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An Introduction to Thermocouples & RTDs

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".

• Type “J” (Iron/Constantan ™)
  Type "J" thermocouples are used in vacuum, oxidizing, inert or reducing atmospheres.Iron element oxidizes rapidly at temperatures exceeding 1000 F (538 C), and therefore require heavier gauge wire to withstand these temperatures.Typically the two wires are red and white.When connecting to the control, the red wire is the negative terminal and the white is the positive terminal.The white wire is also the magnetic wire.
  
  
• Type “K” (Chromel™ /Alumel™)
  Type K thermocouples are used in oxidizing, inert or dry reducing atmospheres. Exposure to vacuum is limited to short time periods.  Type "K" thermocouples must be protected from sulfurous and marginally oxidizing atmospheres. This type is reliable and accurate at high temperatures.  Typically the two wires are yellow and red.  The yellow wire is the positive terminal connection, and the red is the negative terminal connection.  In this case, the red wire is the magnetic wire.
  
  

• GROUNDED THERMOCOUPLES                          
  
  With a grounded thermocouple, the junction is connected to the tip of a metal enclosure. Being connected to the sheath of the enclosure affords protection to the thermocouple and excellent response.

• UNGROUNDED THERMOCOUPLES                    
  
  The thermocouple is isolated from the metal sheath and for this reason gives up a little response time. By being electronically insulated, this design is not subjected to picking up electrical noise. Most newer Van Dorn/Demag, and some other machines, require ungrounded thermocouples.

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?

• Temperature range
  
• Chemical resistance of the thermocouple or sheath material
  
• Abrasion and vibration resistance
  
• Installation requirements (may need to be compatible with existing equipment)
  
• Existing holes may determine probe diameter

What Is An RTD?

RTD, a Resistance Temperature Detector.

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.

Most Common Resistance Materials For RTD's

• Platinum (most popular and accurate)
  
• Nickel
  
• Copper
  
• Balco (rare)
  
• Tungsten (rare)

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.