An Introduction to
Thermocouples & RTDs
|WHAT IS A THERMOCOUPLE?
A thermocouple is a sensor that measures temperature. It consists of two different types of metals, joined together at one end. When the junction of two metals is heated or cooled, a voltage is created that can be correlated to the temperature differences between the two ends. If the temperature at one is is known, the temperature at the other end can be determined.
Thermocouple wire, or extension grade wire, is used to connect the thermocouple to the sensing or control instrumentation. The conditions of measurement determined the type of thermocouple wire and insulation to be used.
WHAT IS RESPONSE TIME?
A time constant has been defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specific set of conditions. Five constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple has the fastest response. Also, the smaller the probe sheath diameter, the faster the response. However, the maximum temperature may be lower. Be aware, however, that sometimes probe sheath can not withstand the full temperature range of the thermocouple type.
TYPES OF THERMOCOUPLES COMMONLY AVAILABLE?
Thermocouples are available in different combinations of metals or calibrations. The most common calibrations are "J" and "K".
DETERMINING THE DIFFERENCE BETWEEN TYPE "J" AND TYPE "K"
Typically for Type "J" the two wires are red and white. The red wire is the negative terminal and the white is the positive terminal. The white wire is also the magnetic wire.
The Type "K" has a red wire and a yellow wire. The yellow wire is the positive terminal connection, and the red is the negative. In this case, the red wire is the magnetic wire.
WHAT IS THE BEST WAY TO CHOOSE A THERMOCOUPLE TYPE?
WHAT IS AN RTD
As the name implies, RTD's are sensors used to measure temperature by correlating the resistance of the RTD element with temperature.
Most RTD elements consist of a length of finely wound platinum wire wrapped around a ceramic or glass core. That displays a linear resistance increase for a corresponding temperature increase. RTD’s are built on the principle that most metals have a positive charge in electrical resistance with a change in temperature.
BENEFITS OF USING AN RTD
The RTD is one of the most accurate temperature sensors. Not only does it provide good accuracy, it also provides excellent stability and repeatability.
RTD’s are also relatively immune to electrical noise and therefore well suited for temperature measurements, especially around motors, generators and high-voltage equipment. Typically these are more expensive than Type "J" and Type "K" style thermocouples.
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