Wrong Thermocouple Extention Cable

> I cannot believe we are discussing the relative merits of doing something in
> various wrong ways as opposed to just fixing it correctly.

That's right, but it is still a good reminder or case study for similar situations since this concept is not fully understood even by many experienced technicians. I've had to explain it to my guys many times in various similar circumstances, for example inside a control cabinet where the T/C extension cable from the field was connected to a terminal strip at the bottom of the cabinet and routed to the PLC modules at the top of the cabinet with non-compensating wire, the temperature gradient (within the same cabinet) between the PLC and the terminal strip created a very sizeable error, more than 10degC.
 
@ Roger, you are correct but i suppose you have misunderstood me. by treating as a single unit and varying the temperature, i meant all the junctions(and that equals the system) always getting equal temperature and that common temperature being varied for the working range.

one possible way to do it practically -take up the bare wire junction (just the junction and small lengths of the two wires) of the k type and attach the wires of the t type(as said in the problem) to the appropriate ends. put this whole thing in the temperature to be measured. and so all 3 junctions are at the same temperature.

well, i haven’t done this thing till now, but i don’t see any reason why it won’t work.
 
C

Curt Wuollet

For once I am in complete agreement with Bob, the one thing that you can say for sure is that it will be wrong under nearly all circumstances. That doesn't inspire confidence. And the workarounds are more time and trouble than fixing it. It wouldn't even be dependable as an indicator because it will be inconsistently wrong depending on season, activity, etc. anything that changes the temperature differential from end to end. It's not something you'd do to someone you like. Much worse than simple inaccuracy.

Regards
cww
 
B

Bruce Durdle

I can quite believe it - especially if the guy responsible for the original design is now responsible for writing the cheques to get things fixed. I was once in a situation where I was responsible for a large furnace management PLC with some issues relating to performance under partial shut-down - I was expected to make it work properly but any modifications had to be approved by the guy who had designed the original program and there was no way he'd admit there was an issue.
 
oh, my mistake.
if all the junctions get to the same temperature, it would just reduce to a t type thermocouple.

what can be done is to keep the outer 2 junctions at some fixed temperature and just vary the temperature of the inner junction only, to see the response.
 
ok forum, one more chance :)

let’s make this whole thing a k type thermocouple.

the actual thermocouple is k type, attach some k type wires and run them a little so they get at atmospheric temperature. now attach the t type wires and run them as long as needed. now at the end, again attach some k type wires at the appropriate ends(that too, at atmospheric temperature). and that goes to the indicator.

so the 4 outer junctions are all at atmospheric temperature and the sensing junction senses the temperature to be measured. this is just a k type thermocouple. theoretically, there is no error with this system (practically, there might, because of the variations of the atmospheric temperature at the 4 outer junctions).
 
C

Curt Wuollet

Aye, there's the rub. The two ends are unlikely to be at the same temperature in the real world. Quite often one end is in the control room and the other end is in the head of a TC well, perhaps stuck into a furnace, or a freezer. From the TC type, probably someplace hot.

Regards
cww
 
Thanks CWW for the reply,

>The two ends are unlikely to be at the same temperature in the real world.

we can always take a few meters of k type wire so that it goes up to a point, it comes to open air and we make junction there. similarly for the k type wire at the indicator side.

if a strictly errorless condition is required, these 2 points can be made artificially isothermal.

regarding the point of making the 3 junctions at the same temperature ( we discussed it @ 2 February 2013 - 6:59 pm, 2 February 2013 - 12:27 pm, just to avoid confusion), if we make it a t type thermocouple then we can expect it to work satisfactorily as a t type. of course, a t type indicator will be needed. so that might also be a possible approach. but here we are not dependent on the atmospheric temperature to be same at two points.
 
> Just how much extension wire that needs to be replaced is involved here?

considering this question as- the amount of the t type wire that has to be replaced by k type wire,

just enough to bring the junction points to open air. the precise amount depends on the precise locations of the thermocouple head and the indicator.
 
Hi, reading through the conversation over - it seems my current problem is discussed over, but I would appreciate if anyone can comment on if I have been able to pinpoint the cause of my problem.

I have a K type thermocouple measuring the throw temperatures of a gas compressor. The measurement is correct before the startup and at the initial start, but then drops and deviates with temperatures when compared to manual reading done with a infrared gun, see reading under:<pre>
MBC Bravo temperature reading HMI Degree C 94 96 114 92 90 115
MBC Bravo Temperature reading with infrared gun Degree C 118 118 117 119 98 96</pre>
All the thermocouples are connected to a field junction box, approx. 45m cable run each. The cable used from the thermocouple to the junction box is a FlexFlame BFOU(i) 250V Thermocouple type K supplied from Draka Norsk Kabel AS. BUT, from this point the multicore cable used between the field junction box to the local control panel is a normal copper cable, approximate 7meter run, the local control panel uses a AB thermocouple card, 1746-NT8. I expect my problem will be solved by replacing the last leg of cable with a type K cable? Anyone knowing where to get a 8 pair thermocouple cable for offshore installation sourced quickly?

Thanks in advance!
 
The junction errors at those temps will only be at +/-2 DegC, worst case.

Copper extension wires are not that uncommon.

Just how precise are you trying to measure. Typically bellow 100 C or so you have to use RTD's, but that is only a judgement call.

That said, you should not be getting what looks like 20-30 C errors peak from the measurement. Those may be real.

my 2 cents,
 
You should keep in mind that infrared thermometers can not read very accurately (emissivity, type of surface, etc.). Usually, the reference to calibrate an IRed thermometer is a thermocouple. If you use copper extension wire or cable, you will get inaccuracies because of the cold junction compensation, which is inside the thermocouple module. This compensation measures the temperature at the module inputs and subtracts this value from the signal coming in. Therefore, if the junction box temperature (where copper and thermocouple wires are connected to each other) and the temperature at the PLC module location are the same, the error will not be significant. If not, the error caused by this will be that of the temperature difference between the 2 locations. If terminal blocks are used, it is best to use K type terminals as well. I hope this helps.
 
>I cannot believe we are discussing the relative merits of
>doing something in various wrong ways as opposed to just
>fixing it correctly.

I have a situation where the wrong TC was installed in a heater bundle. The cabling is of a different type as the TC. We have a contractor that is arriving to assist with a startup of a very critical station and there is no time to 1. Replace the TC in the bundle and 2. Replace the cable back to the house. I can appreciate that someone would want to know the exact symptoms of using a mismatched setup.

In a perfect world everything is available when we want it with as much time as necessary to do it the "Right" way. I would really like to live there...can someone give me an address?
 
I'm old and grumpy enough to remember calibrating thermocouple devices before the days of digital displays.
The measuring device usually consisted of a standard cell, Wheatstone bridge and Null balance galvanometer. They always had a thermometer mounted beside the terminals and another potentiometer which you set for the temperature compensation.

What this has to do with the topic I'm not sure.

Just do it right!
 
And a bucket of ice for the cold junction for pre-startup steam turbine testing

>I'm old and grumpy enough to remember calibrating
>thermocouple devices before the days of digital displays.
>The measuring device usually consisted of a standard cell,
>Wheatstone bridge and Null balance galvanometer. They always
>had a thermometer mounted beside the terminals and another
>potentiometer which you set for the temperature
>compensation.
>
>What this has to do with the topic I'm not sure.
>
>Just do it right!
 
A friend inherited his Dad's process instrument field calibration business and his Dad's notebook.

The notebook lists locations of "Cold Junction" reference points for various plants, building or units, for instance, NW corner of the rolling mill, 6 feet west, 3 feet south, 4 feet deep, 54 Deg F.

Someone had found a stable temperature point, had measured the point and that's where the (separate) cold junction thermocouple junction was landed.

The cold junction thermocouple was run to the instrument, wired in series with the measuring thermocouple and instrument was then calibrated for a 54 Deg F cold junction reference, not a 32 Deg F ice point.

Likewise, not thread relevant, but interesting as to how clever instrument design and implementation was before microprocessors.
 
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