You gotta know it
(It's electric ...Boogie woogie, woogie!)
Now you can't hold it
(It's electric ...Boogie woogie, woogie!)
But you know it's there,
Yeah here there everywhere.

("Electric Boogie" aka "Electric Slide," sung Marcia Griffiths/written by Bunny Wailer, 1982)

Process variable measurements made by electrical instruments are fast but can be impacted by sources of electrical noise and interference.

Eliminating the random generation of emf is necessary when using thermocouple technology to infer a process temperature measurement.

Cold junction compensation and specifically-made extension wire are two technologies used to reduce measuring error when generating an inferred process temperature via thermocouple technology.

 

THE REFERENCE JUNCTION AND VOLTMETER
ARE THE "MEASURING INSTRUMENT" 

In PTOA Segment #107, PTOA Readers and Students learned that a thermocouple has both a temperature measuring junction tip (aka hot junction) and a reference temperature junction (aka cold junction).

In the above graphic, the big black dot on the left hand side is the measuring junction (aka hot junction); the reference junction (aka cold junction) is enclosed in the measuring instrument shown on the right hand side.

The "measuring instrument" is composed of:

• The reference junction (aka cold junction).
• A voltage measuring device.

CAUTION! DO NOT CONFUSE THE TERM "MEASURING INSTRUMENT" WITH "MEASURING JUNCTION OF TWO THERMOCOUPLE WIRES" BECAUSE THAT WOULD BE REAL EASY TO DO!

In the real processing world, the voltage output from the measuring instrument is:

• Amplified ... because millivolts are too weak to transmit as a  standard signal ...
• Converted to a standard 4-20 mA or digital signal ...
• Transmitted into the control room or directly into a nearby DCS component or a device like the Honeywell™ micro-loop temperature controller shown in the below picture.

No matter how modern and uptown the above temperature measuring system looks, always remember and never forget that all thermocouples work just like PTOA Readers and Students recently learned in PTOA Segment #108.

In PTOA Segment #108, PTOA Readers and Students used J thermocouple calibration tables to manually correlate the millivolt outputs from the hot and cold junctions (aka measuring and reference junctions) to a temperature.

The exercise of manually determining an inferred measured temperature makes it easier to understand the below statement:

When the reference junction temperature is impacted by the ambient temperature of its surroundings, the millivolt output read by the voltmeter will be incorrect. Hence the temperature correlated to the millivoltage reading will also be incorrect.

Oh no. Fred is confused again.

Fred just needs to remember that the thermocouple assembly's measuring instrument (the reference junction and voltmeter) are enclosed in some kind of  protective case.

The ambient environment surrounding the case can conduct and convect heat to and from the reference junction.

An emf (millivoltage output) will be generated at the union of the two reference junction wires (aka the cold junction) because ...

as all PTOA Readers and Students already know...

... a millivoltage output is generated whenever the junction of two dissimilar wires are exposed to a temperature change.

Fred is happy again. Yeah!

But now he wants to know how to keep the reference junction temperature (aka cold junction temperature) from straying away from the intended reference temperature.

 

ONE WAY TO CREATE A REFERENCE TEMPERATURE OF 32 °F (0 °C)

One cumbersome way to create a reference junction temperature guaranteed to stay at 32 °F (0 °C) would be to dunk the reference/cold junction well below the surface of an ice bath as shown to the right.

The literally ice-cold reference junction is surrounded by ice water.

As long as ice and water keep being supplied to the container, this cold junction is going to remain steady at 32 °F (0 °C).

 

LET'S GET REAL!

How many PTOA Readers and Students think it is feasible to maintain an ice bath out in the processing units to create a constantly reliable cold junction/reference junction temperature?

In the modern USA processing world, the reference junction temperature is maintained using the magic of electronic circuitry and/or software. An expert understanding of the electronics is beyond the scope of the PTOA to delve into.

Frankly, this PTOA Segment is already wonky enough!

 

Cold Junction Temperature Compensation (aka Reference Junction Temperature Compensation)

The below schematic shows an Iron-Constantan thermocouple.

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order can identify the calibration of this thermocouple because they learned all about it in PTOA Segment #107.

The top picture shows a reference junction (cold junction) maintained with a cumbersome, impractical ice bath.

The lower picture replaces the reference junction ice bath with an  isothermal connecting block which is represented as a rectangle.

The jagged line in a circle represents a thermistor in service as part of the cold junction compensation circuitry.

If the cold junction compensation were removed, an increase in ambient temperature within the instrument case would cause an increase in the reference junction (aka cold junction) temperature and an undesirable emf would be generated.

The difference in voltage sensed between the measuring and reference junctions would thus decrease.

Ergo, the temperature measurement correlated to the millivoltage output detected at the voltmeter would be erroneously low.

Today's modern digital systems do not use hardware electronics for cold junction compensation.

In digital systems, the output from the compensating resistor is digitized into a corresponding voltage that is either added or subtracted directly from the hot junction emf to generate the final emf that is correlated to a temperature measurement.

 

Extension Wires

Unlike most schematics show ...

In the real world the reference junction (aka cold junction) is frequently located away from the rigid thermocouple probe.

As shown in the below graphic, extension wire is used to connect the measuring instrument (aka reference/cold junction and voltmeter ... on the right side) to the thermocouple wire (on the left side).

The interface of the thermocouple wire and extension wire takes place at the logically named connector which is located inside the logically named connection head of the thermocouple assembly.

Factually speaking, the connector itself is also a cold junction because any wire connection made away from the hot junction is yet another cold junction.  But, I digress....

Who amongst us slogging though this PTOA Segment doesn't understand by now that any junction of dissimilar metals will create an emf when exposed to a change in temperature ... and that emf can impact the millivolts read by the voltmeter?

So it's easy to see that the connections in the isothermal block create two more emfs because the thermocouple wire is on one side of the block and the extension wire is on the other.

If you flip the above schematic around so that the measuring junction is on the right side instead of the left, the real world rigid thermocouple (protected in a thermowell) would look like the graphic to the below right.

The graphic shows Oil/Water inside of a container.

The thermocouple (protected by a thermowell) is measuring the temperature of the oil/water.

The thermocouple and well extend outside of the wall of the container.

The connection head is on the left side of the drawing and contains the connector. Extension wires will connect the connector to the measuring instrument.

Extension wires are specifically manufactured and matched to work with specific thermocouple calibrations.

Using the correct thermocouple calibration-extension wire combo makes the emf generated at the connection interfaces insignificant with respect to the final millivoltage measured at the voltmeter and offsets

Unfortunately, the color coding for extension wire has not been standardized throughout the world; what works well in the USA would create an erroneous emf in the UK, etc.

 

TAKE HOME MESSAGES: All electrical measuring devices used to measure process variables can be impacted by electrical noise and unwanted sources of emf.

When thermocouple technology is used to measure a process temperature, undesirable emfs can be generated:

• At the measuring junction/cold junction because it is exposed to conduction and convection heat transfer from its surroundings.
• At the interface of connections made between thermocouple wire and extension wire.

Cold Junction Compensation circuitry/software is used to offset ambient temperature changes that impact the reference junction (aka cold junction).

Specifically made Extension Wire is matched to each type of thermocouple calibration to greatly reduce the emf created:

• At the junction of thermocouple wire and extension wire.
• A the connection between the extension wire and the measuring instrument.

The color coding of extension wire has not been standardized between countries.

The "measuring instrument" of thermocouple technology refers to the instrument case that protects the voltmeter and reference junction (aka cold junction).

©2016 PTOA Segment 00110
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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