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A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. The thermocouple alloys are commonly available as wire.

What are the different thermocouple types?

A thermocouple is available in different combinations of metals. The four most common calibrations are J, K, T and E. There are high temperature calibrations R, S, and B. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Although the thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. That is, a very thin thermocouple may not reach the full  temperature range..

How to select a thermocouple type
Because a thermocouple measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry. The following criteria are used in selecting a thermocouple:

  1. Temperature range
  2. Chemical resistance of the thermocouple or sheath material
  3. Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)

How to select a junction type
Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed (see graphic below: ("Thermocouple Tip Styles"). At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation the thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to dry, noncorrosive and nonpressurized applications..

Thermocouple's composition, temperature , and EMF properties.

ANSI
Letter
Thermocouple Joint Composition Thermocouple
Temperature Range
EMF
positive (+) negative (-)
T Copper Constantan -270°C to +400°C -6.2 to 20.87 mV
E Chromel Constantan -270°C to +1000°C -9.8 to 76.37 mV
J Iron Constantan -210°C to +760°C -8.09 to 69.55 mV
K Chromel Alumel -270°C to +1370°C -6.4 to 54.88 mV
R Pt-13% Rh Platinum -50°C to +1760°C 0.2 to 21.10 mV
B Pt30% Rhodium Pt 6% Rhodium 0°C to + 2000°C 0.0  to 13.82 mV
S Platinum/10% Rhodium Platinum -50°C to +1750°C -0.23 to 18.69 mV
(N) Nicrosil Nisil -270°C to 1300°C -4.34 to 47.51 mV
 

Thermocouple type letters listed above in parenthesis, while industry standard, are non-ANSI types.

** Type B voltage as a function of temperature is non-monotonic, with a minimum of -2.584972uV at a temperature of 21.020262°C. In consequence, function tcb_t() has a lower range limit of about -2.579382uV,
at 22°C.

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 specified set of conditions. Five time constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple is the fastest responding. Also, the smaller the probe sheath diameter, the faster the response, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.

 
grounded junction is recommended for the measurement of static or flowing corrosive gas and liquid temperatures and for high-pressure applications. The junction of a grounded thermocouple is welded to the protective sheath, giving faster response than the ungrounded junction type.
 
An ungrounded junction is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath. The welded wire thermocouple is physically insulated from the thermocouple sheath by MgO powder (soft).
 
An exposed junction is recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response time is required. The junction extends beyond the protective metallic sheath to give accurate fast response. The sheath insulation is sealed where the junction extends to prevent penetration of moisture or gas which could cause errors.
 

Factors to Consider
Choosing the correct thermocouple requires that you look at a number of factors. For example, a thermocouple, which is used only periodically, may be less expensive, but it may have a shorter life span and require servicing more often. If the thermocouple is intended to be used for long periods without service, it may be necessary to use a thicker gauge of wire and it may be necessary to use a design engineered for a greater degree of protection. Below are some general guidelines to consider when choosing a thermocouple. If you need more help making a decision our knowledgeable sales staff is available to recommend a design which will best suit your application.

  1. What are the maximum and minimum temperatures the thermocouple will see?
  2. What error tolerances are necessary for your application?
  3. What is the Measuring atmosphere?(Corrosive/abrasive/vibrating etc)
  4. What is the time response required?
  5. Will the thermocouple be used continuously or periodically?
  6. Will the thermocouple see bending or flexing during it's life?
  7. What is the immersion depth?

General Guidelines

  1. Do not allow excessive bending of the thermocouples. Cold working can lower the insulation resistance at certain points causing short circuits or decreased accuracy.
  2. Use protection tubes for corrosive atmospheres
  3. Always inspect protection tubes when changing thermocouples. Things to look for include cracks, pinholes, contamination, or discoloration.
  4. Do not locate the thermocouple too close to a heating element
  5. Record the life span and cause of failure for each thermocouple to help track down potential problems.
  6. Never apply stress to platinum elements, they are fragile and will break easily at high temperatures.

 
 
 
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