Increasing reliability of screw-down terminal blocks

Ya’akov

Joined Jan 27, 2019
9,071
The trick is to keep the wire from moving around during transportation, not making the connection more secure.
You are assuming that it loosens up during transportation and not just from handling.

It may just be that set up includes tightening the terminals because this is unavoidable.
 

Thread Starter

Paul of Alexandria

Joined Jan 28, 2015
12

strantor

Joined Oct 3, 2010
6,782
Thanks much! I didn't know about those. Ordered some and we'll see what happens.
You don't specify what it was you didn't know about and then ordered, but since you only clicked the "Like" button on one reply to this thread, I'm taking it as a clue that you refer to the ferrules in the quote below.
Generally using crimp connection ferrules solves it.

View attachment 243533
If that's the case, please re-read the following two quotes:
No good for thermocouple connections as this introduces cold joints to the system.
Because the connections were to thermo couples, I didn't suggest ferrules, but certainly otherwise they are always a good idea.
Be advised that any time a thermocouple's mV signal passes through a metal other than the two alloys it's constructed of, a new cold junction is created. You can think of it as creating a new thermocouple in series with the first. The connection between dissimilar metals introduces an offset to your mV signal and renders your measurement inaccurate. Using ferrules on thermocouple wires does exactly that; it creates a junction of dissimilar metals and introduces error to your measurement. I strongly recommend against the use of ferrules on thermocouple wires.

I'm not 100% sure about the following, haven't had time to think deeply or do any research into it, but it may be theoretically "OK" to use ferrules if the thermocouple has a straight run to your board. The terminals on the board probably aren't made of special thermocouple metals, they're probably just ordinary terminals, the measurement device on the board "probably" has CJC (Cold Junction Compensation) circuitry to remove error caused by those terminals, and that circuitry would "probably" remove error introduced by ferrules, but if there are any terminals between the board and the thermocouple tip, those absolutely need to be un-ferruled and connected via the sucky thermocouple terminal blocks.
 

Ya’akov

Joined Jan 27, 2019
9,071
You don't specify what it was you didn't know about and then ordered, but since you only clicked the "Like" button on one reply to this thread, I'm taking it as a clue that you refer to the ferrules in the quote below.

If that's the case, please re-read the following two quotes:



Be advised that any time a thermocouple's mV signal passes through a metal other than the two alloys it's constructed of, a new cold junction is created. You can think of it as creating a new thermocouple in series with the first. The connection between dissimilar metals introduces an offset to your mV signal and renders your measurement inaccurate. Using ferrules on thermocouple wires does exactly that; it creates a junction of dissimilar metals and introduces error to your measurement. I strongly recommend against the use of ferrules on thermocouple wires.

I'm not 100% sure about the following, haven't had time to think deeply or do any research into it, but it may be theoretically "OK" to use ferrules if the thermocouple has a straight run to your board. The terminals on the board probably aren't made of special thermocouple metals, they're probably just ordinary terminals, the measurement device on the board "probably" has CJC (Cold Junction Compensation) circuitry to remove error caused by those terminals, and that circuitry would "probably" remove error introduced by ferrules, but if there are any terminals between the board and the thermocouple tip, those absolutely need to be un-ferruled and connected via the sucky thermocouple terminal blocks.
@strantor the terminal block posted by the TS above in reply to a request from me for photos is actually an active device that includes CJC just behind the ordinary screw terminals.
 

Reloadron

Joined Jan 15, 2015
7,501
I'm suprised our resident expert (@Reloadron) on thermo-couples has not chimed in! :confused:
I am seeing good answers and thinking what could I add? :)

Over the years I worked with plenty of thermocouple alloy terminal strips as well as lugs. Years ago at work we undertook a project to replace all the type J thermocouples with type K. This was done mainly because the Iron in the type J types would rust in the harsh and wet environments in some areas. This was mainly a problem with the thermocouple plugs. That said the terminal strips, many going back to the mid 60s were as good and tight as the day they went into service. These were barrier strips like these. Many had 25 or more thermocouple connections. Those strips had been in place for 40 years plus and while the TC plugs would have environment issues I never recall a barrier strip failing or screws getting loose.
I have a thermo-couple datalogger and amplifier that use screw-down terminal blocks (the clamping type) to hold the connection wires. They work fine in the lab, but can come loose when transported. Any thoughts on how to improve the reliability of these connections, without major modifications to the equipment, would be appreciated.
Since the connection point is at the data logger the logger likely has CJC (Cold Junction Compensation) at the TC connection point. Good quality terminals should not become loose from just average use or shipping and handling. I saw Loctite mentioned and would think that applying any good thread locker or sealant would at least prevent screws from backing out. Obviously a major parts change is not in order so a fix on what is there is needed. That is about all I can add since it's too late to change the terminal block design.

Thinking way back this is a web page I wrote maybe 20 years ago as to basic thermocouple fundamentals. Much of it just covers what strantor already well covered along with similar images. :) Surprised it is still on the server. The web itself is one of two I use mostly as a personal cloud. :)

Ron
 
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The dataforth terminals is crappy. CJC is probably all over the place. I had trouble with a measuremnt Computing device too.
if you blow on the terminals, the temperature changes. Not good.

The terminals need to be isothermal and ideally massive compared to the wires.

I agree, you have to strain relief the thermocouple wires. That could be as simple as az clamp and a piece of foam.

When using fine thermocouples, the connectors usually used teflon washers.

For screws, a bellville washer is best for constant temperature.

Locktite 222 also prevents screws from loosening.

One thing I did was wrapped the extension wire around a pencil to form a spring-like wire at the connector.

I think the problem is wire movement as the first order problem.

With "T" type, I flattened the extention wire under the terminals. The wire is copper. For R&S thermouples, platinium, the teflon washers.
 

strantor

Joined Oct 3, 2010
6,782
@strantor the terminal block posted by the TS above in reply to a request from me for photos is actually an active device that includes CJC just behind the ordinary screw terminals.
Right, I noted that...

...The terminals on the board probably aren't made of special thermocouple metals, they're probably just ordinary terminals, the measurement device on the board "probably" has CJC (Cold Junction Compensation) circuitry to remove error caused by those terminals, and that circuitry would "probably" remove error introduced by ferrules, but ...
...But it's actually not entirely clear. There are two versions, one with CJC and one without:


cjc1.JPGcjc2.JPG

And there are also these:

cjc3.JPG

So it seems to be a configurable system, more than just a device, and I don't think we have enough information to say whether CJC is employed or not.
 

strantor

Joined Oct 3, 2010
6,782
That said the terminal strips, many going back to the mid 60s were as good and tight as the day they went into service. These were barrier strips like these. Many had 25 or more thermocouple connections. Those strips had been in place for 40 years plus and while the TC plugs would have environment issues I never recall a barrier strip failing or screws getting loose.
Thanks for posting that. I am going to give these a try.
 

strantor

Joined Oct 3, 2010
6,782
@Reloadron What do you think about this?
I'm not 100% sure about the following, haven't had time to think deeply or do any research into it, but it may be theoretically "OK" to use ferrules if the thermocouple has a straight run to your board.
Assuming the system/device in question does have CJC, would there be any difference between putting the thermocouple wires directly into the terminals, or putting them into crimp ferrules and then into the terminals? I'm still getting my 5 ton flywheel of a brain up to operating speed but I'm starting to think that it would in fact cause a problem. I'm thinking that the thermocouple-wire-to-ferrule is a cold junction, and the ferrule-to-terminal is an additional one, despite all taking place within the terminal, and the CJC is only compensating for one of those junctions. Unless the ferrule and the terminal are made of the same metal, which they might be. What if it's tinned copper? is the tin>copper a cold joint too? I'm going to hurt myself if I think any more about this right now. Curious about your take on it.
 

Reloadron

Joined Jan 15, 2015
7,501
Will two dissimilar alloys have a Seebeck Effect? Yes as I see it and we could call it a thermoelectric circuit. Will it matter? I doubt it. Most screws and other binding post are nickel plated steel or similar. With CJC right at or really close to the junctionsI doubt it would matter enough to be concerned with. Purely my thinking which is subjective. :)

Ron
 
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