Thermocouple types

I have looked at several web sites and have gotten different ratings from almost all of them as to the temperature ranges of thermocouples.

Here is an example of two sites but there are others with different ratings than these. Some of the sites are stating the specs for the ones they sell but other sites are just providing data, not selling anything. So why the differences?

I understand that the type of material encasing the thermocouple makes a difference but some of these ranges are wildly different. Why do some say the lowest is zero degrees?

And Look at this if you want.. http://www.pyromation.com/Catalog/gen06.pdf

SITE 1 J
J
(0 to 760) °C
[32 to 1400] °F

Site 2 - J
Type J Temperature Range:

Type J Thermocouple Grade WireThermocouple grade wire, -346 to 1,400F (-210 to 760C)

-----------------------------------------------------------------------------------------------
Site 1 K
K
(0 to 1260) °C
[32 to 2300] °F

Site 2 K
Type K Temperature Range:

Type K Thermocouple Grade WireThermocouple grade wire, –454 to 2,300F (–270 to 1260C)

--------------------------------------------------------------------------------------------------
Site 1 T
T
(-200 to 370) °C
[-328 to 700] °F

Site 2 T
Type T Temperature Range:

Type T Thermocouple Grade WireThermocouple grade wire, -454 to 700F (-270 to 370C)

CaspianSage,
From what I am seeing this is mostly the difference between theoretical and working temperature ranges.
Example from several years ago. Application was to hold -300F, a Type K sensor was specified.
In practice Type K is not linear below 0F. Application was not consistent.
Changed sensor to a Type T worked great.
Hope this helps.

Read what Dataq has to say here:

https://www.dataq.com/blog/data-logger/thermocouple-data-loggers/sub-zero-thermocouple-measurements/

In summary, only E or T are really useable - wire should be cert'd for sub zero C service.

Reasons - behaviour of alloys at temperatures below freezing.

You should consider using a Pt100, a 100 ohm platinum RTD for subzero measurements, unless vibration is excessive or the self heating from an RTD will substanially alter the measurement.

You are comparing the rating of the thermocouples to thermocouple _wire_. The thermocouple rating is what sort of process temperatures it is usable. The wire rating is for the ambient environment. Thermocouples generally have fibreglass or teflon insulated leads. Inexpensive thermocouple wire often has PVC insulation...the insulation may not stand up to temperature extremes, but this is OK, as the extension wire is not typically installed in anything worse than outdoors or in a hot factory floor...generally if people are OK there, then pvc insulation is OK.

Thermocouples are used mainly for temperatures > 400-500 ° C.

Some tables may indicate mV versus temperature for low temperatures but rarely will be used in that situation since other systems such as the thermistor PT100 are clearly superior and more precise.

Kevbo said:

You are comparing the rating of the thermocouples to thermocouple _wire_. The thermocouple rating is what sort of process temperatures it is usable. The wire rating is for the ambient environment. Thermocouples generally have fibreglass or teflon insulated leads. Inexpensive thermocouple wire often has PVC insulation...the insulation may not stand up to temperature extremes, but this is OK, as the extension wire is not typically installed in anything worse than outdoors or in a hot factory floor...generally if people are OK there, then pvc insulation is OK.

Click to expand...

Yes I see that now in the example I gave..oops,,, but to the point the site DANW refered to we have more thermocouple ranges. as shown in this image. I had selected a type T for The temps I want measure which are mostly atmospheric and I think it is the best choice overall because of its response time. - But some site list its highesdt temp as 500 deg F and I want to also use it to monitor pilot lights and main burners which reach higher temps. I have seen the high range of T type from 400 to 700 degrees. This all makes me re-think that maybe I should use a J. I don't get why many charts say the lowest of all but T is either 32 or 0 degrees and some say T lowest range is 0 also.

The better vendors show the ranges starting at 0°C, with T and E types for sub-zero:



And they tell you that the material needs to be specifically selected for sub-zero service.

It's easy to believe that because you get a signal, that it's accurate because it's a digital number. Nothing could be further from the truth.

An RTD is a better bet all around.

That is really helpful Dan. Thank you ... I will be monitoring the thermocuples with a 1771-IXE / PLC 5 system. I don't care if it is not absolutely accurate but I would like most of them to be within 5 degrees, 2 would would be better. Obviously 1 degree would be ideal.

I am gong to monitor living space mostly and control heating cooling dampers that is why I was thinking T type as I read they are the type used in refrigeration because of their fast response time. I also want to monitor attic and outdoor temps.

I will probably have to use a second thermocouple card if I want to use more than one type because rs logix will only let you configure one type for the 1771-IXE.

BTW --

It's easy to believe that because you get a signal, that it's accurate because it's a digital number. Nothing could be further from the truth.

Click to expand...

Like the analog clocks that flipped numbers painted on a metal plate mounted on a shaft! people thought they had to be accurate because they were digital ---

So what do you suggest for my application? My temperature ranges for the most part will be no less then minus 20 something to +150 DEG F. and conditions for the wire are not at all hostile.

I think the wire to the thermocouple can be PVC for this application.

What temps do you want?

Usually T is good for near ambient to freezing, but if you are looking to monitor Pilots and burners, you are very close to the upper limit of J, I would go to K.

"K" is a very popular "Base Metal" TC (Chromel - Chromel/Alumel), even for ambient temp readings.
"J" is made from Iron - Constantan....which will rust and cause other issues....so you have to be careful where you use it.

The "Noble metal" TCs are R, S, and B, which are blends of platinum and platinum/rhodium. And as you can see, they are primarily high temp tcs, but last a LONG time due to the material.

Make sure you get the Thermocouples and any extension wire with "special Limits" of calibration. This will give a lot better calibration accuracy (usually <5 deg error). If you do not get "special limits" wire...error could approach 15 deg or more.

What temps do you want?

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Primarily Ambient Nathan, Minus 20 to 150 approximate range. Not sure what you are referring to about special limits though..

CaspianSage said:

Primarily Ambient Nathan, Minus 20 to 150 approximate range. Not sure what you are referring to about special limits though..

Click to expand...

For that range "T" should be fine. However...you also mention "pilots" and "burners", which are 1000 deg up to as high as 2200 degrees F.

Special Limits of Error: These thermocouples are made with a higher grade of thermocouple wire, which increases their accuracy. They are more expensive than standard thermocouples, but not that much.

Standard Limits of Error: These thermocouples use standard “thermocouple grade” wire. They are less expensive and more common, but not as accurate.

Explained better here..
https://www.omega.com/prodinfo/thermocouples.html

and more info here..
https://www.omega.com/techref/colorcodes.html

Also..check the accuracy of the Tc module you are going to use. I have found most modules to be more accurate by a factor of 10. Just the MFG does not want to get into people giving them grief over accuracy of reading. Thermocouples are not the most accurate devices to read for temperature, due to the extremely low voltage output for degree.

As an example... a type "K" tc tops out at 53mV (-454 to 2501 F)
And a type "S" tc tops out at just over 18mV (-58 to 3214 F)

So just 1 degree is a very small change in voltage....and then you get into cold junction errors also.

I agree with all of the above. In addition:
Use Omega Engineering as your reference and source for in-depth information.
Type K is the default first choice for everything above room temp.
Type "C" (from Omega) for high temps to 2300 C.
Use probes with ungrounded tips. Grounded only if you need fast response.
Thermocouples are not suitable for sub-zero temperatures in most cases.
Standard round pin connectors are best. The miniature ones have flat blades that can get intermittent.
Thermocouple is formed at the junction of 2 different metals. That means you get 2 more at the connector of your controller. That's what's referred to as the "Cold Junction".
You must use the same type connectors and extension wire as the thermocouple type so that you don't form any more thermocouples in series. Connectors are color-coded. Yellow for Type K.

Another tip..

Get your TC plugs and jacks for your extension wire here. WAY easier to wire using "Jab In style" connectors.
http://marlinmfg.com/products/connectorsStripanels/fJab2Pole.html
How to know if you need a plug or jack? I use the slang...Plug- Fu**ks, Jack-Sucks..sorry if that was offensive


Omega is a good reference... but I would not necessarily get TCs or connectors from them, if you can help it. They are too big of a company do do a good match for accuracy and I have had quality issues from them on their plugs and jacks.

I do get these TC terminal blocks from them:
https://www.omega.com/pptst/DRTB-2.html
I know they have a "Mini" connector on top, which I would not use for a permanent connection, but it is a nice place to plug in an instrument to verify reading.

CaspianSage said:

BTW -- Like the analog clocks that flipped numbers painted on a metal plate mounted on a shaft! people thought they had to be accurate because they were digital

Click to expand...

I have exactly the same mental picture. When I read what you wrote, I spilled my coffee I laughed so hard.

I'm a fan of Type T for low ranges like this because the inherent error is half that of the other types. It'll work well for you.

You'll want special-limit-of-error Type T for the range you're looking at. Special limit of error is very common and the cost difference between the two is very little. It separates the better alloy from the good enough alloy.

If the 20 degrees below zero is really critical for accuracy then tell the vendor that and pay for a cal cert with a cal point at -20°F.

Dan

Here's a towel for you Dan

This has been a good thread, thank you all for contributing. Looks like some of you found some of the same info I did when I did a search.

I really appreciate the effort all of you put into this.

I will be using a T type for temps I want to monitor in the below 200 degree range and Since I rarely need to read below zero it should be fine. I will use flame sensors for the burners and pilots.

For Dan here is a towel for you to keep close by when you are reading my posts in the future to clean up your coffee spill...