Thermocouples

After connecting a differnt type of thermocouple other than K type u need to change

ur interfacing system according to output type of thermocouple.

No, don't use a different type of thermocouple.

By using a different type, you'll introduce two more junctions into your circuit. This will affect your temperature readings. Even if you could select a different type (assuming that your controller or transmitter allows it), you'll still have inaccurate results.

Always use the same extension cable as the measuring thermocouple.

Some instruments allow the use of differnt thermocouple and need the program adjusted to read the different t/c. The Millivolt curves are different for each type of thermocouple. Each thermocouple is really chosen for the temperature range it will operate in and different types have varied characteristics when exposed to ranges out of their respective prefered application range. ie "T" for lower tmperatures where moisture may be aproblem; Type "E" lower temps where Higher emf is desired; "J" strong in plastic applicvations but become susceptible to oxide problems in moisture area and tekps over 1000 F; "K" plus 1000 to 2000 F can experience curve shift below 1000 F; "R or S" for temperature above 2000 F.

If the application calls for type "K" there is a reason...stick with "K" unless the lead wire is changed to the proper compensating lead Same as the new t/c and instrument.

Hi rmerriel,

Normally, you'd be right. Most modern equipment allow you to select what thermocouple you want to use.

In this case, however, our poster mentioned that he was using thermocouple extension wire. If you want to use a different thermocouple, you'd have to change the extension wire also since they're supposed to match with each other.

All your advice has been correct. Once you understand how thermocouples work, you can derive the answer yourself. The thermocouple is a specific metal junction. Change the metals and you will generate a new calibration curve of voltage v/s temperature. This will cause a calibration shift. Worse yet, When you change the metal, you create new junctions and have additional temperature sensitive areas in locations you had not planned on.

Guest: If you have access to extra cable and welding equipment, you can make the thermocouple you need. There's no magic. It's just a junction of the two materials. DickL

DickL Thermocouple jointing is not just welding two dissimilar metals . Simply by doing welding you can make a thermocouple but it will not generate the correct millivolt cure as per standard calibration charts. It is a specialised jointing to be carried out by specialised welder under controlled conditions. Each thermocouple after manufacture is tested throughly for the functioning before dispatching to the user.That is why we pay such high rates for a thermocouple manufactured by a good manufacturer .Even minute air gaps , cracks and impurity in joints will make the variation of Milli volts generated by it at high temperature inside furnace.So it is not good to advice inexperienced engineers and technicians to carry out such crude engineering methods for specialised furnaces . Even small variation in temperature inside furnace will cost rejection of entire charge and loss of production and loss of money.

V. I.

The original question proposed using an entirely different type of thermocouple in place of chromel/alumel. I believe that a home-made sensor of the proper type could be a better choice. It depends upon the circumstances. Certainly the junctions have to be prepared with care and calibrated appropriately, but there are many error sources to be considered in such a measurement system. Beyond the sensor junction, there are additional junctions at the probe connection to the cable, at the connections to the reference junction, at the reference junction itself and at the connections to the instrumentation. Most of them cannot be controlled to the extent that you describe and each is, potentially, as much of an error source as the sensor junction. Certainly impurities can change the characteristics of the sensor junction but, if present, they can do the same at the other junctions.

I'm not sure where the "specialised furnaces" and inexperienced engineers came from but I would hope that they are capable of preparing and calibrating such a system, regardless of their sensor decision, before they rely on the data.

DickL

You will get at the very least inaccurate readings. Remember, a TC is 2 dissimilar metals. Not 4.

In just changing a thermocouple the system should be re calibrated. There are minor differences in each thermocouple even if the same type.

No Sorry you can't.

1. The connectors are specific to type

2. Ref thermocouple used inside the Meter is also same type to offset the Ambient-temperature.

Unless you change all the items & recalibrate against a STANDARD, it is not possible.

Not convenient at home level.

Hi Guest, Thermocouple is not just an item which can be replaced just like that. If you do not replace properly the very purpose of the temperature controller is defeated . The temperature of a furnace is very important for the hardening ,tempering or normalising of the component placed inside furnace for heat treatment . For heat treatment of some components a variation of 3 °C to 5 °C may cause entire lot of charge to get rejected.Replacement of thermocouple have to be with same type so long as the temperature controller is same or even one that can be calibrated with different thermocouples ,but still the new thermocouple with compensating lead have to be calibrated with respect to the controller .Even the thermocouples in regular use have to be calibrated routinely as per programme and the room temperature correction to be made as per the climatic changes. Maintaining temperature controllers and thermocouples is a specialised job and it is most vital as far as Quality Control and ISO Certification of the heat treatment process of the product .