Regadring thermocouple sensor and calibration 2

[freetown]

Hi all,

I have few questions, would like to understand about thermocouple sensors working:

1- We have some thermoucouple sensors (type T), the distance between sensors and transmitters is about 200m. As the thermoucouple output is few millivolts, is this distance can causes drop voltage and then has an effect on thermocouple's accuracy?

2- What happens if somebody reverses the polarity of a T thermocouple? Will we still have accurate value? It happens once on one sensor and we have 4 thermocouple in almsot the same area. We noticed that the reversed one is only 4 or 5 degree higher than the rest, what is the explanation for that?

3- We use dry-well to calibrate PT100 and thermocouple sensors. When we find a difference between the indication and the value of calibration (let's say dry-well is 60 degree and indication of sensor is 50) and the calibration of transmitter is ok, is the only solution in this case is to replace the sensor as we have no way to adjust sensor internally?

Thank you.

 

[waross]

1:
Modern instruments use a potentiometer circuit which which balances a reference voltage against the TC voltage so that there is no current flow and so no voltage drop.
3:
Poor contact between the dry well and the TC element.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[IRstuff]

2:
Polarity has to be correct; the thermocouple junction produces a potential difference that is a function of the two different metals in the junction, and swapping leads inverts the apparent potential and will result in a bogus reading. This is why thermocouple couple connectors are usually polarized with different sized contacts. Some meters might be able to figure this out and correct the measurement, but having it correct simplifies things.

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[danw2]

1. Better quality instruments with a thermocouple input will spec what the maximum resistance is, per leg, for a thermocouple input. The spec sheet on my desk for a Honeywell controller has a spec of 100 Ohms per leg for any Type thermocouple. There are charts on the web with resistance per foot or per double foot of Type T extension wire. This came up recently and I discovered that Type T extension wire is not readily available in wire heavier than 20G.

There is no explanation of the effect on excessive resistance on the input accuracy, but it probably has something to do with the operation of the analog input's instrumentation amplifiers.

2. If you reverse the polarity of the extension wire running from the thermocouple to the analog input, then you do have an accuracy issue, and there's no documented correction on the error effect of mis-wired thermocouple circuits and I've looked. Extension wire polarity reversal creates an additional junction that contributes an EMF that is not accounted for in the published thermocouple tables, so there's no work around unless you want to do the research to document what magnitude error that particular junction adds or subtracts (without knowing what temperature that added junction is at).

A thermocouple with proper wiring polarity from hot junction to cold junction, but that is wired backwards on the analog input will just drive the signal in the wrong direction.

3. Correct, there is no adjustment of a thermocouple or RTD temperature sensor to correct the sensor output for a given temperature. Any correction adjustment is done at the analog input/system level, using the deviation documented during a sensor calibration using some heat/cooling source, presumably with traceable temperature indicator certification.

Be aware that the ANSI spec is used in North America as the standard for commercial thermocouples. ANSI standard error is ±1°C (-67° to 133°) for a Type T thermocouple and ±0.5°C (over same range) for a limit-of-error thermocouple.

Every place I've been involved with would consider a 10°C sensor error as way out of tolerance for at a Type T thermocouple and would consider it a bad thermocouple and would replace it.

 

[che12345]

I have the following opinion for your consideration.
1. Transmitter is about 200 m would affect the accuracy. It is affected by besides the voltage drop, more importantly is by the electromagnetic interferences; switching and motor etc.
Suggestion: Relocate the transmitter (which is of very small in size) to as close to the thermocouple as possible.
Note: the transmitter output say 4-20 mA to the indicating instruments is NOT very sensitive to interference.
2. Observe the polarity strictly. All thermocouples are either marked + - or with colour code. Attention: Marking and colour code differ between OEMs.
3. Immerse the PT 100 and the thermocouple in a wet-bath of oil or water with a volume of say > 2 L. They shall be at the same level and take reading after say > 30 min in an environment that is not affected by any draft.
Che Kuan Yau (Singapore)

 

[waross]

Other possible sources of error include inadvertently using a different type of thermo-couple, or having the test set set to the wrong type of thermo-couple.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[freetown]

Thanks everyone for the replies, much appreciated.

 

[EdStainless]

If you have extension wire this gets more complicated.
The wire needs to be of the correct grade.
The only time that I used extension wire (as opposed to running the TC wire full distance) is when I was using Pt TCs.
And the connections need to be made using links of the correct TC alloys, you can't just use Cu or brass fittings.
The temperature at the connections matters also.

 

[waross]

A temperature gradient along a length of conductor causes a voltage gradient.
The milli-volts per degree varies with the material of the conductor.
In the event that the cold connection of the thermo-couple is at 40 degrees C and the instrument is at 30 degrees C, the proper extension wires will respond to this difference.
If copper wires were used to extend the leads, the voltage gradient in the copper conductors would cancel and the instrument would be seeing the voltage difference between the hot end of the TC and the cold end of the TC.
The instrument reading would be 10 degrees high.
Mr Che's suggestion to use a local transducer to convert the TC signal to a 4-20 milli-Amp signal is a very good suggestion.
Circuit loading is important with 4-20 ma circuits.
he total resistance of the loop times 20 ma must not exceed the supply voltage.

Anecdote alert.
We had an incinerator to burn off SO2.
The incinerator had a 100 foot high refractory lined steel stack.
We had a TC in the stack to shut off the natural gas if the stack temperature went too high.
There was a knock-out pot to remove hydro-carbon liquid slugs from the CO2 stream.
The fan was left running to cool the incinerator.
The incinerator ran away and melted the thermo well.
The hi-alarm did not function.
All of the circuit components were checked and found to be in good order.
I suggested liquid carry over and was poo pooed by the experts.
"There is a Knock-out pot."
The second time it happened, I dig deeper into the circuit.
There was a real fear that the high temperature may fracture the refractory lining and bring down the stack.
From the published information of all of the circuit components, I added up the resistances and compared them to the maximum resistance that our power supply could support.
The total listed total resistance was high but within the capability of the power supply.
I should mention that the high alarm was set to trigger at around 19 ma.
What to do next?
After years in the field, I had learned not to blindly trust numbers that I had not personally verified.
I measured the actual input resistance of each device in the circuit.
Sure enough the alarm relay was actually about 20% above the listed resistance.
An added resistor in parallel with the input resistance brought the input resistance into specs.
I could have set the instrument span higher, but that would have impacted other devices in the circuit.
I also modified the controls to shut the fan down on high temp alarm.
There is nothing like a powerful blower feeding fresh air into a burning liquid slug to
About a week later the incinerator shut down on high alarm.
The operators told me that they could hear something bubbling and boiling inside the bottom of the stack and there was a wisp of smoke coming out of the stack.

Lessons learned:
Don't blindly trust the spec sheet when working close to the limits.
Don't blindly accept that the knock-out pot will always work properly.

I never did hear what the problem had been with the KO-pot.
I guess the guys didn't want to admit that my very first diagnosis was the correct one. grin,
That was a rare exception in the application of 4-20 ma controls.
There are millions of 4-20 ma systems in use because the system is so dependable and robust.

About a week later, the