thermocouple technology in the processing plant!

However,

Thermocouple technology must be used for the continuous measurement of extremely hot process temperatures above the range that RTDs can successfully operate within.

 

The graph to the left  compares the temperature-measuring ranges of thermocouples (the red line) to the temperature-measuring range of RTDs (the blue line).

The label on the Y axis is labelled "relative output" because:

• The output from thermocouples is in millivolts.
• The output from RTDs is in units of electrical resistance called ohms.

So "relative output" means "millivolts for thermocouples" and "ohms for RTDs."

PTOA Readers and Students will learn all about RTD technology in upcoming PTOA Segments so don't fret about the details now.

Just remember ...

RTD temperature-measuring technology is limited to measuring temperatures below 650 °C (1202 °F).

Yet thermocouples ... meaning one type of specialty calibration or another .. can measure temperatures all the way up to  +2316 °C which is  4200 °F.

 

MORE BENEFITS OF THERMOCOUPLE TECHNOLOGY

Point-Source Temperature Measurement

"Point-source temperature measurement" means being able to distinguish the temperature of one specific point from a different area in the vicinity.

One great industrial use of using thermocouple technology for point-source temperature measurement is explained in a You-Tube video that was produced by Agent JayZ.

Most of the information in Agent JayZ's video entitled "Power Turbines and Thermocouples" will make more sense after experiencing the future PTOA Focus Study Area on Rotating Equipment.

For now just access the below link and let it play in the background until 8 minutes and 30 seconds have passed.

At that point in the video, Agent JayZ describes the purpose of strategically placing multiple thermocouples at specific points in a circle that will measure the temperature of the exhaust gas that exits a gas turbine (aka "GT").

AGENT JAYZ EXPLAINS THERMOCOUPLES AND GTS

Thank you Agent JayZ for illustrating how thermocouple technology point-source temperature measurement can be used to troubleshoot fuel delivery problems that can occur while the GT is operating.

As Agent JayZ explains, using thermocouples to measure temperatures at specific points around the gas exhaust helps Control Board Operators avoid catastrophic GT failure.

 

Thermocouples are more rugged than RTDs

PTOA Readers and Students will soon understand why RTD technology cannot tolerate the vibration that thermocouples can tolerate.

For example, an  RTD would not function for long measuring a GT's exhaust gas temperature!

 

Thermocouples are Simple .... Sorta

PTOA Readers and Students already understand that the thermocouple technology is simple and easy to understand ...

IF ...

the definition of "simple" acknowledges the need to provide:

• Cold Junction Compensation.
• The correctly matched thermocouple wire-extension wire combination.

PTOA Readers and Students learned all about CJC and extension wires in PTOA Segment #110.

The installation cost of RTDs has historically been higher than that for thermocouples.

Yet RTDs are replacing thermocouples where the process service allows the upgrade simply due to the greater accuracy and repeatability in temperature measurement.

Afterall ...

The purpose of installing a means of temperature measurement in the first place is to have confidence that the temperature being measured is accurate and repeatable!

Given the choice between RTD and thermocouple technology with all other factors being equal, thermocouple technology will be used only in applications that can sacrifice accuracy in temperature measurement.

 

Thermocouples have shorter measurement response lag time.

The greater accuracy and repeatability of RTD technology comes at the sacrifice of response time compared to that achieved with thermocouple technology.

The measurement response time for an RTD is good, but the measurement response time for thermocouples is faster.

 

SUMMARY OF THERMOCOUPLE TECHNOLOGY BENEFITS

• High Temperature Range Measurement.
• Point-Temperature Sensing.
• Can be manufactured to be relatively rugged.
• Relatively simple technology (yet still must supply CRC and match extension wire to thermcouple wire).
• Faster Measurement-Response Lag Time.

 

Decisions, Decisions

Selecting the optimal calibrations for the process service requires 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.

So the decision tree to use thermocouple technology can come down to something like this:

Type K, J, or E?

• Type K will outlast Type J (iron-constantan) because iron oxidizes ... just like an iron nail left outside will rust.
• Type K also has a more linear relationship between voltage and temperature than Type E.
• However ... as the graph to the above right shows ...  both Type E and Type J have a more steeply sloped line than the line for Type K or other calibrations. In the real world that translates into meaning that Type E and Type J thermocouples will give a more distinct millivoltage output for each temperature ... and in some temperature range applications that ability can be more important than having the capability to measure a wider temperature range.

When in doubt, go with Type K, the most popular calibration:    

• Type K has one of the widest temperature ranges of all calibrations; Type K can measure temperatures up to 2300 °F (1260  °C), depending upon wire size.
• Type K is reliable and accurate except at the very highest ranges of its calibration.
• Type K is made of nickel so has good corrosion resistance.

Of course the working environment that the thermocouple is exposed to must be well known in order to select the correct thermocouple design.

The hot junction must be protected in a grounded or ungrounded junction probe if the thermocouple will be exposed to a "reducing atmosphere" (aka an atmosphere of hydrogen, or when carbon monoxide or hydrogen sulfide are present with oxygen).

The hot junction must also be protected from high pressure and high flow rates.

And don't forget about Type N!

• Type N is also made of a nickel based alloy that can attain 2300 °F (1260 °C) as does Type K, but Type N also has better resistance to oxidation at high temps and does not oxidize when around sulfur.
• Temperature measurements by Type N will also drift less than those measured with Type K.

 

SUMMARY OF THERMOCOUPLE TECHNOLOGY LIMITATIONS

• PTOA Segment #111 mentioned the inferior accuracy of thermocouple technology compared to RTDs.
• PTOA Segment #111 also mentioned that thermocouple technology is not as repeatable as RTD technology.
• PTOA Segment #109 mentioned the problems grounded-junction thermocouples can have with respect to ground loop error and noise pickup. The junction could even make an alloy with the sheath material!
• Because millivolts are a small output, thermocouples don't measure small temperature changes too well. There must be at least a 33 °C (59.4 °F) anticipated temperature measurement range for thermocouple technology to work.
• Thermocouple alloy wire must be correctly paired with extension wire ... otherwise the temperature measurement output will be gibberish.

This PTOA Segment #112 concludes the PTOA focus on thermocouple technology for measuring processing temperatures. YEAH!

The next PTOA Segment #113 begins the focus on Resistance-Temperature Devices (RTDs).

TAKE HOME MESSAGES: Process temperature measurements that prioritize measurement accuracy and repeatability will not use thermcouples but rather RTDs.

Thermocouples are the better choice temperature-measuring technology for:

• High temperature measurements above 650 °C (1202 °F).
• Point-Source temperature measurement.

 

©2016 PTOA Segment 00112
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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