I had purchased new j-type thermocouple and measured the resistance to cross check it. Found the resistance value of the new one is different than the previous one. Previous T/C has 4.3 ohm and new one has 15 ohm. How to confirm which one is the best quality?

Any other simple testing procedure available other than resistance test so that we can check it without removing it from the machine?

What is the standard resistance value of j and k type thermocouples?

 

Hi,

There is no standard resistance for a thermocouple. It would depend on the gauge of wire it's made of. Also when you are measuring the resistance the reading will be effected if the tip is at a different temperature than the terminals because of the mV generated. An indication of this would be a different resistance reading when you reverse your meter leads. Unless you are looking for super accuracy any thermocouple should give accurate results.

The only time the resistance would effect your reading would be when using a readout that has a low impedence input, e.g. a moving coil meter (voltage drop). Your normal PLC or instrument signal transmitter will have a high impedence input.

Regards,

Roy

 

I wish more people would use Google or Yahoo to search for information about their problems. If you have access to control.com then you have access to search programs on the internet.

I used "type j thermocouples" in Yahoo and had excellent results. One site, http://www.veriteq.com/thermocouple-data-logger/type-j.htm says this:

"Type J thermocouples are made up of a positive iron wire and a negative Constantan wire. They are one of the most popular thermocouple types primarily because of a broad measurement range and superior voltage output (which can result in greater temperature resolution). Note that type J probes, because of the iron component, are susceptible to oxidation and rusting, particularly at sub-zero temperatures."

Note that it is a voltage device, not resistance, so an ohmmeter is an inappropriate test instrument. A typical DVM will show strange resistance values due to the voltage generated by the junction. Reversing the test leads will give a widely different resistance due to polarity reversal.

There are proper test instruments that can read the temperature at the junction as well as simulate a signal.

One company with a wealth of information and test instruments is omega.com. A lot of their information is free or available for a nominal charge. I don't think I have ever been in a US maintenance shop that did not have a shelf of their handbooks. Fortunately many of the books are now on CD.

Hope this helps. I would rather show people how to solve a problem themselves than to solve it for them.

 

As the thermocouple is essentially a "short circuit" device that generates a small voltage (millivolts) proportional to the temperature, you have to be careful of testing it with an ohmmeter. The temperature of the junction will affect the reading on the ohmmeter. The higher the temperature, the higher the voltage, so reading it with an ohmeter will present a different value depending on the polarity of your test leads. The voltage generated by the junction will either add or subtract from the voltage applied by the ohmmeter, thereby falsing your ohmic reading. If you are comparing two devices it is essential that they be at about the same temperature. The values you describe seem high to me for something that is at room temperature. Ohmic values are generally low ohms or fractions of an ohm. The important thing when measuring the resistance is that the values be different when reversing the polarity. This indicates that the junction is indeed generating a millivoltage.

Gerald Beaudoin

 

A thermocouple will only generate a voltage when there is a temperature gradient between its ends. A thermocouple that is isothermal or the same temperature across its length will generate no emf.

Hence, a resistance check of a T/C whose entire length is at one fixed temperature would be a reasonable check.

Thermocouple wire has known resistances.
See page 2 for resistances for various wire sizes in the pdf at
http://www.thermometrics.com/assets/images/thermcpl.pdf

Thermcouples are known to increase in resistance as the process end becomes polluted over time.

Issac

 

Hi,

Thanks.

Can we guess tentative ohmic resistance of J-type thermocouple, because every meterial has its own specific resistance, J-type t/c also made of specific material with specific type of wire.
What I faced problem is, the display temperature was lower then actual temerature, which t/c have higher ohmic resistance value.

Regards,
Prakash

 

>Can we guess tentative ohmic resistance
>of J-type thermocouple <

Why guess? You don't have to guess at the ohmic value, just read the post in this thread from 2 Nov 12:57 AM that provides a direct link to a pdf with a resistance table.

>What I faced problem is, the display
>temperature was lower then actual
>temerature, which t/c have higher ohmic
>resistance value. <

Your attempt to investigate an accuracy issue by measuring resistance of an isothermal T/C is ill advised.

There are a multitude of possiblities for divergent readings, beyond an inaccurate thermocouple:
- wrong type of T/C
- incorrect configuration of the reading device
- poor or in accurate ice point compensation
- bad thermocouple
- grounded T/C producing ground loop
- improper comparison of a known standard to the reading in question due to a thermal lag or location difference.

You claim "display temperature was lower then actual temerature (sic)", but how do you know that?

What are you using to determine the "actual temperature" vs the meter reading?

Why do you believe the "actual temperature" vs the display reading?

What provenance of traceability does the "actual temperature" have that the display does not?

Issac

 

Remember also to apply cold junction compensation.

The thermocouples connections to the multimeter
and or op mamp to amplfy the signal will set up an EMF in opposition to that generated by the
thermocouple end.

 

Hi Issac,

Thanks a lot.

>You claim "display temperature was
>lower then actual temerature (sic)",
>but how do you know that?

We had the temperature module to control the temperature. Calstat is connected in the heater zone for extra control for heater. How I guess the actual temperature is that the Calstat, which has tripping point of 630deg F, was tripped but the temperature reading is 400degF in the monitor.

I also guess the same supplier provided different type sensors. We are going to exchange it to right type.

Thanks,
Prakash

 

respected sir,

i wanted to know the max.resistance of a J-thermocouple. n how it we can measure..also what is the max, length of a thermocouple used for autoclaves?

 

See the thermocouple resistance by wire gauge table at the link below and calculate what the resistance is depending on the length of the wires, understanding that

- resistance measurements of non-isothermal thermocouples is suspect (due to EMF generated)

- resistance increases as the chemical composition of the constituent wires changes.

http://www.conaxbuffalo.com/tech/newpdf/WireSize.pdf

The maximum length of a thermocouple is determined not by the device in which the thermocouple is installed, but by the device reading the thermocouple signal. Issues are

- the maximum resistance the thermocouple input can tolerate (not published by every controller/recorder/PLC/DCS/indicator vendor)

- noise issues

Runs of several hundred feet (100m) of thermocouple wire are not uncommon with today's instrumentation.

Don't autoclaves typically use RTD's?

 

Thanks, well written and very interesting. I also learned something.