Tell him, tell them, tell one and tell `em all.
Tell `em `bout the function at the junction.

("Function at the Junction," by Shorty Long, 1966)

Remember how the industrial uses of temperature measuring devices based on volume and distance expansion were darn limited and therefore rather specific?

Thermocouple technology has been adapted to a widely diverse range of temperature-measuring applications for several reasons; the biggest reason being the generation of an electrical output that can be amplified, converted to a standard signal, and then transmitted into the control room.

Another reason is the creation of man-made metal wire alloys that do a decent job measuring specific temperature ranges in their unique calibrations which were the focus of PTOA Segment #107.

 

In PTOA Segment #109, PTOA Readers and Students learned about three thermocouple probe styles that further expand the use of thermocouple technology to successfully infer a temperature measurement for:

• Home appliances like kitchen ranges and hot water heaters.
• 

  Three "rigid" thermcouple bundles inside their respective thermowells each measuring the top, mid, and bottom of the catalyst bed in the reactor.

  Monitoring the temperature of hot exhaust gas that exits gas turbines used in electrical power production.

• Monitoring the "temperature profile" of fixed catalyst beds using "bundled together" thermocouples in protective wells that are positioned to measure the catalyst bed temperature at the top, middle, and bottom.
• Monitoring the temperature of solid surface metals using thermocouples manufactured with flat and thin hot junctions which provide maximum contact with the solid surface.

The decision-tree related to determining the application of thermocouple technology versus using alternative technology (RTDs and thermistors) begins with considering:

• The temperature range that needs to be measured.
• The required accuracy and reliability of the temperature measurement.
• The stability and ruggedness required while measuring the process temperature in an environment that is corrosive, abrasive and/or vibrating and/or even rotating.
• Whether or not the temperature measuring environment can chemically react with the thermocouple metals by adding oxygen (that's called "an oxidizing atmosphere"), taking away oxygen (that's called "a reducing atmosphere").
• Whether or not the thermocouple will be exposed to moisture ... because electricity and water do not mix!
• Whether or not the installation of the thermocouple is new or must be retrofit to be compatible with existing metals and openings that already exist in equipment.

Yes indeedo, matching the right thermocouple calibration and structure to the process service job depends upon the expected functionality needed for the anticipated temperature range.

Regardless of the application or what it may look like, always remember and never forget that all thermocouples must have:

• A measuring junction (aka hot junction).
• A reference junction (aka cold junction) with a voltmeter.
• A means to "cold junction compensate" the reference junction.
• Thermocouple wire that is sufficiently long to reach the measuring instrument (aka the reference junction and voltmeter) otherwise the thermocouple wire must be correctly paired with extension wire.

Without all of the above, there will either be no function at the junction or there will be a worthless function at the junction.

 

THERMOCOUPLE MEASUREMENT ACCURACY 

Standard and Special Limits of Error

The below table illustrates the error of J, K, E, and T thermocouple  calibrations using standard thermocouple wire (aka the standard limits of error) compared to much more expensive, higher grade special thermocouple wire (aka the special limits of error).

The table states that the Type K thermocouple can measure temperatures from -200 °C to 1250 °C  (-328 to 2282 °F).

Furthermore ...

The table indicates that a Type K thermocouple made with standard thermocouple wire has a stated inherent instrument error of "2.2 °C or 0.75% ... whichever is greater." 

Please invest the time to find that statement of the Type K standard limits of error on the middle column, second row down.

The accuracy of the Type K calibration improves to a special limit of error stated as "1.1 °C or 0.4% ... whichever is greater" when the themocouple is made of higher grade, specialty wire.

Obviously, the temperature measurement error generated by the expensive, higher-grade wire is reduced compared to the error generated by standard thermocouple wire.

But what is meant by the mysterious statement "whichever is greater?"

The mysterious phrase means that the thermocouple calibration establishes the possible temperature-measuring range of the device but the actual high temperature of the range will be determined by the diameter of the wire.

 

Thermocouple Wire Diameter

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned in PTOA Segment #109 that a fast response time will result when the diameter of the probe is small.

A small-diameter thermocouple probe naturally protects two small-diameter themocouple wires.

Pretend the thermocouple wires of the thermocouple shown to the left are made of  0.040 diameter wire.

Pretend the thermocouple starts its day just sitting around at a  room temperature of 70 °F.

Then the measuring junction (aka hot junction) of the thermcouple is  immersed in a hot oil bath that is held at a constant 170 °F with the help of an immersion heater as shown in the picture to the left.

 

The 100 degree difference between the room temperature (70 °F) and the temperature of the hot oil (170 °F) can be  plotted on the y-axis of a graph in ten degree increments (0, 10, 20, 30, ...).

The time in seconds that it takes for the thermocouple to measure an increase of 100 degrees can be plotted on the x axis.

Then the experiment can be repeated with same-calibration thermocouples but with larger and larger diameters, like 0.125 and 0.250.

The result is plotted on the graph shown to the left.

The dotted line helps to focus on the time that it takes for each of the thermocouples to indicate an increase in 62 degrees.

After comparing the time it takes for each thermocouple to measure an increase in 62 degrees, the below conclusions are evident:

The .040 diameter wire on the left takes 1.75 seconds to measure an increase of 62 degrees.

The larger, 0.125 diameter wire in the middle takes 6 seconds to measure an increase of 62 degrees.

The largest, 0.250 diameter wire on the right takes a whopping 24 seconds to measure an increase of 62 degrees!

Proof positive that the following conclusion is accurate:

In a comparison between same-calibration, different-diameter thermocouple hot junctions, the thermocouple with the smallest diameter will measure a change in temperature faster. 

Hey, that's great. All thermocouples should be made with 0.040 diameter wire... Righteeo?

Heck, No!!!

The processing environment more often than not will require a more sturdy, thick wire that can withstand the impact of vibration, corrosion, even chemically reacting with the environment (meaning being oxidized or reduced).

 

Futhermore there is a price to pay for the increased accuracy:

The smaller the diameter of a thermocouple wire, the less capable the thermocouple is at withstanding and measuring the highest temperatures of its calibration range.

So ...

Prior to determining the mathematical accuracy of any type of thermocouple calibration, the following must be known:

• What is the calibration of the thermocouple?
• Is the thermocouple made of standard or specialty wire?
• What is the diameter of the wire?

The accuracy of the millivoltage measuring device also impacts the overall accuracy of the temperature measurement made with thermocouple technology.

Even though it is not possible to calculate an accuracy without more information, it is possible to make a general conclusion about thermocouple accuracy when compared to the other available electronic temperature measuring devices:

Compared to the RTD and thermistor, the  thermocouple is the least accurate temperature measuring device.

 

THERMOCOUPLE MEASUREMENT REPEATABILITY

PTOA Readers and Students learned how to distinguishing measurement accuracy from measurement repeatability in PTOA Segment # 99 entitled "Instrument Tech Must-Knows: Measurement Accuracy."

Unfortunately, the repeatability of thermocouples is not as good as the repeatability of RTDs and thermistors for reasons that will be explained in future PTOA Segments.

 

THERMOCOUPLE MEASUREMENT RESPONSE LAG

Wire Diameter and Response Lag

PTOA Readers and Students already devoted precious time in this very PTOA Segment to learn what the PTOA Department of Redundancy Department repeats below:

In a comparison between same-calibration, different-diameter thermocouple hot junctions, the thermocouple with the smallest diameter will measure a change in temperature faster. 

 

Probe Styles and Response Lag

After reading PTOA Segment #109, PTOA Readers and Students completely understand why the thermocouple probe style also impacts response time.

Once again, The PTOA Department of Redundancy Department lists the three thermocouple probe styles in order of fastest to slowest response time:

• Exposed junction probe.
• Grounded junction probe.
• Ungrounded junction probe.

At least in theory the grounded junction probe has a faster response time than the ungrounded junction probe.

The response time of the ungrounded junction probe will be faster than that of a same-size same-calibration grounded junction probe if the hot junction of the grounded thermocouple has formed at a different place than at the very tip of the probe.

TAKE HOME MESSAGES: The wide versatility of thermocouple technology is due to:

• The electrical output that is generated, amplified, converted to standard signal, and transmitted to the control room.
• The capability to manufacture metal alloys that make thermocouple calibrations which linearly relate to inferred temperatures.
• The ability to protect thermocouples with probes without too much sacrifice in  accuracy, reliability, and measurement lag.

The accuracy of a thermocouple is determined by

• The calibration of the thermocouple.
• The use of standard or special limits wire.
• The diameter of the thermocouple wire.

Smaller diameter wire has the fastest temperature-measuring response time (aka shortest response lag) but sacrifices accuracy measuring the highest temperatures of the calibration range.

Temperature measurements inferred by thermocouple technology are not as accurate nor as reliable as those made with  RTD or thermistor technology.

However, thermocouples can be manufactured to be more rugged and stable. And as PTOA Readers and Students will soon learn ...

Thermocouple technology can measure much higher temperature ranges than ether RTD technology or thermistor technology .

©2016 PTOA Segment 00111
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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