NOW I KNOW MY K, J, Ts … NEXT TIME WON’T YOU SING WITH ME?
A B C D E F G
H I J K L M N O P
Q R S
T U V
W and
X Y and Z!
Now I know my ABCs
Next time won't you sing with me?
("The Alphabet Song," by Charles Bradlee, 1835)
HOW TO CREATE A LITTLE JOLT OF A VOLTAGE
The measuring junction of the thermocouple is on the right. The reference junction is on the left.
1.Take two specially made and selected different metal wires and join them at one end.
2. Make certain that the free ends of the wires are at the same reference temperature.
3. Expose the metal junction to a hot environment... for example a hot process stream. The heat from the process stream will conduct into the conjoined metals.
Voila! A small magnitude of volts ... aka "a millivoltage" ... is generated!
The process also works with colder environments.
4. The magnitude of the millivoltage that is generated can be determined with a voltmeter.
5. Because scientist-engineers of yore spent their lives working on it we know:
The millivolt output reading correlates to a temperature.
The relationship is so dependable that a chart can be made which cross references the millivolt output to a temperature.
Process Operators can infer the temperature on the chart is the same as the temperature of the stuff that is having its temperature measured.
5. Okay. There is actually one more step.
Thermocouples are used in many household appliances that must have temperature control, like an electric kitchen range. The thermocouple in this household water heater is being replaced.
All alert PTOA Readers and Students know the reference end of the wires is also impacted by conduction heat transfer; so the total millivoltage output must be corrected prior to inferring a measured temperature from a chart.
But heck!
Isn't thermocouple technology just a nifty electronic way to determine a process temperature?
CONJUNCTION JUNCTION
Great picture of an exposed junction.
The thermocouple end that is exposed to heat transfer can be referred to as "the junction", "the temperature-measuring end of the thermocouple," and more erroneously as "the hot end" because some thermocouples are actually sensing temperatures that are much colder than the reference temperature.
The straight lines show that the relationship between mVolts generated and Temperature is very linear!
And of course it goes without saying that the two metals selected for the thermocouple are intentionally paired for the ability of their junction to generate that small voltage and reliability correlate that voltage into a temperature because of ...
You know what I'm gonna say...
Ye Olde Linear Relationship between the voltage generated and correlation to temperature makes the thermocouple technology useful with respect to inferring a process temperature measurement!
THERMOCOUPLE CALIBRATION ALPHABET SOUP
Each type of thermocouple has been assigned an alphabet letter that refers to the two metals used in its calibration.
The wires of thermocouple calibrations are color coded.
Knowing the different thermocouple calibrations and why they were chosen for their process service is not going to make or break any Process Operator's ability to perform their job well.
However, if you the future Process Operator actually know your thermocouple calibrations and can talk intelligently to the Instrument Tech about the problem when one fails, you will earn some respect!
The most popular thermocouple calibration is Type K which is made from two alloys of nickel wire, nickel-chromium and nickel-alumel.
Evenly-spaced Type K thermocouples are adhered to the exhaust of this turbine.
Type K has been an all-purpose work horse because of its wide temperature range, accuracy, reliability, and is relatively inexpensive.
However, the popularity of Type N calibration is growing. Why?
Conductivity is an atomic-level physical property of Stuff/Matter/Mass that can be used to measure temperature.
PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned that in PTOA Segment #95 entitled "Temperature Measurement Must-Knows (Part 1)."
The Type N thermocouple (Nicrosil/Nisil ... shall we venture to guess an alloy made of nickel, chromium, and silicon?) has the same accuracy as Type K but can withstand the high temperature range better.
Why?
The wonky way to say it is that Type N is more stable and less prone to chemically interact with its environment (aka "oxidize") than Type K and has less "drift." Drift is evident when the voltage output changes when the temperature stays the same.
Type J calibration (Iron/Constantan) is used in many processing industries, albeit less frequently because frankly Type K has more advantages at the same price.
The graph at the top of the page clearly shows that the slope of Type J's mV versus temperature relationship is steeper than that of Type K or N. The steepness of the slope means the millivolt signal will be more robust.
However, the table above illustrates that Type J cannot measure as high of a temperature range as Type K nor Type N.
The Type J also has the tendency to oxidize above 1000 °F and can be permanently damaged should temperatures creep up to 1400 °F.
Also, the Type J calibration cannot be used in moist environments. That fact is not at all surprising if you remember one of the metal wires is made of iron.
Still, Type J is useful in process services that monitor the high temperatures of plastics manufacturing and special applications in vacuums.
The blue and green dots in the below graphic illustrate where thermocouple technology may be used in a fuels refinery.
Alert PTOA Readers and Students already understand how the crude flow enters the bottom left side of the drawing and is heated through exchangers and then a fired heater before being processed into intermediate and final products.
Type T calibration (Copper/Constantan) is used for low temperature ranges and to measure the extremely low temperatures needed in cryogenic processes.
PTOA Readers and Students were introduced to industrial application of cryogenics in THIS PTOA SEGMENT.
Type T thermocouple and voltmeter reading converted to temperature.
Type T calibrations are also used in the food industry because they can tolerate moisture without interacting with it.
Your Mentor has not worked with the Type E calibration (Nickel-Chromium/Constantan) thermocouple.
The steep slope of its line on the graph infers that Type E would have a stronger millivolt output which should infer greater accuracy than Type J or Type K in the 1000 deg F and lower temperature range.
The major concern of all calibrations mentioned above (K, N, J, T, and E) is whether or not they can sustain the high temperature of their ranges without degrading.
S Type thermocouple is needed in glass and ceramic manufacturing.
Some processing applications require continuously accurate temperature measurement at sustained high temperature while converting raw materials into desired final products.
The thermocouple calibrations shown on the graph with an asterisk are made of extremely expensive metals (e.g. $$$ platinum $$$) which can withstand very high temperatures without losing accuracy and lifespan.
Type S calibration (platinum- 10% rhodium/platinum) is used in biotechnology and pharmaceutical industries.
Type R calibration (platinum-13%rhodium/platinum) is extremely accurate and stable measuring lower temperatures.
Type B calibration (platinum-30%rhodium/platinum-6%rhodium) is used to measure sustained, extremely high temperatures with great accuracy.
The next PTOA Segment will include examples of correlating the voltage output of a thermocouple into a temperature measurement.
TAKE HOME MESSAGES: Using thermocouples to measure temperature works because of the very linear relationship between the millivolt output (generated when a junction of different metals is heated) and temperature.
PTOA Readers and Students gained more practice interpreting a graph that illustrated a real-world linear relationship.
The two wires that are joined to make a thermocouple junction are specifically paired for their ability to generate a millivoltage that linearly correlates to temperature.
The junction of the metals is the end that experiences heat transfer.
The other end of the wires are held at a reference temperature. The millivolt output of the reference junction needs to be considered when determining the temperature measurement.
The paired wires are also called the thermocouple "calibration."
Common process industry thermocouple calibrations are Type K, Type N, and Type J.
Type T is used for low temperatures and cryogenic applications.
Very expensive "Noble Metal" calibrations can endure high temperatures and are very accurate.
Noble metal calibrations are Types R, S, and B. In the event you end up working in an industry that uses these calibrations handle these thermocouples with tender loving care.
©2016 PTOA Segment 00107
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area
You need to login or register to bookmark/favorite this content.