To select between the sensors above, you should consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition, Thermocouples generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but they are not as accurate or stable as RTD’s and thermistors. RTD’s are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Since they require the use of electric current to make measurements, RTD’s are subject to inaccuracies from self-heating. Thermistors tend to be more accurate than RTD’s or thermocouples, but they have a much more limited temperature range. They are also subject to selfheating. Infrared Sensors can be used to measure temperatures higher than any of the other devices and do so without direct contact with the surfaces being measured. However, they are generally not as accurate and are sensitive to surface radiation efficiency (or more precisely, surface emissivity). Using fiber optic cables, they can measure surfaces that are not within a direct line of sight.

Each type of thermocouple has a different output value, but all are in the millivolt range per degree. For example, here are the millivolts of each thermocouple type at 250C and the mV per degree for each type at 250C:

Conductor Size Equivalents:

AWG = American Wire Gage
SWG = (British) Standard Wire Gage

Each thermocouple type has a designated color-code defined in either ANSI/ASTM E230 or IEC60584. The type can be identified by color as follows:

Also, some materials are strongly to slightly magnetic:
Type J Positive (strongly Magnetic), Type K Positive (slightly magnetic).
To determine polarity, connect the thermocouple to a voltmeter capable of measuring millivolts or microvolts and looking for increasing output when the tip is heated slightly.

Choosing the right type of thermocouple is a mater of matching the thermocouple to your measurement requirement. Here are some areas to take into consideration:

Other important considerations are the sheath materials (if immersion probe style), insulation material (if wire or surface sensor) and sensor geometry.

How Do You Know if You Have a Bad Thermocouple? To understand when we have a bad thermocouple, we first have to understand the working principle of a good thermocouple (one that is working).

A thermocouple works through the thermoelectric effect i.e. the direct conversion of temperature differences to an electric voltage. When the probes of a thermocouple are placed on a surface whose temperature we want to measure, the probes are at slightly different temperatures.

Due to this temperature difference, an EMF is produced. And this EMF is proportional to the temperature.

You can measure the generated EMF with the help of millivoltmeter. The millivoltmeter attached with both probes of a thermocouple. Now if you increase the temperature, the generated EMF should also increase.

So if the EMF reading is not varying with respect to the temperature, then thermocouple is bad / not working properly.

Before using a thermocouple, you must have the reference datasheet of the thermocouple you are using. From the datasheet, you can find the table of temperature and corresponding EMF.

But if you specify some performance parameters like cost, range of temperature, ruggedness, and speed of measurement, the thermocouple has a better performance compared to RTD.

The cost of a thermocouple is much less (almost 2.5 to 3 times) compared to RTD. And also, the cost of installation is cheaper. The RTD is designed to measure a limited range of temperatures.

The advantage of an RTD is that it is more accurate compared to the thermocouple. And the repeatability of measurement is more compared to the thermocouple. Hence, RTD is preferred in the application where the most accurate temperature is required.

So, both devices have their advantages and disadvantages. The thermocouple has a wide range of temperature measurements, cheaper, and durable. On the other hand, RTD has better accuracy and reliable measurement.