You got 2 know
You got 2 know

("Got 2 Know," by Flux Pavillion, 2010)

MUST-KNOWS ABOUT THERMOCOUPLE VERSATILITY

PTOA Segments #107 and #108 hint at the wide range of uses that thermocouple technology has been adapted to.

For example, thermocouples are the temperature sensors found in household appliances like ovens and hot water heaters.

And thermocouples are found in rugged industrial applications, like detecting the temperature of a catalyst bed or the temperature of the gases that are exhausted from a  gas turbine.

The variety of thermocouple probe styles make it possible to adapt thermocouple technology to many processing applications.

This PTOA Segment will focus on the three most common thermocouple probe styles.

First PTOA Readers and Students must understand what a thermocouple probe is.

 

 

 

DEFINITION OF "THERMOCOUPLE PROBE"

When the two wires and junction of a thermocouple are inserted into a protective metal tube called a sheath, the assembled device is called a thermocouple probe.

The PTOA Department of Redundancy Department feels the need to repeat:

Thermocouple wires and their junction inserted into a metal sheath  = Thermocouple Probe

The below drawing shows three common styles of thermocouple probes. Note:

• The two wires in the sheath do not touch except at the measuring junction.
• The pink area in the drawing is supposed to represent MI cable (Mineral Insulated cable) that insulates the wires from each other as well as from the sheath, thus preventing corrosion and electrical interference.
• The grayish area in the drawing is supposed to represent MgO powder (Magnesium Oxide powder) that is packed in to further insulate and separate the wires.
• The relationship of the measuring junction to the inner wall of the sheath defines the probe's junction style.

The sheath is commonly fabricated from stainless steel; high temperature applications use inconel.

No PTOA Readers or Students that I know of will be surprised to learn that the temperature measurement response time is greatly impacted by:

• The diameter of the probe.
• The diameter of the wire used to make the measuring junction.

That's because by now all PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order could write a novelette about how conduction heat transfer works.

Thus they understand that the bigger the probe or lower gauge wire (aka larger diameter), the more time it takes for heat to be conducted and convected from the process that is having its temperature measured into the temperature-detecting junction of the two metal wires.

PTOA Readers and Students that may be confused about the above statements can begin reviewing the PTOA Heat Transfer Focus Study Area right here. And you will be glad that you did!

 

THERMOCOUPLE PROBE JUNCTION STYLES

Exposed Junction Thermocouples Probes

A graphic of a thermocouple probe with an exposed junction appears on the bottom of the picture on the left.

The exposed junction thermocouple is logically named because the two wires of the measuring junction are totally exposed bare naked and touching the fluid that is having its temperature measured.

 

No kidding this thermocouple can be made by bead welding the two wires together.

Since the measuring junction is in direct contact with the process stream or surface that is having its temperature measured, the response time is very fast.

Now Imagine inserting this thermocouple into a process stream.

It does not take a genius to visualize that a measuring junction in direct contact with a process stream or surface would not work successfully in applications that have:

• a fast flowing process stream.
• a corrosive-to-metal environment.
• a process stream that yearns to chemically react with the alloy metals that the thermocouple wires are made from.
• a metal surface that creates its own voltage emf resulting in electrical interference and thus a bogus temperature measurement.

All of the above greatly limit the industrial application of exposed junction thermocouples.

One processing application would be measuring the temperature of dry, ambient air that is slowly moving through an air duct.

 

Grounded Junction Probes

A grounded junction thermocouple probe is shown in the top of the picture to the left.

When perfectly fabricated, the tip of the measuring junction is physically attached via weld to the internal end of the sheath.

Shall we pause a minute to consider the craftspersonship involved with making the bead weld joining the two wires and then simultaneously closing the weld on the sheath?

There ain't no elves or little winged blue fairies that magically pop up at midnight to make these things.

So it is not a big surprise that the manufacture of grounded junction probes is limited by how small the wires and sheath can be.

On the positive side, the protective covering of the measuring tip expands its usefulness; grounded thermocouple probes can withstand corrosive environments and higher pressures.

Yet no big surprise that protecting the measuring junction with a more complete sheath increases the measurement response lag.

And since all PTOA Readers and Students immediately understand why the protective covering will increase measurement response lag ... well, there's no need to repeat it.

So let's move on to noticing that the phrase "grounded junction" just means that the junction tip is electrically grounded with respect to the sheath.

Fabricating the grounded tip creates a susceptibility to ground loop interference and this results in a less accurate temperature measurement.

Ground loop interference happens when there is more than one  electrical path available to ground with earth.

Ergo, use of the grounded junction probes requires providing a means for electrical isolation.

So it might seem like the grounded junction probe would have limited processing world usefulness.

Yet Omega's™  "roller-surface thermocouple probe" is useful measuring the outside surface wall temperatures of smooth plastic or steel piping.

And Omega's™ "block style surface probes" are used in processing services that involve measuring the temperature of moving, rotating or stationary surfaces such as are found in the paper, fiber, printing, iron and steel  industries.

The architecture of these probes do not at all look like the rigid thermocouple style that had been shown in PTOA Segment #108 to measure catalyst bed temperatures.

These probes will be manufactured flat and thin to provide maximum contact with the surface of a solid. These probes are a great example of the versatility of thermocouple technology.

 

Ungrounded Junction Probes

Like grounded junction probes, ungrounded junction probes are protected from corrosive and chemically reactive processes by being enclosed in a sheath.

Ungrounded junction probes can be additionally protected from the process stream by being inserted into a thermowell.

The ungrounded junction probe  is the middle probe in the picture to the right.

Although it may not sound right to the ear, the graphic shows that the ungrounded junction style is electrically isolated from the sheath.

 

You guessed it!

The intentional isolation from the sheath increases the measurement response time.

But the sacrifice in response time is worth it because the ungrounded junction probe gives more accurate temperature measurements since it does not have to mess around with ground loop interference.

Even better, the ungrounded junction probe interfaces easily with field instrumentation and microprocessor-based controls in DCS systems (which are electrically grounded).

And these devices transmit the temperature measurements (represented as standard analog or digital signals) into the control board very quickly.

Ungrounded junction probes are typically protected in thermowells and can be used to measure the very high temperatures that are required to make important chemical reactions occur.

 

 

 

 

TAKE HOME MESSAGES: A thermocouple probe is created when the two wires and junction of the thermocouple are inserted into a metal sheath.

The thermocouple wires are separated and insulated with MI cable and MgO powder.

The probes of thermocouples can be made with the following junction types (listed in order from fastest to slowest measurement response time):

• Exposed Junction
• Grounded Junction
• Ungrounded Junction

Probe size and wire size also impact the temperature measurement response time.

Exposed junction probes can be used to measure the temperature of still or slowly moving gases that are dry and under no pressure.

Grounded junction probes have a tendency to be impacted by ground loop interference which hinders the accuracy of their temperature measurements. However they can be used to measure the temperature of solid surfaces ... even while rotating!

Ungrounded junction probes have the most measurement response lag yet their temperature measurements are more accurate. They can interface easily with microprocessors and other DCS components.

 

©2016 PTOA Segment 00109
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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