"Good words are worth much, and cost little." -George Herbert

PTOA Segment #107: Now I Know My K, J, Ts ... Next Time Won't You Sing With Me? 

(Thermocouple) Calibrations: Okay the true definition is a really wonky:

The temperature vs. emf characteristics of the thermocouple as agreed to by the ITS-90 committee and assigned a specific letter of the alphabet.

In every day lingo ... the alphabet letter assigned to the thermocouple that infers which two metal or metal alloy wires were used to fabricate the thermocouple.

In the real world, most working Instrument Techs will not even use the jargon "thermocouple calibration" but PTOA Readers and Students will know what that means when reading thermocouple literature.

 

(Temperature Measuring) Junction (of a Thermocouple): The union of two thermocouple wires that is exposed to a changing temperature and which thereupon generates a corresponding millivoltage output.

There are actually two junctions in a fabricated thermocouple ... the temperature-measuring junction and the reference junction.

However anybody referencing a junction with respect to a thermocouple is talking about the two wires (typically enclosed in a probe) that are immersed in the media that is having its temperature sensed and measured.

 

Reference Temperature: The temperature surrounding the thermocouple's reference junction.

The reference temperature causes the thermocouple's reference junction to generate an emf that is not related to the temperature change sensed by the temperature-measuring junction. This emf must be corrected to achieve an accurate thermocouple temperature measurement.

 

Type B (Thermocouple): A thermocouple fabricated from a platinum wire and a wire made from an alloy of 30%rhodium/platinum and 6%rhodium.

 

Type E (Thermocouple): A thermocouple fabricated from a wire made of nickel-chromium and a wire made from an alloy called constantan.

 

Type J (Thermocouple):A thermocouple fabricated from a wire made of iron and a wire made from a metal alloy called constantan.

 

Type K (Thermocouple):A thermocouple fabricated from a wire made from an alloy of nickel and chromium and wire made from an alloy of nickel and alumel.

Alumel is also a metal alloy.

 

Type N (Thermocouple):A thermocouple fabricated from a wire made from a man-made material called nicrosil and a wire made from another man-made material called nisil.

 

Type R (Thermocouple):A thermocouple fabricated from a wire made of platinum and a wire made from an alloy of 13%rhodium and platinum.

 

Type S (Thermocouple):A thermocouple fabricated from a wire made of platinum and a wire made from an alloy of 10% rhodium and platinum.

 

Type T (Thermocouple):A thermocouple fabricated from a wire made of copper and a wire made from an alloy called constantan.

 

 

PTOA Segment #108: Show Me How This Thing Works

 

Type J Calibration Table (supplied by Intech): A table which cross references the temperature that any accurately fabricated Type J thermocouple would sense to the millivolt output which the thermocouple will generate at that temperature.

 

 

PTOA Segment #109: Instrument Tech Must-Knows: Thermocouple Probe Types

Exposed Junction (Thermocouple Probe):A thermocouple probe wherein the two wires of the temperature-measuring junction are totally exposed bare naked and touching the media that is having its temperature measured.

 

Grounded Junction (Thermocouple Probe): A thermocouple probe that is fabricated such that the tip of the measuring junction is physically attached via weld to the interior nose-end of the sheath.

 

Ground Loop Interference: The deleterious result that results when the tip of the Grounded Junction Probe IS NOT correctly fabricated and therefore the measuring junction of the thermocouple IS NOT attached to the interior nose-end of the sheath but elsewhere on the interior of the sheath.

The fabrication error creates more than one electrical path available to ground with earth and therefore causes the temperature measurement of the instrument to be useless.

 

MgO Powder: Magnesium Oxide powder is packed into thermocouple probes to insulate and separate the two thermocouple wires so that they only contact each other at the measuring junction.

 

MI Cable: Mineral Insulated Cable prevents corrosion and electrical interference by insulating the two thermocouple wires from each other and likewise insulates the wires from the sheath of the thermocouple probe.

 

Thermocouple Probe: The two thermocouple wires and their junction inserted into a metal sheath.

The three probe styles are:

• Exposed Junction Probe
• Grounded Junction Probe
• Ungrounded Junction Probe

 

(Thermocouple Probe) Sheath: The protective metal tube into which the two metal wires and temperature-measuring junction of the thermocouple are inserted.

 

Ungrounded Junction (Thermocouple Probe): A thermocouple probe style fabricated such that the temperature-measuring junction is electrically isolated from the sheath (as opposed to the Grounded Junction Probe that is fabricated such that the temperature-measuring junction has a single point of contact with the interior of the sheath). 

 

 

PTOA Segment #110: Instrument Tech Must-Knows: Cold Junction Compensation & Extension Wires 

Cold Junction (of a Thermocouple) aka Reference Junction (of a Thermocouple):  The connection of the two thermocouple wires that is far away from the temperature-sensing junction (which had been made by connecting the other ends of the two thermocouple wires).

The term "Cold Junction" is commonly in use but would be factually inaccurate when thermocouples are used to sense temperatures that are colder than the ambient temperature.  The term "Reference Junction" is universally more accurate because it accurately applies to all thermocouple services and situations.

 

Cold Junction Compensation (for a Thermocouple): A technical strategy that must be provided by electrical circuitry or software for the purpose of offsetting the ambient temperature changes that impact the thermocouple's reference junction (aka cold junction).

In the absence of Cold Junction Compensation, a milliVoltage that is not related to the changing process temperature would be generated. Why? Because ambient temperature changes would impact the reference junction wires and the result would cause the thermocouple's temperature measurement to be erroneous.

 

(Dedicated Thermocouple) Extension Wire: Specifically manufactured wire for each type of thermocouple calibration that is used to connect expensive thermocouple wire to the thermocouple's measuring instrument and which minimizes undesirable emfs that would otherwise be generated by substituting any 'ole wire that is lying around.

 

Measuring Instrument (of a Thermocouple): The  measuring instrument of a thermocouple refers to the hardware that comprises the reference junction (aka cold junction) and a voltage measuring device.

The term "measuring instrument" must not be confused with the term "measuring junction" which is the temperature sensor of the thermocouple.

 

©2016 PTOA Segment 0136
PTOA Process Industry Jargon Recap 4-4

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